modernish: a shell moderniser library

modernish is an ambitious, as-yet experimental, cross-platform POSIX shell feature detection and language extension library. It aims to extend the shell language with extensive feature testing and language enhancements, using the power of aliases and functions to extend the shell language using the shell language itself.

The name is a pun on Modernizr, the JavaScript feature testing library, -sh, the common suffix for UNIX shell names, and -ish, still not quite a modern programming language but perhaps a little closer. jQuery is another source of general inspiration; like it, modernish adds a considerable feature set by using the power of the language it's implemented in to extend/transcend that same language.

That said, the aim of modernish is to build a better shell language, and not to make the shell language into something it's not. Its feature set is aimed at solving specific and commonly experienced deficits and annoyances of the shell language, and not at adding/faking things that are foreign to it, such as object orientation or functional programming. (However, since modernish is modular, nothing stops anyone from adding a module attempting to implement these things.)

The library builds on pure POSIX 2013 Edition (including full C-style shell arithmetics with assignment, comparison and conditional expressions), so it should run on any POSIX-compliant shell and operating system. But it does not shy away from using non-standard extensions where available to enhance performance or robustness.

Some example programs are in share/doc/modernish/examples.

Modernish also comes with a suite of regression tests to detect bugs in modernish itself. See Appendix B.

Table of contents

• Getting started
• Two basic forms of a modernish program
  • Important notes regarding the system locale
• Interactive use
• Non-interactive command line use
  • Non-interactive usage examples
• Internal namespace
• Feature testing
• Modernish system constants
  • Control character, whitespace and shell-safe character constants
• Legibility aliases
• Enhanced exit
• Reliable emergency halt
• Low-level shell utilities
• Quoting strings for subsequent parsing by the shell
• The stack
  • The shell options stack
  • The trap stack
    • Trap stack compatibility considerations
  • Positional parameters stack
  • Stack query and maintenance functions
• Hardening: emergency halt on error
  • Important note on variable assignments
  • Hardening while allowing for broken pipes
  • Tracing the execution of hardened commands
• Simple tracing of commands
• External commands without full path
• Outputting strings
• Enhanced dot scripts
• Testing numbers, strings and files
  • Integer number arithmetic tests and operations
  • String tests
  • File type tests
  • File comparison tests
  • File status tests
  • I/O tests
  • File permission tests
• Basic string operations
  • toupper/tolower
• Basic system utilities
• Modules
  • use safe
  • use var/arith
    • Arithmetic operator shortcuts
    • Arithmetic comparison shortcuts
  • use var/mapr
    • Differences from mapfile
  • use var/readf
  • use var/setlocal
  • use var/string
  • use sys/base
    • use sys/base/readlink
    • use sys/base/which
    • use sys/base/mktemp
    • use sys/base/seq
    • use sys/base/rev
  • use sys/dir
    • use sys/dir/traverse
    • use sys/dir/countfiles
  • use sys/term
    • use sys/term/readkey
  • use opts/parsergen
  • use loop/cfor
  • use loop/sfor
  • use loop/with
  • use loop/select
• Appendix A
  • Capabilities
  • Quirks
  • Bugs
  • Warning IDs
• Appendix B
  • Difference between bug/quirk/feature tests and regression tests
  • Testing modernish on all your shells

Getting started

Run install.sh and follow instructions, choosing your preferred shell and install location. After successful installation you can run modernish shell scripts and write your own. Run uninstall.sh to remove modernish.

Both the install and uninstall scripts are interactive by default, but support fully automated (non-interactive) operation as well. Command line options are as follows:

install.sh [ -n ] [ -s shell ] [ -f ] [ -d installroot ] [ -D prefix ]

• -n: non-interactive operation
• -s: specify default shell to execute modernish
• -f: force unconditional installation on specified shell
• -d: specify root directory for installation
• -D: extra destination directory prefix (for packagers)

uninstall.sh [ -n ] [ -f ] [ -d installroot ]

• -n: non-interactive operation
• -f: delete */modernish directories even if files left
• -d: specify root directory of modernish installation to uninstall

Two basic forms of a modernish program

The simplest way to write a modernish program is to source modernish as a dot script. For example, if you write for bash:

#! /bin/bash
. modernish
use safe
use sys/base
...your program starts here...

The modernish 'use' command load modules with optional functionality. safe is a special module that introduces a new and safer way of shell programming, with field splitting (word splitting) and pathname expansion (globbing) disabled by default. The sys/base module contains modernish versions of certain basic but non-standardised utilities (e.g. readlink, mktemp, which), guaranteeing that modernish programs all have a known version at their disposal. There are many other modules as well. See below for more information.

The above method makes the program dependent on one particular shell (in this case, bash). So it is okay to mix and match functionality specific to that particular shell with modernish functionality.

The most portable way to write a modernish program is to use the special generic hashbang path for modernish programs. For example:

#! /usr/bin/env modernish
#! use safe
#! use sys/base
...your program begins here...

A program in this form is executed by whatever shell the user who installed modernish on the local system chose as the default shell. Since you as the programmer can't know what shell this is (other than the fact that it passed some rigorous POSIX compliance testing executed by modernish), a program in this form must be strictly POSIX compliant -- except, of course, that it should also make full use of the rich functionality offered by modernish.

Note that modules are loaded in a different way: the use commands are part of hashbang comment (starting with #! like the initial hashbang path). Only such lines that immediately follow the initial hashbang path are evaluated; even an empty line in between causes the rest to be ignored.

Important notes regarding the system locale

• modernish, like most shells, fully supports two locales: POSIX (a.k.a. C, a.k.a. ASCII) and Unicode's UTF-8. It will work in others, but things like converting to upper/lower case, and matching single characters in patterns, are not guaranteed.
  Caveat: some shells or operating systems have bugs that prevent (or lack features required for) full locale support. If portability is a concern, check for thisshellhas BUG_MULTIBYTE or thisshellhas BUG_NOCHCLASS where needed. See Appendix A under Bugs.
• Scripts/programs should not change the locale (LC_* or LANG) after initialising modernish. Doing this might break various functions, as modernish sets specific versions depending on your OS, shell and locale. (Temporarily changing the locale is fine as long as you don't use modernish features that depend on it -- for example, setting a specific locale just for an external command. However, if you use harden(), see the important note in its documentation below!)

Interactive use

Modernish is primarily designed to enhance shell programs/scripts, but also offers features for use in interactive shells. For instance, the new with loop construct from the loop/with module can be quite practical to repeat an action x times, and the safe module on interactive shells provides convenience functions for manipulating, saving and restoring the state of field splitting and globbing.

To use modernish on your favourite interactive shell, you have to add it to your .profile, .bashrc or similar init file.

Important: Upon initialising, modernish adapts itself to other settings, such as the locale. It also removes certain aliases that may keep modernish from initialising properly. So you have to organise your .profile or similar file in the following order:

• first, define general system settings (PATH, locale, etc.);
• then, . modernish and use any modules you want;
• then define anything that may depend on modernish, and set your aliases.

Non-interactive command line use

After installation, the modernish command can be invoked as if it were a shell, with the standard command line options from other shells (such as -c to specify a command or script directly on the command line), plus some enhancements. The effect is that the shell chosen at installation time will be run enhanced with modernish functionality. It is not possible to use modernish as an interactive shell in this way.

Usage:

1. modernish [ --use=module | option ... ] [ scriptfile ] [ arguments ]
2. modernish [ --use=module | option ... ] -c [ script [ me-name [ arguments ] ] ]
3. modernish --test
4. modernish --version

In the first form, the --use long-form option preloads any given modernish modules, any given short or long-form shell options are set or unset (the syntax is identical to that of POSIX shells and the shell options supported depend on the shell executing modernish), and then scriptfile is loaded and executed with any arguments assigned to the positional parameters.

The module argument to each specified --use option is split using standard shell field splitting. The first field is the module name and any further fields become arguments to that module's initialisation routine.

Using the shell option -e or -o errexit is an error, because modernish does not support it and would break. If the shell option -x or -o xtrace is given, modernish sets the PS4 prompt to a useful value that traces the line number and exit status, as well as the current file and function names if the shell is capable of this.

In the second form, after pre-loading any modules and setting any shell options as in the first form, -c executes the specified modernish script, optionally with the me-name assigned to $ME and the arguments assigned to the positional parameters. This is identical to the -c option on POSIX shells, except that the me-name is assigned to $ME and not $0 (because POSIX shells do not allow changing $0).

The --test option runs the regression test suite and exits. This verifies that the modernish installation is functioning correctly. See Appendix B for more information.

The --version option outputs the version of modernish and exits.

Non-interactive usage examples

• Count to 10 using loop/with:
  modernish --use=loop/with -c 'with i=1 to 10; do putln "$i"; done'
• Run a portable-form modernish program using zsh and enhanced-prompt xtrace:
  zsh /usr/local/bin/modernish -o xtrace /path/to/program.sh

Internal namespace

Function-local variables are not supported by the standard POSIX shell; only global variables are provided for. Modernish needs a way to store its internal state without interfering with the program using it. So most of the modernish functionality uses an internal namespace _Msh_* for variables, functions and aliases. All these names may change at any time without notice. Any names starting with _Msh_ should be considered sacrosanct and untouchable; modernish programs should never directly use them in any way. Of course this is not enforceable, but names starting with _Msh_ should be uncommon enough that no unintentional conflict is likely to occur.

Feature testing

Modernish includes a battery of shell bug, quirk and feature tests, each of which is given a special ID. See Appendix A below for a list of shell capabilities, quirks and bugs that modernish currently tests for, as well as further general information on the feature testing framework.

thisshellhas is the central function of the modernish feature testing framework. It not only tests for the presence of modernish shell capabilities/quirks/bugs on the current shell, but can also test for the presence of specific shell built-in commands, shell reserved words (a.k.a. keywords), shell options (short or long form), and signals.

Modernish itself extensively uses feature testing to adapt itself to the shell it's running on. This is how it works around shell bugs and takes advantage of efficient features not all shells have. But any script using the library can do this in the same way, with the help of this function.

For those tests that have no standardised way of performing them, thisshellhas knows the shell-specific methods on all supported shells and automatically initialises the correct version for the current shell.

Test results are cached in memory, so repeated checks using thisshellhas are efficient and there is no need to avoid calling it to optimise performance.

Usage:

thisshellhas [ --cache | --show ] item [ item ... ]

• If item contains only ASCII capital letters A-Z, digits 0-9 or _, return the result status of the associated modernish feature, quirk or bug test.
• If item is an ASCII all-lowercase word, check if it's a shell reserved word or built-in command on the current shell.
• If item starts with --rw= or --kw=, check if the identifier immediately following these characters is a shell reserved word (a.k.a. shell keyword).
• If item starts with --bi=, similarly check for a shell built-in command.
• If item starts with --sig=, check if the shell knows about a signal (usable by kill, trap, etc.) by the name or number following the =. If a number > 128 is given, the remainder of its division by 128 is checked. If the signal is found, its canonicalised signal name is left in the REPLY variable, otherwise REPLY is unset. (If multiple --sig= items are given and all are found, REPLY contains only the last one.)
• If item is -o followed by a separate word, check if this shell has a long-form shell option by that name.
• If item is any other letter or digit preceded by a single -, check if this shell has a short-form shell option by that character.
• The --cache option runs all external modernish bug/quirk/feature tests that have not yet been run, causing the cache to be complete.
• The --show option performs a --cache and then outputs all the IDs of positive results, one per line.

thisshellhas continues to process items until one of them produces a negative result or is found invalid, at which point any further items are ignored. So the function only returns successfully if all the items specified were found on the current shell. (To check if either one item or another is present, use separate thisshellhas invocations separated by the || shell operator.)

Exit status: 0 if this shell has all the items in question; 1 if not; 2 if an item was encountered that is not recognised as a valid identifier.

Note: The tests for the presence of reserved words, built-in commands, shell options, and signals are different from feature/quirk/bug tests in an important way: they only check if an item by that name exists on this shell, and don't verify that it does the same thing as on another shell.

Modernish system constants

Modernish provides certain constants (read-only variables) to make life easier. These include:

• $MSH_VERSION: The version of modernish.
• $MSH_PREFIX: Installation prefix for this modernish installation (e.g. /usr/local).
• $ME: Path to the current program. Replacement for $0. This is necessary if the hashbang path #!/usr/bin/env modernish is used, or if the program is launched like sh /path/to/bin/modernish /path/to/script.sh, as these set $0 to the path to bin/modernish and not your program's path.
• $MSH_SHELL: Path to the default shell for this modernish installation, chosen at install time (e.g. /bin/sh). This is a shell that is known to have passed all the modernish tests for fatal bugs. Cross-platform scripts should use it instead of hard-coding /bin/sh, because on some operating systems (NetBSD, OpenBSD, Solaris) /bin/sh is not POSIX compliant.
• $SIGPIPESTATUS: The exit status of a command killed by SIGPIPE (a broken pipe). For instance, if you use grep something somefile.txt | more and you quit more before grep is finished, grep is killed by SIGPIPE and exits with that particular status. Some modernish functions, such as harden and traverse, need to handle such a SIGPIPE exit specially to avoid unduly killing the program. The exact value of this exit status is shell-specific, so modernish runs a quick test to determine it at initialisation time.
  If SIGPIPE was set to ignore by the process that invoked the current shell, SIGPIPESTATUS can't be detected and is set to the special value 99999. See also the description of the WRN_NOSIGPIPE ID for thisshellhas.
• $DEFPATH: The default system path guaranteed to find compliant POSIX utilities, as given by getconf PATH.

