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Zsh Native Scripting Handbook

This document has been created after 2.5 years of creating software for Zshell and receiving help from IRC channel #zsh. Avoiding forks was the main guideline when creating the projects and this lead to multiple discoveries of Zsh constructs that are fast, robust and do not depend on external tools. Such code is more like Ruby or Perl code, less like top-down shell scripts.

Information

@ is about splitting

How to access all array elements? — use @. However, Zshell completion (echo ${(<TAB>) for @ says: @ — double-quoted splitting of scalars and that’s the true meaning of this flag / subscript. If you quote e.g. an array, like declare -a array; array=( a b c ); print "$array", you create a string, a scalar "a b c". @ is to go back to non-scalar data by doing splitting.

Two forms are available, "$array[@]" and "${(@)array}". First form has additional effect – when option KSH_ARRAYS is set, it indeed induces referencing to whole array instead of first element only. It should then use braces, i.e. ${array[@]}, "${array[@]}" (KSH_ARRAYS requirement).

extended_glob

Glob-flags #b and #m require setopt extended_glob. Patterns utilizing ~ and ^ also require it. Extended-glob is one of the main features of Zsh.

Constructs

Reading a file

declare -a lines; lines=( "${(@f)"$(<path/file)"}" )

This preserves empty lines because of double-quoting (the outside one). @-flag is used to obtain array instead of scalar. If you don’t want empty lines preserved, you can also skip @-splitting, as it is explained in Information section:

declare -a lines; lines=( ${(f)"$(<path/file)"} )

Note: $(<…​) construct strips trailing empty lines.

Skipping grep

declare -a lines; lines=( "${(@f)"$(<path/file)"}" )
declare -a grepped; grepped=( ${(M)lines:#*query*} )

To have grep -v effect, skip M-flag. To grep case insensitively, use #i glob flag (…​:#(#i)*query*}).

Pattern matching in AND-fashion

[[ "abc xyz efg" = *abc*~^*efg* ]] && print Match found

The ~ is a negation — match *abc* but not …​. Then, ^ is also a negation. The effect is: *abc* but not those that don’t have *efg* which equals to: *abc* but those that have also *efg*. This is a regular pattern and it can be used with :# above to search arrays, or with R-subscript flag to search hashes (${hsh[(R)*pattern*]}), etc. Inventor of those patterns is Mikael Magnusson.

Skipping tr

declare -A map; map=( a 1 b 2 );
text=( "ab" "ba" )
text=( ${text[@]//(#m)?/${map[$MATCH]}} )
print $text ▶ 12 21

#m flag enables the $MATCH parameter. At each // substitution, $map is queried for character-replacement. You can substitute a text variable too, just skip [@] and parentheses in assignment.

Ternary expressions with +,-,:+,:- substitutions

HELP="yes"; print ${${HELP:+help enabled}:-help disabled} ▶ help enabled
HELP=""; print ${${HELP:+help enabled}:-help disabled} ▶ help disabled

Ternary expression is known from C language but exists also in Zsh, but directly only in math context, i.e. (( a = a > 0 ? b : c )). Flexibility of Zsh allows such expressions also in normal context. Above is an example. :+ is "if not empty, substitute …" :- is "if empty, substitute …". You can save great number of lines of code with those substitutions, it’s normally at least 4-lines if condition or lenghty &&/|| use.

Ternary expressions with :# substitution

var=abc; print ${${${(M)var:#abc}:+is abc}:-not abc} ▶ is abc
var=abcd; print ${${${(M)var:#abc}:+is abc}:-not abc} ▶ not abc

An one-line "if var = x, then …, else …". Again, can spare a great amount of boring code that makes 10-line function a 20-line one.

Using built-in regular expressions engine

[[ "aabbb" = (#b)(a##)*(b(#c2,2)) ]] && print ${match[1]}-${match[2]} ▶ aa-bb

## is: "1 or more". (#c2,2) is: "exactly 2". A few other constructs: # is "0 or more", ? is "any character", (a|b|) is "a or b or empty match". #b enables the $match parameters. There’s also #m but it has one parameter $MATCH for whole matched text, not for any parenthesis.

Zsh patterns are basically a custom regular expressions engine. They are slightly faster than zsh/regex module (used for =~ operator) and don’t have that dependency (regex module can be not present, e.g. in default static build of Zsh). Also, they can be used in substitutions, for example in // substitution.

Skipping uniq

declare -aU array; array=( a a b ); print $array ▶ a b
declare -a array; array=( a a b ); print ${(u)array} ▶ a b

Enable -U flag for array so that it guards elements to be unique, or use u-flag to uniquify elements of any array.

