esceval - Escape Evaluation

esceval is a couple small CLI tools that let us:

1. Escape strings so that they evaluate back to the same string in all Bourne-like/POSIX shells.

2. Safely check if arbitrary strings are in "esceval format" before evaluating them.

Usage

1. esceval

esceval quotes/escapes all arguments it receives and prints them on standard output, separated by spaces:

$ esceval foo 'bar  qux' "abc'def" "'zyx'''wvu'"
'foo' 'bar  qux' 'abc'\''def' \''zyx'\'\'\''wvu'\'

2. escevalcheck

escevalcheck checks each of its arguments - if any of them is not a validly esceval-formatted string, exit status is 1; otherwise, exit status is 0.

$ escevalcheck "'foo' 'bar  qux' 'abc'\''def' \''zyx'\'\'\''wvu'\'"; echo $?
0
$ escevalcheck "'foo"; echo $?  # note the missing closing quote
1

3. escevalenv and escevalcheckenv

escevalenv and escevalcheckenv are the same as esceval and escevalcheck, but they take environment variable names as arguments:

$ escevalenv PATH
'/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'

When handling sensitive data (private or identifying information, cryptographic keys, passwords, and so on),

x="$SECRET" escevalenv x

is more secure than

esceval "$SECRET"

since on most operating systems, environment variables are kept more private from other processes than arguments.

If a given environment variable is not set, escevalenv and escevalcheckenv treat it as if it's set to the empty string:

$ escevalenv EXAMPLE_NAME_WHICH_IS_NOT_SET
''
$ escevalcheckenv EXAMPLE_NAME_WHICH_IS_NOT_SET; echo $?
0

Example

A portable pushd and popd in just a few lines:

DIRECTORY_STACK="$(esceval "$(pwd)")"

pushd()
{
    cd "$1" &&
    DIRECTORY_STACK="$(esceval "$1") $DIRECTORY_STACK"
}

popd()
{
    if ! escevalcheck "$DIRECTORY_STACK"
    then
        printf '%s\n' 'popd: directory stack corrupted' 1>&2
        return 1
    fi
    eval "_popd $DIRECTORY_STACK"
}

_popd()
{
    if [ "$#" -lt 2 ]
    then
        printf '%s\n' 'popd: directory stack empty' 1>&2
        return 1
    fi
    shift
    DIRECTORY_STACK="$(esceval "$@")"
    cd "$1" && pwd
}

Lists, Stacks, Queues

esceval (combined with eval, a shell function or set --, and shift) makes it really easy to write shell scripts which work with lists, stacks, and queues, even on shells without native arrays.

The above example shows how we can implement stack push and pop. For FIFO queue behavior, we just need to append the new esceval'd values instead of prepending them.

(Of course, these are still O(n) operations, not O(1) like we'd get from native stacks and queues, because the shell has to evaluate the string each time, but this gets us much of the simplicity, ease, and code clarity of those data structures.)

Versus printf's %q

If you just want an unambiguous echo for debugging, logging, or error messages, printf with %q may be better at revealing stray control characters, differences in whitespace, and so on. (At least in shells like bash and zsh, where %q can print escape sequences like $'\t').

If you need outputs that can be parsed back to the same value by all Bourne-like/POSIX shells, and are simple to verify as safe to evaluate, use esceval.

esceval Format Specification

1. A single-quote (') is encoded as a backslash and then a single-quote (\').

2. Any substrings of characters other than single-quotes are encoded by wrapping them in single-quotes. (For example, abc is encoded as 'abc'.) This includes the empty string (encoded as '').

3. Separate strings are separated by one or more spaces ( ).

(Command-line implementations of esceval should write a newline after the last encoded string for convenience, but this is not part of the esceval format, and escevalcheck implementations must reject unescaped newlines since they would parse as command separators when evaluated by most shells in most contexts.)

Redundant encodings (for example, encoding a''b into 'a'\'''\''b' instead of 'a'\'\''b') are permitted to allow very simple implementations.