Control character, whitespace and shell-safe character constants

POSIX does not provide for the quoted C-style escape codes commonly used in bash, ksh and zsh (such as $'\n' to represent a newline character), leaving the standard shell without a convenient way to refer to control characters. Modernish provides control character constants (read-only variables) with hexadecimal suffixes $CC01 .. $CC1F and $CC7F, as well as $CCe, $CCa, $CCb, $CCf, $CCn, $CCr, $CCt, $CCv (corresponding with printf backslash escape codes). This makes it easy to insert control characters in double-quoted strings.

More convenience constants, handy for use in bracket glob patterns for use with case or modernish match:

• $CONTROLCHARS: All ASCII control characters.
• $WHITESPACE: All ASCII whitespace characters.
• $ASCIIUPPER: The ASCII uppercase letters A to Z.
• $ASCIILOWER: The ASCII lowercase letters a to z.
• $ASCIIALNUM: The ASCII alphanumeric characters 0-9, A-Z and a-z.
• $SHELLSAFECHARS: Safelist for shell-quoting.
• $ASCIICHARS: The complete set of ASCII characters (minus NUL).

Legibility aliases

A few aliases that seem to make the shell language look slightly friendlier:

alias not='! '              # more legible synonym for '!'
alias so='[ "$?" -eq 0 ]'   # test preceding command's success with
                            # 'if so;' or 'if not so;'
alias forever='while :;'    # indefinite loops: forever do <stuff>; done

Enhanced exit

exit: extended usage: exit [ -u ] [ status [ message ] ]

• If the -u option is given, the shell function showusage is called, which has a simple default but can be redefined by your script. This function is intended to print a message showing how the command should be invoked. The function is run in a subshell and its output is redirected to standard error.
• Any remaining arguments after status are combined, separated by spaces, and taken as a message to print on exit. It is printed to standard error, preceded by the name of the current program ($ME minus directories).

Reliable emergency halt

die: reliably halt program execution, even from within subshells, optionally printing an error message. Note that die is meant for an emergency program halt only, i.e. in situations were continuing would mean the program is in an inconsistent or undefined state. Shell scripts running in an inconsistent or undefined state may wreak all sorts of havoc. They are also notoriously difficult to terminate correctly, especially if the fatal error occurs within a subshell: exit won't work then. That's why die is optimised for killing all the program's processes (including subshells and external commands launched by it) as quickly as possible. It should never be used for exiting the program normally.

On interactive shells, die behaves differently. It does not kill or exit your shell; instead, it issues SIGINT to the shell to abort the execution of your running command(s), which is equivalent to pressing Ctrl+C.

Usage: die [ message ]

A special DIE pseudosignal can be trapped (using plain old trap or pushtrap) to perform emergency cleanup commands upon invoking die. On interactive shells, DIE is simply an alias for SIGINT. On non-interactive shells, DIE traps are separate. In order to kill the malfunctioning program as quickly as possible (hopefully before it has a chance to delete all your data), die doesn't wait for those traps to complete before killing the program. Instead, it executes each DIE trap simultaneously as a background job, then gathers the process IDs of the main shell and all its subprocesses, sending SIGKILL to all of them except any DIE trap processes.

(One case where die is limited is when the main shell program has exited, but several runaway background processes that it forked are still going. If die is called by one of those background processes, then it will kill that background process and its subshells, but not the others. This is due to an inherent limitation in the design of POSIX operating systems. When the main shell exits, its surviving background processes are detached from the process hierarchy and become independent from one another, with no way to determine that they once belonged to the same program.)

Low-level shell utilities

insubshell: easily check if you're currently running in a subshell environment (usually called simply subshell), that is, a copy of the parent shell that starts out as an exact duplicate except for traps. This is not to be confused with a newly initialised shell that is merely a child process of the current shell, which is sometimes (erroneously) called a "subshell" as well.
Usage: insubshell [ -p | -u ]
This function returns success (0) if it was called from within a subshell and non-success (1) if not. One of two options can be given:

• -p: Store the process ID (PID) of the current subshell or main shell in REPLY. (Note that on AT&T ksh93, which does not fork a new process for non-background subshells, that PID is same as the main shell's except for background jobs.)
• -u: Store an identifier in REPLY that is useful for determining if you've entered a subshell relative to a previously stored identifier. The content and format are unspecified and shell-dependent.

setstatus: manually set the exit status $? to the desired value. The function exits with the status indicated. This is useful in conditional constructs if you want to prepare a particular exit status for a subsequent 'exit' or 'return' command to inherit under certain circumstances.

isvarname: Check if argument is valid portable identifier in the shell, that is, a portable variable name, shell function name or long-form shell option name. (Modernish requires portable names everywhere; for example, accented or non-Latin characters in variable names are not supported.)

isset: check if a variable, shell function or option is set. Usage:

• isset varname: Check if a variable is set.
• isset -v varname: Id.
• isset -x varname: Check if variable is exported.
• isset -r varname: Check if variable is read-only.
• isset -f funcname: Check if a shell function is set.
• isset -optionletter (e.g. isset -C): Check if shell option is set.
• isset -o optionname: Check if shell option is set by long name.

Exit status: 0 if the item is set; 1 if not; 2 if the argument is not recognised as a syntactically valid identifier.

When checking a shell option, a nonexistent shell option is not an error, but returns the same result as an unset shell option. (To check if a shell option exists, use thisshellhas.

Note: just isset -f checks if shell option -f (a.k.a. -o noglob) is set, but with an extra argument, it checks if a shell function is set. Similarly, isset -x checks if shell option -x (a.k.a -o xtrace) is set, but isset -x varname checks if a variable is exported. If you use unquoted variable expansions here, make sure they're not empty, or the shell's empty removal mechanism will cause the wrong thing to be checked (even in use safe mode).

unexport: the opposite of export. Unexport a variable while preserving its value, or (while working under set -a) don't export it at all. Usage is like export, with the caveat that variable assignment arguments containing non-shellsafe characters or expansions must be quoted as appropriate, unlike in some specific shell implementations of export. (To get rid of that headache, use safe.)

Quoting strings for subsequent parsing by the shell

shellquote: Quote the values of specified variables in such a way that the values are suitable for parsing by the shell as string literals. This is essential for the safe use of eval or any other context where the shell must parse untrusted input. shellquote only uses quoting mechanisms specified by POSIX, so the quoted values it produces are safe to parse in any POSIX shell.
Usage: shellquote [ -f|+f ] varname [ [ -f|+f ] varname ... ]
The values of the variables specified by name are shell-quoted and stored back into those variables. By default, a value is only quoted if it contains characters not present in $SHELLSAFECHARS. An -f argument forces unconditional quoting for subsequent variables; an +f argument restores default behaviour. shellquote returns success (0) if all variables were processed successfully, and non-success (1) if any undefined (unset) variables were encountered. In the latter case, any set variables still get their values quoted.

shellquoteparams: shell-quote the current shell's positional parameters in-place.

storeparams: store the positional parameters, or a sub-range of them, in a variable, in a shellquoted form suitable for restoration using eval "set -- $varname". For instance: storeparams -f2 -t6 VAR quotes and stores $2 to $6 in VAR.

The stack

push & pop: every variable and shell option gets its own stack. For variables, both the value and the set/unset state is (re)stored. Usage:

• push [ --key=value ] item [ item ... ]
• pop [ --keepstatus ] [ --key=value ] item [ item ... ]

where item is a valid portable variable name, a short-form shell option (dash plus letter), or a long-form shell option (-o followed by an option name, as two arguments). The precise shell options supported (other than the ones guaranteed by POSIX) depend on the shell modernish is running on. For cross-shell compatibility, nonexistent shell options are treated as unset.

Before pushing or popping anything, both functions check if all the given arguments are valid and pop checks all items have a non-empty stack. This allows pushing and popping groups of items with a check for the integrity of the entire group. pop exits with status 0 if all items were popped successfully, and with status 1 if one or more of the given items could not be popped (and no action was taken at all).

The --key= option is an advanced feature that can help different modules or funtions to use the same variable stack safely. If a key is given to push, then for each item, the given key value is stored along with the variable's value for that position in the stack. Subsequently, restoring that value with pop will only succeed if the key option with the same key value is given to the pop invocation. Similarly, popping a keyless value only succeeds if no key is given to pop. If there is any key mismatch, no changes are made and pop returns status 2. For instance, if a function pushes all its values with something like --key=myfunction, it can do a loop like while pop --key=myfunction var; do ... even if var already has other items on its stack that shouldn't be tampered with. Note that this is a robustness/convenience feature, not a security feature; the keys are not hidden in any way. (The var/setlocal module, which provides stack-based local variables, internally makes use of this feature.)

If the --keepstatus option is given, pop will exit with the exit status of the command executed immediately prior to calling pop. This can avoid the need for awkward workarounds when restoring variables or shell options at the end of a function. However, note that this makes failure to pop (stack empty or key mismatch) a fatal error that kills the program, as pop no longer has a way to communicate this through its exit status.

The shell options stack

The shell options stack allows saving and restoring the state of any shell option available to the set builtin using push and pop commands with a syntax similar to that of set.

Long-form shell options are matched to their equivalent short-form shell options, if they exist. For instance, on all POSIX shells, -f is equivalent to -o noglob, and push -o noglob followed by pop -f works correctly. (This works even for shell-specific short & long option equivalents; modernish internally does a check to find any equivalent.)

On shells with a dynamic no option name prefix, that is on ksh, zsh and yash (where, for example, noglob is the opposite of glob), the no prefix is ignored, so something like push -o glob followed by pop -o noglob does the right thing. But this depends on the shell and should never be used in cross-shell scripts.

To facilitate cross-shell scripting, it is fine to push and pop shell options that may not exist on the current shell; nonexistent options are considered unset for stack purposes.

The trap stack

pushtrap and poptrap: traps are now also stack-based, so that each program component or library module can set its own trap commands without interfering with others.

Note an important difference between the trap stack and stacks for variables and shell options: pushing traps does not save them for restoring later, but adds them alongside other traps on the same signal. All pushed traps are active at the same time and are executed from last-pushed to first-pushed when the respective signal is triggered. Traps cannot be pushed and popped using push and pop but use dedicated commands as follows.

Usage:

• pushtrap [ --key=value ] [ -- ] command sigspec [ sigspec ... ]
• poptrap [ --key=value ] [ -- ] sigspec [ sigspec ... ]

pushtrap works like regular trap, with the following exceptions:

• Adds traps for a signal without overwriting previous ones. (However, any traps set prior to initialising modernish, or by bypassing the modernish 'trap' alias to access the system command directly, will be overwritten by a pushtrap for the same signal. To remedy this, you can issue a simple trap command; as modernish prints the traps, it will quietly detect ones it doesn't yet know about and make them work nicely with the trap stack.)
• Unlike regular traps, a stack-based trap does not cause a signal to be ignored. Setting one will cause it to be executed upon the shell receiving that signal, but after the stack traps complete execution, modernish re-sends the signal to the main shell, causing it to behave as if no trap were set (unless a regular POSIX trap is also active). Thus, pushtrap does not accept an empty command as it would be pointless.
• Each stack trap is executed in a new subshell to keep it from interfering with others. This means a stack trap cannot change variables except within its own environment, and 'exit' will only exit the trap and not the program.
• Each stack trap is executed with $? initially set to the exit status that was active at the time the signal was triggered.
• Stack traps do not have access to the positional parameters.
• pushtrap stores current $IFS (field splitting) and $- (shell options) along with the pushed trap. Within the subshell executing each stack trap, modernish restores IFS and the shell options f (noglob), u (nounset) and C (noclobber) to the values in effect during the corresponding pushtrap. This is to avoid unexpected effects in case a trap is triggered while temporary settings are in effect.
• The --key option applies the keying functionality inherited from plain push to the trap stack. It works the same way, so the description is not repeated here.

poptrap takes just signal names or numbers as arguments. It takes the last-pushed trap for each signal off the stack, storing the commands that was set for those signals into the REPLY variable, in a format suitable for re-entry into the shell. Again, the --key option works as in plain pop.

Stack-based traps, like native shell traps, are reset upon entering a subshell (such as a command substitution or a series of commands enclosed in parentheses). However, commands for printing traps will print the traps for the parent shell, until another trap, pushtrap or poptrap command is invoked, at which point all memory of the parent shell's traps is erased.