Skipping awk

declare -a list; list=( "a,b,c,1,e" "p,q,r,2,t" );
print "${list[@]/(#b)([^,]##,)(#c3,3)([^,]##)*/${match[2]}}" ▶ 1 2

The pattern specifies 3 blocks of [^,]##, so 3 "not-comma multiple times, then comma", then single block of "not-comma multiple times" in second parentheses — and then replaces this with second parentheses. Result is 4th column extracted from multiple lines of text, something awk is often used for. Other method is use of s-flag. For single line of text:

text="a,b,c,1,e"; print ${${(s:,:)text}[4]} ▶ 1

Thanks to in-substitution code-execution capabilities it’s possible to use s-flag to apply it to multiple lines:

declare -a list; list=( "a,b,c,1,e" "p,q,r,2,t" );
print "${list[@]/(#m)*/${${(s:,:)MATCH}[4]}}" ▶ 1 2

Searching arrays

declare -a array; array=( a b " c1" d ); print ${array[(r)[[:space:]][[:alpha:]]*]} ▶  c1

[[:space:]] contains unicode spaces. This is often used in conditional expression like [[ -z ${array[(r)…​]} ]].

Code execution in // substitution

append() { gathered+=( $array[$1] ); }
functions -M append 1 1 append
declare -a array; array=( "Value 1" "Other data" "Value 2" )
declare -a gathered; integer idx=0
: ${array[@]/(#b)(Value ([[:digit:]]##)|*)/$(( ${#match[2]} > 0 ? append(++idx) : ++idx ))}
print $gathered ▶ Value 1 Value 2

Use of #b glob flag enables math-code execution (and not only) in / and // substitutions. Implementation is very fast.

Serializing data

declare -A hsh deserialized; hsh=( key value )
serialized="${(j: :)${(qkv@)hsh}}"
deserialized=( "${(Q@)${(z@)serialized}}" )
print ${(kv)deserialized} ▶ key value

j-flag means join — by spaces, in this case. Flags kv mean: keys and values, interleaving. Important q-flag means: quote. So what is obtained is each key and value quoted, and put into string separated by spaces.

z-flag means: split as if Zsh parser would split. So quoting (with backslashes, double quoting and other) is recognized. Obtained is array ( "key" "value") which is then dequoted with Q-flag. This yields original data, assigned to hash deserialized. Use this to e.g. implement array of hashes.

Note: to be compatible with setopt ksharrays, use [@] instead of (@), e.g.: …​( "${(Q)${(z)serialized[@]}[@]}" )

Tip: serializing with Bash
array=( key1 key2 )
printf -v serialized "%q " "${array[@]}"
eval "deserialized=($serialized)"

This method works also with Zsh. The drawback is use of eval, however it’s impossible that any problem will occurr unless someone compromises variable’s value, but as always, eval should be avoided if possible.

Real world examples

Following code checks if there is a git subcommand $mysub:

if git help -a | grep "^  [a-z]" | tr ' ' '\n' | grep -x $mysub > /dev/null > /dev/null; then

That are 4 forks. The code can be replaced according to this guide:

local -a lines_list
lines_list=( ${(f)"$(git help -a)"} )
lines_list=( ${(M)${(s: :)${(M)lines_list:#  [a-z]*}}:#$mysub} )
if (( ${#lines_list} > 0 )); then

The result is just 1 fork.

Tips and Tricks

Parsing INI file

With Zshell’s extended_glob parsing an ini file is an easy task. It will not result in a nested-arrays data structure (Zsh doesn’t support nested hashes), but the hash keys like $DB_CONF[db1_<connection>_host] are actually really intuitive.

The code should be placed in file named read-ini-file, in $fpath, and autoload read-ini-file should be invoked.

read-ini-file
# Copyright (c) 2018 Sebastian Gniazdowski
#
# $1 - path to the ini file to parse
# $2 - name of output hash
# $3 - prefix for keys in the hash
#
# Writes to given hash under keys built in following way: ${3}<section>_field.
# Values are values from ini file. Example invocation:
#
# read-ini-file ./database1-setup.ini DB_CONF db1_
# read-ini-file ./database2-setup.ini DB_CONF db2_
#

setopt localoptions extendedglob

local __ini_file="$1" __out_hash="$2" __key_prefix="$3"
local IFS='' __line __cur_section="void" __access_string
local -a match mbegin mend

[[ ! -r "$__ini_file" ]] && { builtin print -r "read-ini-file: an ini file is unreadable ($__ini_file)"; return 1; }

while read -r -t 1 __line; do
    if [[ "$__line" = [[:blank:]]#\;* ]]; then
        continue
    # Match "[Section]" line
    elif [[ "$__line" = (#b)[[:blank:]]#\[([^\]]##)\][[:blank:]]# ]]; then
        __cur_section="${match[1]}"
    # Match "string = string" line
    elif [[ "$__line" = (#b)[[:blank:]]#([^[:blank:]=]##)[[:blank:]]#[=][[:blank:]]#(*) ]]; then
        match[2]="${match[2]%"${match[2]##*[! $'\t']}"}" # severe trick - remove trailing whitespace
        __access_string="${__out_hash}[${__key_prefix}<$__cur_section>_${match[1]}]"
        : "${(P)__access_string::=${match[2]}}"
    fi
done < "$__ini_file"

return 0