Trap stack compatibility considerations

Modernish tries hard to avoid incompatibilities with existing trap practice. To that end, it intercepts the regular POSIX 'trap' command using an alias, reimplementing and interfacing it with the shell's builtin trap facility so that plain old regular traps play nicely with the trap stack. You should not notice any changes in the POSIX 'trap' command's behaviour, except for the following:

• The regular 'trap' command does not overwrite stack traps (but does overwrite previous regular traps).
• Unlike zsh's native trap command, signal names are case insensitive.
• Unlike dash's native trap command, signal names may have the SIG prefix; that prefix is quietly accepted and discarded.
• Setting an empty trap action to ignore a signal only works fully (passing the ignoring on to child processes) if there are no stack traps associated with the signal; otherwise, an empty trap action merely suppresses the signal's default action for the current process -- e.g., after executing the stack traps, it keeps the shell from exiting.
• The 'trap' command with no arguments, which prints the traps that are set in a format suitable for re-entry into the shell, now also prints the stack traps as 'pushtrap' commands. (bash users might notice the SIG prefix is not included in the signal names written.)
• The bash/yash-style '-p' option, including its yash-style --print equivalent, is now supported on all shells. If further arguments are given after that option, they are taken as signal specifications and only the commands to recreate the traps for those signals are printed.
• Saving the traps to a variable using command substitution (as in: var=$(trap)) now works on every shell supported by modernish, including (d)ash, mksh and zsh.
• To reset (unset) a trap, the modernish 'trap' command accepts both valid POSIX syntax and legacy bash/(d)ash/zsh syntax, like trap INT to unset a SIGINT trap (which only works if the 'trap' command is given exactly one argument). Note that this is for compatibility with existing scripts only.

POSIX traps for each signal are always executed after that signal's stack-based traps; this means they should not rely on modernish modules that use the trap stack to clean up after themselves on exit, as those cleanups would already have been done.

Modernish introduces a new DIE pseudosignal whose traps are executed upon invoking die. See the die description for more information.

On interactive shells, INT traps (both POSIX and stack) are cleared out after executing them once. This is because die uses SIGINT for cleanup and command interruption on interactive shells, with the DIE signal name turned into an alias for INT. As a side effect of this special handling, INT traps on interactive shells do not have access to the positional parameters and cannot return from shell functions.

Positional parameters stack

pushparams and popparams: push and pop the complete set of positional parameters. No arguments are supported.

Stack query and maintenance functions

For the four functions below, item can be:

• a valid portable variable name
• a short-form shell option: dash plus letter
• a long-form shell option: -o followed by an option name (two arguments)
• @ to refer to the positional parameters stack
• --trap=SIGNAME to refer to the trap stack for the indicated signal

stackempty [ --key=value ] [ --force ] item: Tests if the stack for an item is empty. Returns status 0 if it is, 1 if it is not. The key feature works as in pop: by default, a key mismatch is considered equivalent to an empty stack. If --force is given, this function ignores keys altogether.

stacksize [ --silent | --quiet ] item: Leaves the size of a stack in the REPLY variable and, if option --silent or --quiet is not given, writes it to standard output. The size of the complete stack is returned, even if some values are keyed.

printstack [ --quote ] item: Outputs a stack's content. Option --quote shell-quotes each stack value before printing it, allowing for parsing multi-line or otherwise complicated values. Column 1 to 7 of the output contain the number of the item (down to 0). If the item is set, column 8 and 9 contain a colon and a space, and if the value is non-empty or quoted, column 10 and up contain the value. Sets of values that were pushed with a key are started with a special line containing --- key:value. A subsequent set pushed with no key is started with a line containing --- (key off). Returns status 0 on success, 1 if that stack is empty.

clearstack [ --key=value ] [ --force ] item [ item ... ]: Clears one or more stacks, discarding all items on it. If (part of) the stack is keyed or a --key is given, only clears until a key mismatch is encountered. The --force option overrides this and always clears the entire stack (be careful, e.g. don't use within setlocal ... endlocal). Returns status 0 on success, 1 if that stack was already empty, 2 if there was nothing to clear due to a key mismatch.

Hardening: emergency halt on error

harden: modernish's replacement for set -e a.k.a. set -o errexit (which is fundamentally flawed, not supported and will break the library).

harden installs a shell function that hardens a particular command by checking its exit status against values indicating error or system failure. Exactly what exit statuses signify an error or failure depends on the command in question; this should be looked up in the POSIX specification (under "Utilities") or in the command's man page or other documentation.

If the command fails, the function installed by harden calls die, so it will reliably halt program execution, even if the failure occurred within a subshell (for instance, in a pipe construct or command substitution).

harden (along with use safe) is an essential feature for robust shell programming that current shells lack. In shell programs without modernish, proper error checking is too inconvenient and therefore rarely done. It's often recommended to use set -e a.k.a set -o errexit, but that is broken in various strange ways (see links above) and the idea is often abandoned. So, all too often, shell programs simply continue in an inconsistent state after a critical error occurs, occasionally wreaking serious havoc on the system. Modernish harden was designed to help solve that problem properly.

Usage:

harden [ -f funcname ] [ -[cpXtPE] ] [ -e testexpr ] [ var=value ... ] [ -u var ... ] command_name_or_path [ command_argument ... ]

The -f option hardens the command as the shell function funcname instead of defaulting to command_name_or_path as the function name. (If the latter is a path, that's always an invalid function name, so the use of -f is mandatory.)

The -c option causes command_name_or_path to be hardened and run immediately instead of setting a shell function for later use. This option is meant for commands that run once; it is not efficient for repeated use. It cannot be used together with the -f option.

The -e option, which defaults to >0, indicates the exit statuses corresponding to a fatal error. It depends on the command what these are; consult the POSIX spec and the manual pages. The status test expression testexpr, argument to the -e option, is like a shell arithmetic expression, with the binary operators == != <= >= < > turned into unary operators referring to the exit status of the command in question. Assignment operators are disallowed. Everything else is the same, including && (logical and) and || (logical or) and parentheses. Note that the expression needs to be quoted as the characters used in it clash with shell grammar tokens.

The -X option causes harden to always search for and harden an external command, even if a built-in command by that name exists.

The -E option causes the hardening function to consider it a fatal error if the hardened command writes anything to the standard error stream. This option allows hardening commands (such as bc) where you can't rely on the exit status to detect an error. The text written to standard error is passed on as part of the error message printed by die. Note that:

• Intercepting standard error necessitates that the command be executed from a subshell. This means any builtins or shell functions hardened with -E cannot influence the calling shell (e.g. harden -E cd renders cd ineffective).
• -E does not disable exit status checks; by default, any exit status greater than zero is still considered a fatal error as well. If your command does not even reliably return a 0 status upon success, then you may want to add -e '>125', limiting the exit status check to reserved values indicating errors launching the command and signals caught.

The -p option causes harden to search for commands using the system default path (as obtained with getconf PATH) as opposed to the current $PATH. This ensures that you're using a known-good external command that came with your operating system. By default, the system-default PATH search only applies to the command itself, and not to any commands that the command may search for in turn. But if the -p option is specified at least twice, or if the command is a shell function (hardened under another name using -f), the command is run in a subshell with PATH exported as the default path, which is equivalent to adding a PATH=$DEFPATH assignment argument (see below).

Examples:

harden make                           # simple check for status > 0
harden -f tar '/usr/local/bin/gnutar' # id.; be sure to use this 'tar' version
harden -e '> 1' grep                  # for grep, status > 1 means error
harden -e '==1 || >2' gzip            # 1 and >2 are errors, but 2 isn't (see manual)

Important note on variable assignments

As far as the shell is concerned, hardened commands are shell functions and not external or builtin commands. This essentially changes one behaviour of the shell: variable assignments preceding the command will not be local to the command as usual, but will persist after the command completes. (POSIX technically makes that behaviour optional but all current shells behave the same in POSIX mode.)

For example, this means that something like

harden -e '>1' grep
# [...]
LC_ALL=C grep regex some_ascii_file.txt

should never be done, because the meant-to-be-temporary LC_ALL locale assignment will persist and is likely to cause problems further on.

To solve this problem, harden supports adding these assignments as part of the hardening command, so instead of the above you do:

harden -e '>1' LC_ALL=C grep
# [...]
grep regex some_ascii_file.txt

With the -u option, harden also supports unsetting variables for the duration of a command, e.g.:

harden -e '>1' -u LC_ALL grep

Pitfall alert: if the -u option is used, this causes the hardened command to run in a subshell with those variables unset, because using a subshell is the only way to avoid altering those variables' state in the main shell. This is usually fine, but note that a builtin command hardened with use of -u cannot influence the calling shell. For instance, something like harden -u LC_ALL cd renders cd ineffective: the working directory is only changed within the subshell which is then immediately left.

The same happens if you harden a shell function under another name using -f while adding environment variable assignments (or using the -p option, which effectively adds PATH=$DEFPATH). The hardened function will not be able to influence the main shell. Also note that the hardening function will export the assigned environment variables for the duration of that subshell, so those variables will be inherited by any external command run from the hardened function. (While hardening shell functions using harden is possible, it's not really recommended and it's better to call die directly in your shell function upon detecting an error.)

Hardening while allowing for broken pipes

If you're piping a command's output into another command that may close the pipe before the first command is finished, you can use the -P option to allow for this:

harden -e '==1 || >2' -P gzip       # also tolerate gzip being killed by SIGPIPE
gzip -dc file.txt.gz | head -n 10	# show first 10 lines of decompressed file

head will close the pipe of gzip input after ten lines; the operating system kernel then kills gzip with the PIPE signal before it's finished, causing a particular exit status that is greater than 128. This exit status would normally make harden kill your entire program, which in the example above is clearly not the desired behaviour. If the exit status caused by a broken pipe were known, you could specifically allow for that exit status in the status expression. The trouble is that this exit status varies depending on the shell and the operating system. The -p option was made to solve this problem: it automatically detects and whitelists the correct exit status corresponding to SIGPIPE termination on the current system.

Tolerating SIGPIPE is an option and not the default, because in many contexts it may be entirely unexpected and a symptom of a severe error if a command is killed by a broken pipe. It is up to the programmer to decide which commands should expect SIGPIPE and which shouldn't.

Tip: It could happen that the same command should expect SIGPIPE in one context but not another. You can create two hardened versions of the same command, one that tolerates SIGPIPE and one that doesn't. For example:

harden -f hardGrep -e '>1' grep     # hardGrep does not tolerate being aborted
harden -f pipeGrep -e '>1' -P grep  # pipeGrep for use in pipes that may break

Note: If SIGPIPE was set to ignore by the process invoking the current shell, the -p option has no effect, because no process or subprocess of the current shell can ever be killed by SIGPIPE. However, this may cause various other problems and you may want to refuse to let your program run under that condition. thisshellhas WRN_NOSIGPIPE can help you easily detect that condition so your program can make a decision. See the WRN_NOSIGPIPE description for more information.

Tracing the execution of hardened commands

The -t option will trace command output. Each execution of a command hardened with -t causes the full command line to be output to standard error, in the following format:

[functionname]> commandline

where functionname is the name of the shell function used to harden the command and commandline is the complete and actual command executed. The commandline is properly shell-quoted in a format suitable for re-entry into the shell (which is an enhancement over the builtin tracing facility on most shells). If standard error is on a terminal that supports ANSI colours, the tracing output will be colourised.

The -t option was added to harden because the commands that you harden are often the same ones you would be particularly interested in tracing. The advantage of using harden -t over the shell's builtin tracing facility (set -x or set -o xtrace) is that the output is a lot less noisy, especially when using a shell library such as modernish.

Note: Internally, -t uses the shell file descriptor 9, redirecting it to standard error (using exec 9>&2). This allows tracing to continue to work normally even for commands that redirect standard error to a file (which is another enhancement over set -x on most shells). However, this does mean harden -t conflicts with any other use of the file descriptor 9 in your shell program.

If file descriptor 9 is already open before harden is called, harden does not attempt to override this. This means tracing may be redirected elsewhere by doing something like exec 9>trace.out before calling harden. (Note that redirecting FD 9 on the harden command itself will not work as it won't survive the run of the command.)

Simple tracing of commands

Sometimes you just want to trace the execution of some specific commands as in harden -t (see above) without actually hardening them against command errors; you might prefer to do your own error handling. trace makes this easy. It is modernish's replacement or complement for set -x a.k.a. set -o xtrace.

trace is actually a shortcut for harden -t -P -e '>125 && !=255' commandname. The result is that the indicated command is automatically traced upon execution. Other options, including -f, -c and environment variable assignments, are as in harden.

A bonus is that you still get minimal hardening against fatal system errors. Errors in the traced command itself are ignored, but your program is immediately halted with an informative error message if the traced command:

• cannot be found (exit status 127);
• was found but cannot be executed (exit status 126);
• was killed by a signal other than SIGPIPE (exit status > 128, except the shell-specific exit status for SIGPIPE, and except 255 which is used by some utilities, such as ssh and rsync, to return an error).

Note: The caveat for command-local variable assignments for harden also applies to trace. See Important note on variable assignments above.

External commands without full path

extern is like command but always runs an external command, without having to know or determine its location. This provides an easy way to bypass a builtin, alias or function. It does the same $PATH search the shell normally does when running an external command. For instance, to guarantee running external printf just do: extern printf ...

Usage: extern [ -p ] [ -v ] command [ argument ... ]

• -p: use the operating system's default PATH (as determined by getconf PATH) instead of your current $PATH for the command search. This guarantees a path that finds all the standard utilities defined by POSIX, akin to command -p but still guaranteeing an external command. (Note that extern -p is more reliable than command -p because many shell binaries don't ask the OS for the default path and have a wrong default path hard-coded in.)
• -v: don't execute command but show the full path name of the command that would have been executed. Any extra arguments are taken as more command paths to show, one per line. extern exits with status 0 if all the commands were found, 1 otherwise. This option can be combined with -p.

Outputting strings

putln: prints each argument on a separate line. There is no processing of options or escape codes. (Modernish constants $CCn, etc. can be used to insert control characters in double-quoted strings. To process escape codes, use printf instead.)

put: prints each argument separated by a space, without a trailing separator or newline. Again, there is no processing of options or escape codes.

echo: This command is notoriously unportable and kind of broken, so is deprecated in favour of put and putln. Modernish does provide its own version of echo, but it is only activated if modernish is in the hashbang path or otherwise is itself used as the shell (the "most portable" way of running programs explained above). If your script runs on a specific shell and sources modernish as a dot script (. modernish), or if you use modernish interactively in your shell profile, the shell-specific version of echo is left intact. This is to make it possible to add modernish to existing shell-specific scripts without breaking anything, while still providing one consistent echo for cross-shell scripts.

The modernish version of echo, if active, does not interpret any escape codes and supports only one option, -n, which, like BSD echo, suppresses the final newline. However, unlike BSD echo, if -n is the only argument, it is not interpreted as an option and the string -n is printed instead. This makes it safe to output arbitrary data using this version of echo as long as it is given as a single argument (using quoting if needed).

Enhanced dot scripts

source: bash/zsh-style source command now available to all POSIX shells, complete with optional positional parameters given as extra arguments (which is not supported by POSIX .).

Testing numbers, strings and files

Complete replacement for test/[ in the form of speed-optimised shell functions, so modernish scripts never need to use that [ botch again. Instead of inherently ambiguous [ syntax (or the nearly-as-confusing [[ one), these familiar shell syntax to get more functionality, including:

Integer number arithmetic tests and operations

let: implementation of let as in ksh, bash and zsh, now available to all POSIX shells. This makes C-based signed integer arithmetic evaluation available to every supported shell, with the exception of the unary "++" and "--" operators (which have been given the capability designation ARITHPP). This means let should be used for operations and tests, e.g. both let "x=5" and if let "x==5"; then... are supported (note single = for assignment, double == for comparison). See POSIX 2.6.4 Arithmetic Expansion for more information on the supported operators.

isint: test if a given argument is a decimal, octal or hexadecimal integer number in valid POSIX shell syntax, ignoring leading (but not trailing) spaces and tabs.

String tests

empty:        test if string is empty
identic:      test if 2 strings are identical
contains:     test if string 1 contains string 2
startswith:   test if string 1 starts with string 2
endswith:     test if string 1 ends with string 2
match:        test if string matches a glob pattern
ematch:       test if string matches an extended regex

File type tests

These avoid the snags with symlinks you get with [ and [[. By default, symlinks are not followed. Add -L to operate on files pointed to by symlinks instead of symlinks themselves (the -L makes no difference if the operands are not symlinks).

is present:    test if file exists (yields true even if invalid symlink)
is -L present: test if file exists and is not an invalid symlink
is sym:        test if file is symlink
is -L sym:     test if file is a valid symlink
is reg:        test if file is a regular file
is -L reg:     test if file is regular or a symlink pointing to a regular
is dir:        test if file is a directory
is -L dir:     test if file is dir or symlink pointing to dir
is fifo, is -L fifo, is socket, is -L socket, is blockspecial,
               is -L blockspecial, is charspecial, is -L charspecial:
               same pattern, you figure it out :)

File comparison tests

By default, symlinks are not followed. Again, add -L to follow them.

is newer:      test if file 1 is newer than file 2 (or if file 1 exists,
               but file 2 doesn't)
is older:      test if file 1 is older than file 2 (or if file 1 doesn't
               exist, but file 2 does)
is samefile:   test if file 1 and file 2 are the same file (hardlinks)
is onsamefs:   test if file 1 and file 2 are on the same file system (for
               non-regular, non-directory files, test the parent directory)
is -L newer, is -L older, is -L samefile, is -L onsamefs:
               same as above, but after resolving symlinks

File status tests

Symlinks are followed.

is nonempty:   test is file exists, is not an invalid symlink, and is
               not empty (also works for dirs with read permission)
is setuid:     test if file has user ID bit set
is setgid:     test if file has group ID bit set

I/O tests

is onterminal: test if file descriptor is associated with a terminal

File permission tests

These use a more straightforward logic than [ and [[. Any symlinks given are resolved, as these tests would be meaningless for a symlink itself.

can read:      test if we have read permission for a file
can write:     test if we have write permission for a file or directory
               (for directories, only true if traverse permission as well)
can exec:      test if we have execute permission for a regular file
can traverse:  test if we can enter (traverse through) a directory

Basic string operations

The main modernish library contains functions for a few basic string manipulation operations (because they are needed by other functions in the main library). Currently these are:

toupper/tolower

toupper:       convert all letters to upper case
tolower:       convert all letters to lower case

If no arguments are given, toupper and tolower copy standard input to standard output, converting case.

If one or more arguments are given, they are taken as variable names (note: they should be given without the $) and case is converted in the contents of the specified variables, without reading input or writing output.

toupper and tolower try hard to use the fastest available method on the particular shell your program is running on. They use built-in shell functionality where available and working correctly, otherwise they fall back on running an external utility.

Which external utility is chosen depends on whether the current locale uses the Unicode UTF-8 character set or not. For non-UTF-8 locales, modernish assumes the POSIX/C locale and tr is always used. For UTF-8 locales, modernish tries hard to find a way to correctly convert case even for non-Latin alphabets. A few shells have this functionality built in with typeset. The rest need an external utility. Even in 2017, it is a real challenge to find an external utility on an arbitrary POSIX-compliant system that will correctly convert case for all applicable UTF-8 characters. Modernish initialisation tries tr, awk, GNU awk and GNU sed before giving up and issuing a warning ID. If thisshellhas WRN_2UP2LOW, it means modernish is in a UTF-8 locale but has not found a way to convert Case for NON ASCII characters, so toupper and tolower will convert only ASCII characters and leave any other characters in the string alone.

Basic system utilities

Small utilities that should have been part of the standard shell, but aren't. Since their implementation is inexpensive, they are part of the main library instead of a module.

mkcd: make one or more directories, then, upon success, change into the last-mentioned one. mkcd inherits mkdir's usage, so options depend on your system's mkdir; only the POSIX options are guaranteed.

Modules

use: use a modernish module. It implements a simple Perl-like module system with names such as 'safe', 'var/setlocal' and 'loop/select'. These correspond to files 'safe.mm', 'var/setlocal.mm', etc. which are dot scripts defining functionality. Any extra arguments to the use command are passed on to the dot script unmodified, so modules can implement option parsing to influence their initialisation.

use safe

Does IFS=''; set -f -u -C, that is: field splitting and globbing are disabled, variables must be defined before use, and output redirection using > will not overwrite existing files (use >| to allow overwriting).

Essentially, this is a whole new way of shell programming, eliminating most variable quoting headaches, protects against typos in variable names wreaking havoc, and protects files from being accidentally overwritten by output redirection.

Of course, you don't get field splitting and globbing. But modernish provides various ways of enabling one or both only for the commands that need them, setlocal...endlocal blocks chief among them (see use var/setlocal below).

On interactive shells (or if use safe -i is given), also loads convenience functions fsplit and glob to control and inspect the state of field splitting and globbing in a more user friendly way.

It is highly recommended that new modernish scripts start out with use safe. But this mode is not enabled by default because it will totally break compatibility with shell code written for default shell settings.

use var/arith

These shortcut functions are alternatives for using 'let'.

Arithmetic operator shortcuts

inc, dec, mult, div, mod: simple integer arithmetic shortcuts. The first argument is a variable name. The optional second argument is an arithmetic expression, but a sane default value is assumed (1 for inc and dec, 2 for mult and div, 256 for mod). For instance, inc X is equivalent to X=$((X+1)) and mult X Y-2 is equivalent to X=$((X*(Y-2))).

ndiv is like div but with correct rounding down for negative numbers. Standard shell integer division simply chops off any digits after the decimal point, which has the effect of rounding down for positive numbers and rounding up for negative numbers. ndiv consistently rounds down.

Arithmetic comparison shortcuts

These have the same name as their test/[ option equivalents. Unlike with test, the arguments are shell integer arith expressions, which can be anything from simple numbers to complex expressions. As with $(( )), variable names are expanded to their values even without the $.

Function:         Returns successfully if:
eq <expr> <expr>  the two expressions evaluate to the same number
ne <expr> <expr>  the two expressions evaluate to different numbers
lt <expr> <expr>  the 1st expr evaluates to a smaller number than the 2nd
le <expr> <expr>  the 1st expr eval's to smaller than or equal to the 2nd
gt <expr> <expr>  the 1st expr evaluates to a greater number than the 2nd
ge <expr> <expr>  the 1st expr eval's to greater than or equal to the 2nd

use var/mapr

mapr (map records): Read delimited records from the standard input, invoking a callback command with each input record as an argument and with up to quantum arguments at a time. By default, an input record is one line of text.

Usage: mapr [ -d delimiter | -D ] [ -n count ] [ -s count ] [ -c quantum ] callback [ argument ... ]

Options:

• -d delimiter: Use the single character delimiter to delimit input records, instead of the newline character. A NUL (0) character and multibyte characters are not supported.
• -P: Paragraph mode. Input records are delimited by sequences consisting of a newline plus one or more blank lines, and leading or trailing blank lines will not result in empty records at the beginning or end of the input. Cannot be used together with -d.
• -s number: Skip and discard the first count records read.
• -n number: Stop processing after passing a total of number records to invocation(s) of callback. If -n is not supplied or number is 0, all records are passed, except those skipped using -s.
• -m length: Pass at most length bytes of arguments to each call to callback. The default value depends on constraints set by the operating system. The length of each argument is rounded up to a multiple of 8 bytes. If length is 0, this limit is disabled.
• -c quantum: Pass at most quantum arguments at a time to each call to callback. If -c is not supplied or if quantum is 0, the number of arguments per invocation is not limited except by -m; whichever limit is reached first applies.

Arguments:

• callback: Call the callback command with the collected arguments each time quantum lines are read. The callback command may be a shell function or any other kind of command, and is executed from the same shell environment that invoked mapr. If the callback command exits with status 255, mapr aborts processing and exits with status 1. If the callback command is interrupted by the SIGPIPE signal, mapr aborts processing and exits with status $SIGPIPESTATUS. Other than that, it is a fatal error for the callback command to exit with a status > 0.
• argument: If there are extra arguments supplied on the mapr command line, they will be added before the collected arguments on each invocation on the callback command.

Differences from mapfile

mapr was inspired by the bash 4.x builtin command mapfile a.k.a. readarray, and uses similar options, but there are important differences.

• mapr passes all the records as arguments to the callback command.
• mapr does not support assigning records directly to an array. Instead, all handling is done through the callback command (which could be a shell function that assigns its arguments to an array.)
• The callback command is specified directly instead of with a -C option, and it may consist of several arguments (as with xargs).
• The record separator itself is never included in the arguments passed to the callback command (so there is no -t option to remove it).
• mapr supports paragraph mode.
• If the callback command exits unsuccessfully, this is treated as a fatal error, except that status 255 merely aborts processing.

Differences from xargs

mapr shares important characteristics with xargs while avoiding its myriad pitfalls.

• Instead of being an external utility, mapr is fully integrated into the shell. The callback command can be a shell function or builtin, which can directly modify the shell environment.
• mapr is line-oriented by default, so it is safe to use for input arguments that contain spaces or tabs.
• mapr does not parse or modify the input arguments in any way, e.g. it does not process and remove quotes from them like xargs does.
• mapr supports paragraph mode.
• If the callback command exits unsuccessfully, this is treated as a fatal error, except that (like with xargs) status 255 merely aborts processing.

use var/readf

readf reads arbitrary data from standard input into a variable until end of file, converting it into a format suitable for passing to the printf utility. For example, readf var <foo; printf "$var" >bar will copy foo to bar. Thus, readf allows storing both text and binary files into shell variables in a textual format suitable for manipulation with standard shell facilities.

All non-printable, non-ASCII characters are converted to printf octal or one-letter escape codes, except newlines. Not encoding newline characters allows for better processing by line-based utilities such as grep, sed, awk, etc. However, if the file ends in a newline, that final newline is encoded to \\n to protect it from being stripped by command substitutions.

Usage: readf varname

Caveats:

• Best for small-ish files. The encoded file is stored in memory (a shell variable). For a binary file, encoding in printf format typically about doubles the size, though it could be up to four times as large.
• If the shell executing your program does not have printf as a builtin command, the external printf command will fail if the encoded file size exceeds the maximum length of arguments to external commands (getconf ARG_MAX will obtain this limit for your system). Shell builtin commands do not have this limit. Check for a printf builtin using thisshellhas if you need to be sure, and always harden printf!

use var/setlocal

Defines a new setlocal...endlocal shell code block construct with arbitrary local variables and arbitrary local shell options, as well as safe field splitting and pathname expansion operators.

Usage: setlocal [ localitem ... ] [ [ --split | --split=characters ] [ --glob ] -- [ word ... ] ] ; do commands ; endlocal

The commands are executed with the specified settings applied locally to the setlocal...endlocal block.

Each localitem can be:

• A variable name with or without a = immediately followed by a value. This renders that variable local to the block, initially either unsetting it or assigning the value, which may be empty.
• A shell option letter immediately preceded by a - or + sign. This locally turns that shell option on or off, respectively. This follows the counterintuitive syntax of set. Long-form shell options like -o optionname and +o optionname are also supported. It depends on the shell what options are supported. Specifying a nonexistent option is a fatal error. Use thisshellhas to check for a non-POSIX option's existence on the current shell before using it.

The return command exits the block, causing the global variables and settings to be restored and resuming execution at the point immmediately following endlocal. This is like a shell function. In fact, internally, setlocal blocks are one-time shell functions that use the stack to save and restore variables and settings. Like any shell function, a setlocal block exits with the exit status of the last command executed within it or, with the status passed on by or given as an argument to return.

The break and continue commands, when not used within a loop within the block, also exit the block, but always with exit status 0. It's preferred to use return instead. Note that setlocal creates a new loop context and you cannot use break or continue to resume or break from enclosing loops outside the setlocal block. (Shells with QRK_BCDANGER do in fact allow this, preventing endlocal from restoring the global settings! Shells without this quirk automatically protect against this.)

Within the block, the positional parameters ($@, $1, etc.) are always local. By default, a copy is inherited from outside the block. Any changes to the positional parameters made within the block will be discarded upon exiting it.

However, if a -- is present, the set of words after -- becomes the positional parameters instead, after being modified by the --split or --glob operators if present. The --split operator subjects the words to default field splitting, whereas --split=string subjects them to field splitting based on the characters given in string. The --glob operator subjects them to pathname expansion. These operators do not enable field splitting or pathname expansion within the block itself, but only subject the words after the -- to them. If field splitting and globbing are disabled globally, this provides a safe way to perform field splitting or globbing without actually enabling them for any code. To illustrate this advantage, note the difference:

# Field splitting is enabled for all unquoted expansions within the
# setlocal block, which may be unsafe, so must quote "$foo" and "$bar".
setlocal dir IFS=':'; do
    for dir in $PATH; do
    somestuff "$foo" "$bar"
done
endlocal

# The value of PATH is split at ':' and assigned to the positional
# parameters, without enabling field splitting within the setlocal block.
setlocal dir --split=':' -- $PATH; do
    for dir do
        somestuff $foo $bar
    done
endlocal

Important: The --split and --glob operators are designed to be used along with safe mode. If they are used in traditional mode, i.e. with field splitting and/or pathname expansion globally active, you must make sure the words after the -- are properly quoted as with any other command, otherwise you will have unexpected duplicate splitting or pathname expansion.

Other usage notes:

• Although the setlocal declaration ends with ; do as in a while or until loop, the code block is terminated with endlocal and not with done. Terminating it with done results in a misleading shell syntax error (end of file, or missing }), a side effect of how setlocal is implemented.
• setlocal blocks do not mix well with LOCAL (shell-native functionality for local variables), especially not on shells with QRK_LOCALUNS or QRK_LOCALUNS2. Use one or the other, but not both.
• For maximum compatibility with shell bugs (particularly BUG_FNSUBSH on ksh93, and an alias parsing oddity on mksh [up to R54 2016/11/11] that triggers a spurious syntax error), setlocal blocks should not be used within subshells, including command substitution subshells. There is usually not much point to this anyway; the point of setlocal is to have certain settings local and keep the rest global, all without the performance hit of forking a subshell process. (Forking new subshells within a setlocal block is fine.)
• A note of caution concerning loop constructs: Care should be taken not to use break and continue in ways that would cause execution to continue outside the setlocal block. Some shells do not allow break and continue to break out of a shell function (including the internal one-time shell function employed by setlocal), so thankfully this fails on those shells. But on others this succeeds, so global settings are not restored, wreaking havoc on the rest of your program. One way to avoid the problem is to envelop the entire loop in a setlocal block. Another is to exit the internal shell function using return 1 and then add || break or || continue immediately after endlocal.
• zsh programmers may recognise setlocal as pretty much the equivalent of zsh's anonymous functions -- functionality that is hereby brought to all POSIX shells, albeit with a rather different syntax.

use var/string

String comparison and manipulation functions.

sortsbefore and sortsafter: test if string 1 sorts before or after string 2. The POSIX shell provides no standard built-in way of doing this. These functions use built-in ways where available, or fall back on the expr utility.
Usage: sortsbefore|sortsafter string1 string2

trim: strip whitespace from the beginning and end of a variable's value. Whitespace is defined by the [:space:] character class. In the POSIX locale, this is tab, newline, vertical tab, form feed, carriage return, and space, but in other locales it may be different. (On shells with BUG_NOCHCLASS, $WHITESPACE is used to define whitesapce instead.) Optionally, a string of literal characters can be provided in the second argument. Any characters appearing in that string will then be trimmed instead of whitespace. Usage: trim varname [ characters ]

replacein: Replace leading, -trailing or -all occurrences of a string by another string in a variable.
Usage: replacein [ -t | -a ] varname oldstring newstring

append and prepend: Append or prepend zero or more strings to a variable, separated by a string of zero or more characters, avoiding the hairy problem of dangling separators. Usage: append|prepend [ --sep=separator ] [ -Q ] varname [ string ... ]
If the separator is not specified, it defaults to a space character. If the -Q option is given, each string is shell-quoted before appending or prepending.

use sys/base

Some very common external commands ought to be standardised, but aren't. For instance, the which and readlink commands have incompatible options on various GNU and BSD variants and may be absent on other Unix-like systems. This module provides a complete re-implementation of such basic utilities written as modernish shell functions. Scripts that use the modernish version of these utilities can expect to be fully cross-platform. They also have various enhancements over the GNU and BSD originals.

use sys/base/readlink

readlink: Read the target of a symbolic link. Robustly handles weird filenames such as those containing newline characters. Stores result in the $REPLY variable and optionally writes it on standard output. Optionally canonicalises each path, following all symlinks encountered (for this mode, all but the last component must exist). Optionally shell-quote each item of output for later parsing by the shell, separating multiple items with spaces instead of newlines.

use sys/base/which

which: Outputs, and/or stores in the REPLY variable, either the first available directory path to each given command, or all available paths, according to the current $PATH or the system default path. Exits successfully if at least one path was found for each command, or unsuccessfully if none were found for any given command.

Usage: which [ -[apqsnQ1] ] [ -P number ] program [ program ... ]

• -a: List all executables found, not just the first one for each argument.
• -p: Search the system default path, not the current $PATH. This is the minimal path, specified by POSIX, that is guaranteed to find all the standard utilities.
• -q: Be quiet: suppress all warnings.
• -s: Silent operation: don't write output, only store it in the REPLY variable. Suppress warnings except, if you run which -s in a subshell, the warning that the REPLY variable will not survive the subshell.
• -n: When writing to standard output, do not write a final newline.
• -Q: Shell-quote each unit of output. Separate by spaces instead of newlines. This generates a list of arguments in shell syntax, guaranteed to be suitable for safe parsing by the shell, even if the resulting pathnames should contain strange characters such as spaces or newlines and other control characters.
• -1 (one): Output the results for at most one of the arguments in descending order of preference: once a search succeeds, ignore the rest. Suppress warnings except a subshell warning for -s. This is useful for finding a command that can exist under several names, for example, in combination with harden:
  harden -P -f tar $(which -f -1 gnutar gtar tar)
  This option modifies which's exit status behaviour: which -1 returns successfully if any match was found.
• -f: Consider it a fatal error if at least one of the given programs is not found. But if option -1 is also given, only throw a fatal error if none are found.
• -P: Strip the indicated number of pathname elements from the output, starting from the right. -P1: strip /program; -P2: strip /*/program, etc. This is useful for determining the installation root directory for an installed package.

use sys/base/mktemp

A cross-platform shell implementation of 'mktemp' that aims to be just as safe as native mktemp(1) implementations, while avoiding the problem of having various mutually incompatible versions and adding several unique features of its own.

Creates one or more unique temporary files, directories or named pipes, atomically (i.e. avoiding race conditions) and with safe permissions. The path name(s) are stored in $REPLY and optionally written to stdout.

Usage: mktemp [ -dFsQCt ] [ template ... ]

• -d: Create a directory instead of a regular file.
• -F: Create a FIFO (named pipe) instead of a regular file.
• -s: Silent. Store output in $REPLY, don't write any output or message.
• -Q: Shell-quote each unit of output. Separate by spaces, not newlines.
• -C: Automated cleanup. Pushes a trap to remove the files on exit. On an interactive shell, that's all this option does. On a non-interactive shell, the following applies: Clean up on receiving SIGPIPE and SIGTERM as well. On receiving SIGINT, clean up if the option was given at least twice, otherwise notify the user of files left. On the invocation of die, clean up if the option was given at least three times, otherwise notify the user of files left.
• -t: Prefix one temporary files directory to all the templates: $TMPDIR/ if TMPDIR is set, /tmp/ otherwise. The templates may not contain any slashes. If the template has neither any trailing Xes nor a trailing dot, a dot is added before the random suffix.

The template defaults to /tmp/temp.. An suffix of random shell-safe ASCII characters is added to the template to create the file. For compatibility with other mktemp implementations, any optional trailing X characters in the template are removed. The length of the suffix will be equal to the amount of Xes removed, or 10, whichever is more. The longer the random suffix, the higher the security of using mktemp in a shared directory such as tmp.

Since /tmp is a world-writable directory shared by other users, for best security it is recommended to create a private subdirectory using mktemp -d and work within that.

Option -C cannot be used while invoking mktemp in a subshell, such as in a command substitution. Modernish will detect this and treat it as a fatal error. The reason is that a typical command substitution like tmpfile=$(mktemp -C) is incompatible with auto-cleanup, as the cleanup EXIT trap would be triggered not upon exiting the program but upon exiting the command substitution subshell that just ran mktemp, thereby immediately undoing the creation of the file. Instead, do something like: mktemp -sC; tmpfile=$REPLY

use sys/base/seq

A cross-platform implementation of seq that is more powerful and versatile than native GNU and BSD seq(1) implementations. The core is written in bc, the POSIX arbitrary-presision calculator language. That means this seq inherits the capacity to handle numbers with a precision and size only limited by computer memory, as well as the ability to handle input numbers in any base from 1 to 16 and produce output in any base 1 and up.

Usage: seq [ -w ] [ -f format ] [ -s string ] [ -S scale ] [ -B base ] [ -b base ] [ first [ incr ] ] last

seq prints a sequence of arbitrary-precision floating point numbers, one per line, from first (default 1), to as near last as possible, in increments of incr (default 1). If first is larger than last, the default incr is -1.

• -w: Equalise width by padding with leading zeros. The longest of the first, incr or last arguments is taken as the length that each output number should be padded to.
• -f: printf-style floating-point format. The format string is passed on (with an added \n) to awk's builtin printf function. Because of that, the -f option can only be used if the output base is 10. Note that awk's floating point precision is limited, so very large or long numbers will be rounded.
• -s: Use string to separate numbers. Default: newline. The terminator character remains a newline in any case (which is like GNU seq and differs from BSD seq).
• -S: Explicitly set the scale (number of digits after decimal point). Defaults to the largest number of digits after the decimal point among the first, incr or last arguments.
• -B: Set input and output base from 1 to 16. Defaults to 10.
• -b: Set arbitrary output base from 1. Defaults to input base. See the bc(1) manual for more infromation on the output format for bases greater than 16.

The -S, -B and -b options take shell integer numbers as operands. This means a leading 0X or 0x denotes a hexadecimal number and (except on shells with BUG_NOOCTAL) a leading 0 denotes an octal numnber.

For portability reasons, modernish seq always uses a dot (.) for the floating point, never a comma, regardless of the system locale. This applies both to command arguments and to output.

The -w, -f and -s options are inspired by GNU and BSD seq, mostly emulating GNU where they differ. The -S, -B and -b options are modernish enhancements based on bc(1) functionality.

use sys/base/rev

rev copies the specified files to the standard output, reversing the order of characters in every line. If no files are specified, the standard input is read.

Usage: like rev on Linux and BSD, which is like cat except that - is a filename and does not denote standard input. No options are supported.

use sys/dir

Functions for working with directories. So far I have:

use sys/dir/traverse

traverse is a fully cross-platform, robust replacement for find without the snags of the latter. It is not line oriented but handles all data internally in the shell. Any weird characters in file names (including whitespace and even newlines) "just work", provided either use safe is active or shell expansions are properly quoted. This avoids many hairy common pitfalls with find while remaining compatible with all POSIX systems.

traverse recursively walks through a directory, executing a command for each file and subdirectory found. That command is usually a handler shell function in your program.

Unlike find, which is so smart its command line options are practically their own programming language, traverse is dumb: it has minimal functionality of its own. However, with a shell function as the command, any functionality of 'find' and anything else can be programmed in the shell language. Flexibility is unlimited. The install.sh script that comes with modernish provides a good example of its practical use. See also the traverse-test example program.

Usage: traverse [ -d ] [ -F] [ -X ] directory command

traverse calls command, once for each file found within the directory, with one parameter containing the full pathname relative to the directory. Any directories found within are automatically entered and traversed recursively unless the command exits with status 1. Symlinks to directories are not followed.

find's -prune functionality is implemented by testing the command's exit status. If the command indicated exits with status 1 for a directory, this means: do not traverse the directory in question. For other types of files, exit status 1 is the same as 0 (success). Exit status 2 means: stop the execution of traverse and resume program execution. An exit status greater than 2 indicates system failure and causes the program to abort.

find's -depth functionality is implemented using the -d option. By default, traverse handles directories first, before their contents. The -d option causes depth-first traversal, so all entries in a directory will be acted on before the directory itself. This applies recursively to subdirectories. That means depth-first traversal is incompatible with pruning, so returning status 1 for directories will have no effect.

find's -xdev functionality is implemented using the -F option. If this is given, traverse will not descend into directories that are on another file system than that of the directory given in the argument.

xargs-like functionality is implemented using the -X option. As many items as possible are saved up before being passed to the command all at once. This is also incompatible with pruning. Unlike xargs, the command is only executed if at least one item was found for it to handle.

use sys/dir/countfiles

countfiles: Count the files in a directory using nothing but shell functionality, so without external commands. (It's amazing how many pitfalls this has, so a library function is needed to do it robustly.)

Usage: countfiles [ -s ] directory [ globpattern ... ]

Count the number of files in a directory, storing the number in REPLY and (unless -s is given) printing it to standard output. If any globpatterns are given, only count the files matching them.

use sys/term

Utilities for working with the terminal.

use sys/term/readkey

readkey: read a single character from the keyboard without echoing back to the terminal. Buffering is done so that multiple waiting characters are read one at a time.

Usage: readkey [ -E ERE ] [ -t timeout ] [ -r ] [ varname ]

-E: Only accept characters that match the extended regular expression ERE (the type of RE used by grep -E/egrep). readkey will silently ignore input not matching the ERE and wait for input matching it.

-t: Specify a timeout in seconds (one significant digit after the decimal point). After the timeout expires, no character is read and readkey returns status 1.

-r: Raw mode. Disables INTR (Ctrl+C), QUIT, and SUSP (Ctrl+Z) processing as well as translation of carriage return (13) to linefeed (10).

The character read is stored into the variable referenced by varname, which defaults to REPLY if not specified.

use opts/parsergen

Parsing of command line options for shell functions is a hairy problem. Using getopts in shell functions is problematic at best, and manually written parsers are very hard to do right. That's why this module provides generateoptionparser, a command to generate an option parser: it takes options specifying what variable names to use and what your function should support, and outputs code to parse options for your shell function. Options can be specified to require or not take arguments. Combining/stacking options and arguments in the traditional UNIX manner is supported.

Only short (one-character) options are supported. Each option gets a corresponding variable with a name with a specified prefix, ending in the option character (hence, only option characters that are valid in variables are supported, namely, the ASCII characters A-Z, a-z, 0-9 and the underscore). If the option was not specified on the command line, the variable is set, otherwise it is set to the empty value, or, if the option requires an argument, the variable will contain that argument.

use loop/cfor

A C-style for loop akin to for (( )) in bash/ksh/zsh, but unfortunately not with the same syntax. For example, to count from 1 to 10:

cfor 'i=1' 'i<=10' 'i+=1'; do
    echo "$i"
done

(Note that ++i and i++ can only be used on shells with ARITHPP, but i+=1 or i=i+1 can be used on all POSIX-compliant shells.)

use loop/sfor

A C-style for loop with arbitrary shell commands instead of arithmetic expressions. For example, to count from 1 to 10 with traditional shell commands:

sfor 'i=1' '[ "$i" -le 10 ]' 'i=$((i+1))'; do
    print "$i"
done

or, with modernish commands:

sfor 'i=1' 'le i 10' 'inc i'; do
    print "$i"
done

use loop/with

The shell lacks a very simple and basic loop construct, so this module provides for an old-fashioned MS BASIC-style for loop, renamed a with loop because we can't overload the reserved shell keyword for. Integer arithmetic only. Usage:

with <varname>=<value> to <limit> [ step <increment> ]; do
   # some commands
done

To count from 1 to 10:

with i=1 to 10; do
    print "$i"
done

The value for step defaults to 1 if limit is equal to or greater than value, and to -1 if limit is less than value. The latter is a slight enhancement over the original BASIC for construct. So counting backwards is as simple as with i=10 to 1; do (etc).

use loop/select

A complete and nearly accurate reimplementation of the select loop from ksh, zsh and bash for POSIX shells lacking it. Modernish scripts running on any POSIX shell can now easily use interactive menus.

(All the new loop constructs have one bug in common: as they start with an alias that expands to two commands, you can't pipe a command's output directly into such a loop. You have to enclose it in {...} as a workaround. I have not found a way around this limitation that doesn't involve giving up the familiar do...done syntax.)

---

Appendix A

This is a list of shell capabilities and bugs that modernish tests for, so that both modernish itself and scripts can easily query the results of these tests. The all-caps IDs below are all usable with the thisshellhas function. This makes it easy for a cross-platform modernish script to write optimisations taking advantage of certain non-standard shell features, falling back to a standard method on shells without these features. On the other hand, if universal compatibility is not a concern for your script, it is just as easy to require certain features and exit with an error message if they are not present, or to refuse shells with certain known bugs.

Most feature/quirk/bug tests have their own little test script in the libexec/modernish/cap directory. These tests are executed on demand, the first time the capability or bug in question is queried using thisshellhas. See README.md in that directory for further information. The test scripts also document themselves in the comments.

Tests that are essential to the functioning of modernish are incorporated into the main bin/modernish script; these are considered built-in tests, and are immediately run and cached at initialisation time. In the lists below, an ID in ITALICS denotes such a built-in test.

Capabilities

Non-standard shell capabilities currently tested for are:

• LEPIPEMAIN: execute last element of a pipe in the main shell, so that things like somecommand | read somevariable work. (zsh, AT&T ksh, bash 4.2+)
• RANDOM: the $RANDOM pseudorandom generator. Modernish seeds it if detected. The variable is then set it to read-only whether the generator is detected or not, in order to block it from losing its special properties by being unset or overwritten, and to stop it being used if there is no generator. This is because some of modernish depends on RANDOM either working properly or being unset.
  (The use case for non-readonly RANDOM is setting a known seed to get reproducible pseudorandom sequences. To get that in a modernish script, use awk's srand(yourseed) and int(rand()*32768).)
• LINENO: the $LINENO variable contains the current shell script line number.
• LOCAL: function-local variables, either using the local keyword, or by aliasing local to typeset (mksh, yash).
• KSH88FUNC: define ksh88-style shell functions with the 'function' keyword, supporting dynamically scoped local variables with the 'typeset' builtin. (mksh, bash, zsh, yash, et al)
• KSH93FUNC: the same, but with static scoping for local variables. (ksh93 only) See Q28 at the ksh93 FAQ for an explanation of the difference.
• ARITHPP: support for the ++ and -- unary operators in shell arithmetic.
• ARITHCMD: standalone arithmetic evaluation using a command like ((expression)).
• ARITHFOR: ksh93/C-style arithmetic 'for' loops of the form for ((exp1;exp2;exp3)) docommands; done.
• CESCQUOT: Quoting with C-style escapes, like $'\n' for newline.
• ADDASSIGN: Add a string to a variable using additive assignment, e.g. VAR+=string
• PSREPLACE: Search and replace strings in variables using special parameter substitutions with a syntax vaguely resembling sed.
• ROFUNC: Set functions to read-only with readonly -f. (bash, yash)
• DOTARG: Dot scripts support arguments.
• HERESTR: Here-strings, an abbreviated kind of here-document.
• TESTO: The test/[ builtin supports the -o unary operator to check if a shell option is set.
• PRINTFV: The shell's printf builtin has the -v option to print to a variable, which avoids forking a command substitution subshell.
• ANONFUNC: zsh anonymous functions (basically the native zsh equivalent of modernish's var/setlocal module)
• KSHARRAY: ksh88-style arrays. Supported on bash, zsh (under emulate sh), mksh, pdksh and ksh93.
• KSHARASGN: ksh93-style mass array assignment in the style of array=(one two three). Supported on the same shells as KSHARRAY except pdksh.
• TRAPZERR: This feature ID is detected if the ERR trap is an alias for the ZERR trap. According to the zsh manual, this is the case for zsh on most systems, i.e. those that don't have a SIGERR signal. (The trap stack uses this feature test.)
• TRAPPRSUBSH: The ability to obtain a list of the current shell's native traps from a command substitution subshell, for example: var=$(trap). Note that modernish transparently reimplements this feature on shells without this native capability, so this feature ID is only relevant if you are bypassing modernish to access the trap builtin directly. Also, in order to be useful to modernish, this feature test only yields a positive if the trap command in var=$(trap) can be replaced by a shell function that in turn calls the builtin trap command.
• OPTNOPREFIX: Long-form shell option names use a dynamic no prefix for all options (including POSIX ones). For instance, glob is the opposite of noglob, and nonotify is the opposite of notify. (ksh93, yash, zsh)

Quirks

Shell quirks currently tested for are:

• QRK_IFSFINAL: in field splitting, a final non-whitespace IFS delimiter character is counted as an empty field (yash < 2.42, zsh, pdksh). This is a QRK (quirk), not a BUG, because POSIX is ambiguous on this.
• QRK_32BIT: mksh: the shell only has 32-bit arithmetics. Since every modern system these days supports 64-bit long integers even on 32-bit kernels, we can now count this as a quirk.
• QRK_APIPEMAIN: On zsh < 5.3, any element of a pipeline (not just the last element) that is nothing but a simple variable assignment is executed in the current shell environment, instead of a subshell. For instance, the assignment var=foo survives SomeCommands | var=foo | SomeMoreCommands.
• QRK_ARITHEMPT: In yash, with POSIX mode turned off, a set but empty variable yields an empty string when used in an arithmetic expression, instead of 0. For example, foo=''; echo $((foo)) outputs an empty line.
• QRK_ARITHWHSP: In yash and FreeBSD /bin/sh, trailing whitespace from variables is not trimmed in arithmetic expansion, causing the shell to exit with an 'invalid number' error. POSIX is silent on the issue. The modernish isint function (to determine if a string is a valid integer number in shell syntax) is QRK_ARITHWHSP compatible, tolerating only leading whitespace.
• QRK_BCDANGER: break and continue can affect non-enclosing loops, even across shell function barriers (zsh, Busybox ash; older versions of bash, dash and yash). (This is especially dangerous when using var/setlocal which internally uses a temporary shell function to try to protect against breaking out of the block without restoring global parameters and settings.)
• QRK_EMPTPPFLD: Unquoted $@ and $* do not discard empty fields. POSIX says for both unquoted $@ and unquoted $* that empty positional parameters may be discarded from the expansion. AFAIK, just one shell (yash) doesn't.
• QRK_EMPTPPWRD: POSIX says that empty "$@" generates zero fields but empty '' or "" or "$emptyvariable" generates one empty field. But it leaves unspecified whether something like "$@$emptyvariable" generates zero fields or one field. Zsh, pdksh/mksh and (d)ash generate one field, as seems logical. But bash, AT&T ksh and yash generate zero fields, which we consider a quirk. (See also BUG_PP_01)
• QRK_EVALNOOPT: eval does not parse options, not even --, which makes it incompatible with other shells: on the one hand, (d)ash does not accept
  eval -- "$command" whereas on other shells this is necessary if the command starts with a -, or the command would be interpreted as an option to eval. A simple workaround is to prefix arbitrary commands with a space. Both situations are POSIX compliant, but since they are incompatible without a workaround,the minority situation is labeled here as a QuiRK.
• QRK_EXECFNBI: In pdksh and zsh, exec looks up shell functions and builtins before external commands, and if it finds one it does the equivalent of running the function or builtin followed by exit. This is probably a bug in POSIX terms; exec is supposed to launch a program that overlays the current shell, implying the program launched by exec is always external to the shell. However, since the POSIX language is rather vague and possibly incorrect, this is labeled as a shell quirk instead of a shell bug.
• QRK_HDPARQUOT: Double quotes within certain parameter substitutions in here-documents aren't removed (FreeBSD sh; bosh). For instance, if var is set, ${var+"x"} in a here-document yields "x", not x. POSIX considers it undefined to use double quotes there, so they should be avoided for a script to be fully POSIX compatible. (Note this quirk does not apply for substitutions that remove pattens, such as ${var#"$x"} and ${var%"$x"}; those are defined by POSIX and double quotes are fine to use.) (Note 2: single quotes produce widely varying behaviour and should never be used within any form of parameter substitution in a here-document.)
• QRK_LOCALINH: On a shell with LOCAL, local variables, when declared without assigning a value, inherit the state of their global namesake, if any. (dash, FreeBSD sh)
• QRK_LOCALSET: On a shell with LOCAL, local variables are immediately set to the empty value upon being declared, instead of being initially without a value. (zsh)
• QRK_LOCALSET2: Like QRK_LOCALSET, but only if the variable by the same name in the global/parent scope is unset. If the global variable is set, then the local variable starts out unset. (bash 2 and 3)
• QRK_LOCALUNS: On a shell with LOCAL, local variables lose their local status when unset. Since the variable name reverts to global, this means that unset will not necessarily unset the variable! (yash, pdksh/mksh. Note: this is actually a behaviour of typeset, to which modernish aliases local on these shells.)
• QRK_LOCALUNS2: This is a more treacherous version of QRK_LOCALUNS that is unique to bash. The unset command works as expected when used on a local variable in the same scope that variable was declared in, however, it makes local variables global again if they are unset in a subscope of that local scope, such as a function called by the function where it is local. (Note: since QRK_LOCALUNS2 is a special case of QRK_LOCALUNS, modernish will not detect both.)
• QRK_OPTCASE: Long-form shell option names are case-insensitive. (yash, zsh)
• QRK_OPTDASH: Long-form shell option names ignore the -. (ksh93, yash)
• QRK_OPTULINE: Long-form shell option names ignore the _. (yash, zsh)
• QRK_PPIPEMAIN: On zsh, in all elements of a pipeline, parameter expansions are evaluated in the current environment (with any changes they make surviving the pipeline), though the commands themselves of every element but the last are executed in a subshell. For instance, given unset or empty v, in the pipeline cmd1 ${v:=foo} | cmd2, the assignment to v survives, though cmd1 itself is executed in a subshell.
• QRK_SPCBIXP: Variable assignments directly preceding special builtin commands are exported, and persist as exported. (bash; yash)
• QRK_UNSETF: If 'unset' is invoked without any option flag (-v or -f), and no variable by the given name exists but a function does, the shell unsets the function. (bash)

Bugs

Non-fatal shell bugs currently tested for are:

• BUG_APPENDC: When set -C (noclobber) is active, "appending" to a nonexistent file with >> throws an error rather than creating the file. (zsh < 5.1) This is a bug making use safe less convenient to work with, as this sets the -C (-o noclobber) option to reduce accidental overwriting of files. The safe module requires an explicit override to tolerate this bug.
• BUG_ARITHINIT: Using unset or empty variables (dash <= 0.5.9.1) or unset variables (yash <= 2.44) in arithmetic expressions causes the shell to exit, instead of taking them as a value of zero.
• BUG_ARITHLNNO: The shell supports $LINENO, but the variable is considered unset in arithmetic contexts, like $(( LINENO > 0 )). This makes it error out under set -u and default to zero otherwise. Workaround: use shell expansion like $(( $LINENO > 0 )). (FreeBSD sh)
• BUG_ARITHSPLIT: Unquoted $((arithmetic expressions)) are not subject to field splitting as expected. (zsh, pdksh, mksh<=R49)
• BUG_ARITHTYPE: In zsh, arithmetic assignments (using let, $(( )), etc.) on unset variables assign a numerical/arithmetic type to a variable, causing subsequent normal variable assignments to be interpreted as arithmetic expressions and fail if they are not valid as such.
• BUG_BRACQUOT: shell quoting within bracket patterns has no effect (zsh < 5.3; ksh93) This bug means the - retains it special meaning of 'character range', and an initial ! (and, on some shells, ^) retains the meaning of negation, even in quoted strings within bracket patterns, including quoted variables.
• BUG_CASECC: glob patterns as in case cannot match an escaped ^A ($CC01) or DEL ($CC7F) control character. Found on: bash 2.05b, 3.0, 3.1
• BUG_CASELIT: If a case pattern doesn't match as a pattern, it's tried again as a literal string, even if the pattern isn't quoted. This can result in false positives when a pattern doesn't match itself, like with bracket patterns. This contravenes POSIX and breaks use cases such as input validation. (AT&T ksh93) Note: modernish match works around this.
• BUG_CASESTAT: The 'case' conditional construct prematurely clobbers the exit status $?. (found in zsh < 5.3, Busybox ash <= 1.25.0, dash < 0.5.9.1)
• BUG_CC7F: Quoted expansions delete the DEL control character ($CC7F), except if that character is directly preceded in the value by the \1 ($CC01) control character. Note that this bug removes DEL from $CONTROLCHARS and $ASCIICHARS. (bash 2.05b, 3.0)
• BUG_CMDOPTEXP: the command builtin does not recognise options if they result from expansions. For instance, you cannot conditionally store -p in a variable like defaultpath and then do command $defaultpath someCommand. (found in zsh < 5.3)
• BUG_CMDPV: command -pv does not find builtins ({pd,m}ksh), does not accept the -p and -v options together (zsh < 5.3) or ignores the '-p' option altogether (bash 3.2); in any case, it's not usable to find commands in the default system PATH.
• BUG_CMDSPASGN: preceding a special builtin with 'command' does not stop preceding invocation-local variable assignments from becoming global. (AT&T ksh, 2010-ish versions; bash 2.05b and 3.0 in POSIX mode have a variant of this bug that only applies to the eval, ., and source builtins)
• BUG_CMDSPEXIT: preceding a special builtin with 'command' does not stop it from exiting the shell if the builtin encounters error. (zsh < 5.2; mksh < R50e)
• BUG_CMDVRESV: 'command -v' does not find reserved words such as "if". (pdksh, mksh). This necessitates a workaround version of thisshellhas().
• BUG_CSCMTQUOT: unbalanced single and double quotes and backticks in comments within command substitutions cause obscure and hard-to-trace syntax errors later on in the script. (ksh88; pdksh, incl. {Open,Net}BSD ksh; bash 2.05b)
• BUG_CSNHDBKSL: Backslashes within non-expanding here-documents within command substitutions are incorrectly expanded to perform newline joining, as opposed to left intact. (bash <= 4.4, and pdksh)
• BUG_DOLRCSUB: parsing problem where, inside a command substitution of the form $(...), the sequence $$'...' is treated as $'...' (i.e. as a use of CESCQUOT), and $$"..." as $"..." (bash-specific translatable string). (Found in bash up to 4.4)
• BUG_DQGLOB: globbing is not properly deactivated within double-quoted strings. Within double quotes, a * or ? immediately following a backslash is interpreted as a globbing character. This applies to both pathname expansion and pattern matching in case. Found in: dash. (The bug is not triggered when using modernish match.)
• BUG_EMPTYBRE is a case pattern matching bug in zsh < 5.0.8: empty bracket expressions eat subsequent shell grammar, producing unexpected results. This is particularly bad if you want to pass a bracket expression using a variable or parameter, and that variable or parameter could be empty. This means the grammar parsing depends on the contents of the variable!
• BUG_EVALCOBR: break and continue do not work if they are within eval, wrongly causing loop execution to continue. (pdksh; mksh < R55 2017/04/12)
• BUG_FNREDIRP: I/O redirections on function definitions are forgotten if the function is called as part of a pipeline with at least one |. (bash 2.05b)
• BUG_FNSUBSH: Function definitions within subshells (including command substitutions) are ignored if a function by the same name exists in the main shell, so the wrong function is executed. unset -f is also silently ignored. ksh93 (all current versions as of June 2015) has this bug.
• BUG_HASHVAR: On zsh, $#var means the length of $var - other shells and POSIX require braces, as in ${#var}. This causes interesting bugs when combining $#, being the number of positional parameters, with other strings. For example, in arithmetics: $(($#-1)), instead of the number of positional parameters minus one, is interpreted as ${#-} concatenated with 1. So, for zsh compatibility, always use ${#} instead of $# unless it's stand-alone or followed by a space.
• BUG_HDOCBKSL: Line continuation using backslashes in expanding here-documents is handled incorrectly. (zsh up to 5.4.2)
• BUG_HDOCMASK: Here-documents (and here-strings, see HERESTRING) use temporary files. This fails if the current umask setting disallows the user to read, so the here-document can't read from the shell's temporary file. Workaround: ensure user-readable umask when using here-documents. (bash, pdksh, mksh, zsh)
• BUG_IFSGLOBC: In glob pattern matching (such as in case and [[), if a wildcard character is part of IFS, it is matched literally instead of as a matching character. This applies to glob characters *, ?, [ and ]. Since nearly all modernish functions use case for argument validation and other purposes, nearly every modernish function breaks on shells with this bug if IFS contains any of these three characters! (Found in bash < 4.4)
• BUG_IFSGLOBP: In pathname expansion (filename globbing), if a wildcard character is part of IFS, it is matched literally instead of as a matching character. This applies to glob characters *, ?, [ and ]. (Bug found in bash, all versions up to at least 4.4)
• BUG_IFSGLOBS: in glob pattern matching (as in case or paramter substitution with # and %), if IFS starts with ? or * and the "$*" parameter expansion inserts any IFS separator characters, those characters are erroneously interpreted as wildcards when quoted "$*" is used as the glob pattern. (AT&T ksh93)
• BUG_IFSISSET: AT&T ksh93 (recent versions): ${IFS+s} always yields 's' even if IFS is unset. This applies to IFS only.
• BUG_ISSETLOOP: AT&T ksh93: Expansions like ${var+set} remain static when used within a for, while or until loop; the expansions don't change along with the state of the variable, so they cannot be used to check whether a variable is set within a loop if the state of that variable may change in the course of the loop.
• BUG_KUNSETIFS: ksh93: Can't unset IFS under very specific circumstances. unset -v IFS is a known POSIX shell idiom to activate default field splitting. With this bug, the unset builtin silently fails to unset IFS (i.e. fails to activate field splitting) if we're executing an eval or a trap and a number of specific conditions are met. See BUG_KUNSETIFS.t for more information.
• BUG_LNNOALIAS: The shell has LINENO, but $LINENO is always expanded to 0 when used within an alias. (pdksh variants, including mksh and oksh)
• BUG_LNNOEVAL: The shell has LINENO, but $LINENO is always expanded to 0 when used in 'eval'. (pdksh variants, including mksh and oksh)
• BUG_MULTIBIFS: We're on a UTF-8 locale and the shell supports UTF-8 characters in general (i.e. we don't have BUG_MULTIBYTE) -- however, using multibyte characters as IFS field delimiters still doesn't work. For example, "$*" joins positional parameters on the first byte of $IFS instead of the first character. (ksh93, mksh, FreeBSD sh, Busybox ash)
• BUG_MULTIBYTE: We're in a UTF-8 locale but the shell does not have multi-byte/variable-length character support. (Non-UTF-8 variable-length locales are not yet supported.) Dash is a recent shell with this bug.
• BUG_NOCHCLASS: POSIX-mandated character [:classes:] within bracket [expressions] are not supported in glob patterns. (pdksh, mksh, and family)
• BUG_NOEXPRO: Cannot export read-only variables. (zsh 5.0.8-5.5.1 as sh)
• BUG_NOOCTAL: Shell arithmetic does interpret numbers with leading zeroes as octal numbers; these are interpreted as decimal instead, though POSIX specifies octal. (older mksh, 2013-ish versions)
• BUG_NOUNSETEX: Cannot assign export attribute to variables in an unset state; exporting a variable immediately sets it to the empty value. (zsh < 5.3)
• BUG_NOUNSETRO: Cannot freeze variables as readonly in an unset state. This bug in zsh < 5.0.8 makes the readonly command set them to the empty string instead.
• BUG_OPTNOLOG: on dash, setting -o nolog causes $- to wreak havoc: trying to expand $- silently aborts parsing of an entire argument, so e.g. "one,$-,two" yields "one,". (Same applies to -o debug.)
• BUG_PARONEARG: When IFS is empty on bash 3.x and 4.x (i.e. field splitting is off), ${1+"$@"} is counted as a single argument instead of each positional parameter as separate arguments. To avoid this bug, simply use "$@" instead. (${1+"$@"} is an obsolete workaround for a fatal shell bug, FTL_UPP.)
• BUG_PFRPAD: Negative padding value for strings in the printf builtin does not cause blank padding on the right-hand side, but inserts blank padding on the left-hand side as if the value were positive, e.g. printf '[%-4s]' hi outputs [  hi], not [hi  ]. (zsh 5.0.8)
• BUG_PP_01: POSIX says that empty "$@" generates zero fields but empty '' or "" or "$emptyvariable" generates one empty field. This means concatenating "$@" with one or more other, separately quoted, empty strings (like "$@""$emptyvariable") should still produce one empty field. But on bash 3.x, this erroneously produces zero fields. (See also QRK_EMPTPPWRD)
• BUG_PP_02: Like BUG_PP_01, but with unquoted $@ and only with "$emptyvariable"$@, not $@"$emptyvariable". (pdksh)
• BUG_PP_03: When IFS is unset or empty (zsh 5.3.1) or empty (pdksh), assigning var=$* only assigns the first field, failing to join and discarding the rest of the fields. Workaround: var="$*" (POSIX leaves var=$@, etc. undefined, so we don't test for those.)
• BUG_PP_03A: When IFS is unset, assignments like var=$* incorrectly remove leading and trailing spaces (but not tabs or newlines) from the result. Workaround: quote the expansion. Found on: bash 4.3 and 4.4.
• BUG_PP_03B: When IFS is unset, assignments like var=${var+$*}, etc. incorrectly remove leading and trailing spaces (but not tabs or newlines) from the result. Workaround: quote the expansion. Found on: bash 4.3 and 4.4.
• BUG_PP_03C: When IFS is unset, assigning var=${var-$*} only assigns the first field, failing to join and discarding the rest of the fields. (zsh 5.3, 5.3.1) Workaround: var=${var-"$*"}
• BUG_PP_04: If IFS is set and empty, assigning the positional parameters to a variable using a conditional assignment within a parameter substitution, such as : ${var=$*}, discards everything but the last field from the assigned value while incorrectly generating multiple fields for the expansion. (pdksh, mksh)
• BUG_PP_04_S: When IFS is null (empty), the result of a substitution like ${var=$*} is incorrectly field-split on spaces. The difference with BUG_PP_04 is that the assignment itself succeeds normally. Found on: bash 4.2, 4.3
• BUG_PP_04A: Like BUG_PP_03A, but for conditional assignments within parameter substitutions, as in : ${var=$*} or : ${var:=$*}. Workaround: quote either $* within the expansion or the expansion itself. Found on: bash 2.05b through 4.4.
• BUG_PP_04B: When assigning the positional parameters ($*) to a variable using a conditional assignment within a parameter substitution, e.g. : ${var:=$*}, the fields are always joined and separated by spaces, regardless of the content or state of IFS. Workaround as in BUG_PP_04A. (bash 2.05b)
• BUG_PP_04D: When field-splitting the result of an expansion such as ${var:=$*}, if the first positional parameter starts with a space, an initial empty field is incorrectly generated. (mksh <= R50)
• BUG_PP_04E: Like BUG_PP_04B but not if IFS is set and empty. (bash 4.3)
• BUG_PP_05: POSIX says that empty $@ and $* generate zero fields, but with null IFS, empty unquoted $@ and $* yield one empty field. Found on: dash 0.5.9 and 0.5.9.1; Busybox ash.
• BUG_PP_06: POSIX says that unquoted $@ initially generates as many fields as there are positional parameters, and then (because $@ is unquoted) each field is split further according to IFS. With this bug, the latter step is not done. Found on: zsh < 5.3
• BUG_PP_06A: POSIX says that unquoted $@ and $* initially generate as many fields as there are positional parameters, and then (because $@ or $* is unquoted) each field is split further according to IFS. With this bug, the latter step is not done if IFS is unset (i.e. default split). Found on: zsh < 5.4
• BUG_PP_07: unquoted $* and $@ (including in substitutions like ${1+$@} or ${var-$*}) do not perform default field splitting if IFS is unset. Found on: zsh (up to 5.3.1) in sh mode
• BUG_PP_07A: When IFS is unset, unquoted $* undergoes word splitting as if IFS=' ', and not the expected IFS=" ${CCt}${CCn}". Found on: bash 4.4
• BUG_PP_08: When IFS is empty, unquoted $@ and $* do not generate one field for each positional parameter as expected, but instead join them into a single field without a separator. Found on: yash < 2.44 and dash < 0.5.9 and Busybox ash < 1.27.0
• BUG_PP_08B: When IFS is empty, unquoted $* within a substitution (e.g. ${1+$*} or ${var-$*}) does not generate one field for each positional parameter as expected, but instead joins them into a single field without a separator. Found on: bash 3 and 4
• BUG_PP_09: When IFS is non-empty but does not contain a space, unquoted $* within a substitution (e.g. ${1+$*} or ${var-$*}) does not generate one field for each positional parameter as expected, but instead joins them into a single field separated by spaces (even though, as said, IFS does not contain a space). Found on: bash 2
• BUG_PP_10: When IFS is null (empty), assigning var=$* removes any $CC01 (^A) and $CC7F (DEL) characters. (bash 3, 4)
• BUG_PP_10A: When IFS is non-empty, assigning var=$* prefixes each $CC01 (^A) and $CC7F (DEL) character with a $CC01 character. (bash 4.4)
• BUG_PP_11: If IFS is set and empty (no field separator for $*), assigning unquoted $* to a variable (i.e.: foo=$*) causes the fields to be separated by a space in the variable, instead of joined together without a separator. (bash 2.05b, 3.0)
• BUG_PSUBBKSL1: A backslash-escaped } character within a quoted parameter substitution is not unescaped. (bash 2 & 3, standard dash, Busybox ash)
• BUG_PSUBNEWLN: Due to a bug in the parser, parameter substitutions spread over more than one line cause a syntax error. Workaround: instead of a literal newline, use $CCn. (found in dash <= 0.5.9.1 and Busybox ash <= 1.28.1)
• BUG_PSUBSQUOT: in pattern matching parameter substitutions (${param#pattern}, ${param%pattern}, ${param##pattern} and ${param%%pattern}), if the whole parameter substitution is quoted with double quotes, then single quotes in the pattern are not parsed. POSIX says they are to keep their special meaning, so that glob characters may be quoted. For example: x=foobar; echo "${x#'foo'}" should yield bar but with this bug yields foobar. (dash; Busybox ash)
• BUG_PSUBSQHD: Like BUG_PSUBSQUOT, but included a here-document instead of quoted with double quotes. (dash, pdksh, mksh)
• BUG_READWHSP: If there is more than one field to read, read does not trim trailing IFS whitespace (dash 0.5.7, 0.5.8) or any IFS whitespace (dash 0.5.6, 0.5.6.1).
• BUG_REDIRIO: the I/O redirection operator <> (open a file descriptor for both read and write) defaults to opening standard output (i.e. is short for 1<>) instead of defaulting to opening standard input (0<>) as POSIX specifies. (AT&T ksh93)
• BUG_REDIRPOS: Buggy behaviour occurs if a redirection is positioned in between to variable assignments in the same command. On zsh 5.0.x, a parse error is thrown. On zsh 5.1 to 5.4.2, anything following the redirection (other assignments or command arguments) is silently ignored.
• BUG_SCLOSEDFD: bash < 5.0 and dash <= 0.5.9.1 fail to save a closed file descriptor onto the shell-internal stack when added at the end of a block or loop (e.g. } 8<&- or done 7>&-), so any 'exec' of that descriptor will leak out of the block. However, pushing an open file descriptor works fine. Workaround: enclose in another block that pushes the FD in an open state.
• BUG_SELECTEOF: in a shell-native 'select' loop, the REPLY variable is not cleared if the user presses Ctrl-D to exit the loop. (zsh)
• BUG_SELECTRPL: in a shell-native 'select' loop, input that is not a menu item is not stored in the REPLY variable as it should be. (mksh R50 2014)
• BUG_SETOUTVAR: The set builtin (with no arguments) only prints native function-local variables when called from a shell function. (yash <= 2.46)
• BUG_SPCBILOC: Variable assignments preceding special builtins create a partially function-local variable if a variable by the same name already exists in the global scope. (bash < 5.0 in POSIX mode)
• BUG_TESTERR0: mksh: test/[ exits successfully (exit status 0) if an invalid argument is given to an operator. (mksh R52 fixes this)
• BUG_TESTERR1A: AT&T ksh: test/[ exits with a non-error 'false' status (1) if an invalid argument is given to an operator.
• BUG_TESTERR1B: zsh: test/[ exits with status 1 (false) if there are too few or too many arguments, instead of a status > 1 as it should do.
• BUG_TESTILNUM: On dash (up to 0.5.8), giving an illegal number to test -t or [ -t causes some kind of corruption so the next test/[ invocation fails with an "unexpected operator" error even if it's legit.
• BUG_TESTONEG: The test/[ builtin supports a -o unary operator to check if a shell option is set, but it ignores the no prefix on shell option names, so something like [ -o noclobber ] gives a false positive. Bug found on yash up to 2.43. (The TESTO feature test implicitly checks against this bug and won't detect the feature if the bug is found.)
• BUG_TESTPAREN: Incorrect exit status of test -n/-z with values (, ) or ! in zsh 5.0.6 and 5.0.7. This can make scripts that process arbitrary data (e.g. the shellquote function) take the wrong action unless workarounds are implemented or modernish equivalents are used instead. Also, spurious error message with both test -n and test -z.
• BUG_TESTRMPAR: zsh: in binary operators with test/[, if the first argument starts with ( and the last with `)', both the first and the last argument are completely removed, leaving only the operator, and the result of the operation is incorrectly true because the operator is incorrectly parsed as a non-empty string. This applies to any operator.
• BUG_TRAPEMPT: The trap builtin does not quote empty traps in its output, rendering the output unsuitable for shell re-input. For instance, trap '' INT; trap outputs "trap -- INT" instead of "trap -- '' INT". (found in pdksh, mksh)
• BUG_TRAPEXIT: the shell's trap builtin does not know the EXIT trap by name, but only by number (0). Using the name throws a "bad trap" error. Found in klibc 2.0.4 dash. Note that the modernish trap and pushtrap commands effectively work around this bug, so it only affects scripts if they bypass modernish.
• BUG_TRAPRETIR: Using 'return' within 'eval' triggers infinite recursion if both a RETURN trap and the 'functrace' shell option are active. This bug in bash-only functionality triggers a crash when using modernish, so to avoid this, modernish automatically disables the functrace shell option if a RETURN trap is set or pushed and this bug is detected. (bash 4.3, 4.4)

Warning IDs

Warning IDs do not identify any characteristic of the shell, but instead warn about a potentially problematic system condition that was detected at initalisation time.

• WRN_2UP2LOW: This warning ID is issued if modernish is initialised in a UTF-8 locale and the modernish functions toupper and tolower cannot convert non-ASCII letters to upper or lower case -- e.g. accented Latin letters, Greek, cyrillic. Modernish tries hard to make it work, falling back to using the external tr, awk, gawk or GNU sed command if the shell can't convert non-ASCII (or any) characters itself. So if the shell cannot do it, then this warning ID is only issued if none of these external commands can convert them either. But if the shell can, then this warning ID is not issued even if the external commands cannot. This means that the result of thisshellhas WRN_2UP2LOW only applies to the modernish toupper and tolower functions and not to your shell or any external command in particular.
• WRN_NOSIGPIPE: Modernish has detected that the process that launched the current program has set SIGPIPE to ignore, an irreversible condition that is in turn inherited by any process started by the current shell, and their subprocesses, and so on. This makes it impossible to detect $SIGPIPESTATUS; it is set to the special value 99999 which is impossible as an exit status. But it also makes it irrelevant what that status is, because neither the current shell nor any process it spawns is now capable of receiving SIGPIPE. The -P option to harden is also rendered irrelevant. Note that a command such as yes | head -n 10 now never ends; the only way yes would ever stop trying to write lines is by receiving SIGPIPE from head, which is being ignored. Programs that use commands in this fashion should check if thisshellhas WRN_NOSIGPIPE and either employ workarounds or refuse to run if so.

Appendix B

Modernish comes with a suite of regression tests to detect bugs in modernish itself, which can be run using modernish --test after installation. By default, it will run all the tests verbosely but without tracing the command execution. The install.sh installer will run the suite quietly on the selected shell before installation.

A few options are available to specify after --test:

• -e: run expensive (i.e. slow or memory-hogging) tests that are disabled by default. Some tests run without this option but are more thorough if this option is given.
• -q: quieter operation; report expected fails [known shell bugs] and unexpected fails [bugs in modernish]). Add -q again for quietest operation (report unexpected fails only).
• -s: entirely silent operation.
• -t: run only specific test sets or tests. Test sets are those listed in the full default output of modernish --test. This option requires an option-argument in the following format:
  testset1:num1,num2,…/testset2:``*num1*,*num2*,…/… The colon followed by numbers is optional; if omitted, the entire set will be run, otherwise the given numbered tests will be run in the given order. Example:modernish --test -t match:2,4,7/arith/shellquote:1runs test 2, 4 and 7 from thematchset, the entirearithset, and only test 1 from theshellquote` set.
• -x: trace each test using the shell's xtrace facility. Each trace is stored in a separate file in a specially created temporary directory. By default, the trace is deleted if a test does not produce an unexpected fail. Add -x again to keep expected fails as well, and again to keep all traces regardless of result. If any traces were saved, modernish will tell you the location of the temporary directory at the end, otherwise it will silently remove the directory again.

These short options can be combined so, for example, --test -qxx is the same as --test -q -x -x.

Difference between bug/quirk/feature tests and regression tests

Note the difference between these regression tests and the tests listed above in Appendix A. The latter are tests for whatever shell is executing modernish: they test for capabilities (features, quirks, bugs) of the current shell. They are meant to be run via thisshellhas and are designed to be taken advantage of in scripts. On the other hand, these tests run by modernish --test are regression tests for modernish itself. It does not make sense to use these in a script.

New/unknown shell bugs can still cause modernish regression tests to fail, of course. That's why some of the regression tests also check for consistency with the results of the feature/quirk/bug tests: if there is a shell bug in a widespread release version that modernish doesn't know about yet, this in turn is considered to be a bug in modernish, because one of its goals is to know about all the shell bugs in all released shell versions currently seeing significant use.

Testing modernish on all your shells

The testshells.sh program in share/doc/modernish/examples can be used to run the regression test suite on all the shells installed on your system. You could put it as testshells in some convenient location in your $PATH, and then simply run:

testshells modernish --test

(adding any further options you like -- for instance, you might like to add -q to avoid very long terminal output). On first run, testshells will generate a list of shells it can find on your system and it will give you a chance to edit it before proceeding.

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