Language features compared

This page compares what the shell language of dash, bash, zsh and luish can do beyond the basics that all four share: quoting, parameter expansion, arrays, arithmetic, conditionals, globbing, redirections, control flow and the variables the shell sets. Built-in commands compared compares the built-in commands.

Each feature was tried in dash 0.5.12, bash 5.2, zsh 5.9 (started with -f) and luish 0.7.0. The zsh column is native zsh. Where luish takes a feature from zsh, it usually follows zsh’s sh emulation (zsh --emulate sh), which turns some of zsh’s features off and counts array indices from 0; the notes say where that matters.

In the tables, ✓ marks a feature that works, – one that doesn’t, and opt one that needs an option turned on.

luish turns on everything that gives a meaning to what is a syntax error in dash, such as a=(x y) or [[, so a script can use it without any setting. What would change the behaviour of a POSIX script, such as **/ or glob qualifiers, needs an option, as in bash and zsh.

Quoting

Feature	dash	bash	zsh	luish	Notes
'...', "...", \	✓	✓	✓	✓
$'a\tb'	–	✓	✓	–	In POSIX since 2024
$"..." (translated)	–	✓	–	–

Parameter expansion

All four have POSIX’s forms: ${x:-word}, ${x:=word}, ${x:?word}, ${x:+word}, ${#x}, ${x#pattern}, ${x##pattern}, ${x%pattern} and ${x%%pattern}.

Feature	dash	bash	zsh	luish	Notes
Substrings, ${x:1:3}, ${x: -3}	–	✓	✓	✓
Replacement, ${x/pat/rep}, ${x//pat/rep}	–	✓	✓	✓	& in the replacement is literal, as in zsh (bash 5.2 replaces it with the match)
Anchored replacement, ${x/#pat/rep}, ${x/%pat/rep}	–	✓	✓	✓
Case, ${x^^}, ${x^}, ${x,,}	–	✓	–	–	zsh and luish: ${(U)x}, ${(L)x}, ${(C)x}
Transformations, ${x@Q}, ${x@U} …	–	✓	–	–
Indirection, ${!x}	–	✓	–	✓	zsh: ${(P)x}
Names by prefix, ${!prefix*}, ${!prefix@}	–	✓	–	✓	zsh: ${(k)parameters[(I)prefix*]}
zsh’s flags @ k v j s f F L U C u o O i n a, as in ${(j:,:)a[@]}, ${(o)a[@]}, ${(s:,:)x}, ${(f)x}	–	–	✓	✓
zsh’s other flags, such as P, q, e, l:n:, r:n:, %	–	–	✓	–
Nested, ${${x#a}%b}	–	–	✓	–
Modifiers, ${x:h}, ${x:t}, ${x:r}, ${x:e}, ${x:A}	–	–	✓	✓	luish has h, t, r, e, a, A, u and l
Lengths and offsets in characters	–	✓	✓	–	luish, as dash, counts bytes: in a UTF-8 locale, x=é; echo ${#x} prints 2 (bash and zsh: 1)

Arrays

Feature	dash	bash	zsh	luish	Notes
a=(x y z), "${a[@]}", ${#a[@]}, a+=(w)	–	✓	✓	✓
First index		0	1	0	zsh’s sh emulation counts from 0, as bash and luish
$a		First element	All elements	First element
a[i]=x, ${a[i]}, ${a[-1]}	–	✓	✓	✓
Holes: a=(); a[5]=x; echo ${#a[@]}	–	1	5	6	bash’s arrays are sparse; zsh and luish fill the gap with empty elements
Indices, ${!a[@]}	–	✓	–	✓
Slices, ${a[@]:1:2}	–	✓	✓	✓
zsh’s ranges, $a[2,3]	–	–	✓	–
On each element, ${a[@]/x/y}, ${a[@]#p}	–	✓	✓	✓
local a=(x y), typeset -a	–	✓	✓	✓
Read into an array, read -a (bash), read -A (zsh)	–	✓	✓	✓	luish takes both
mapfile, readarray	–	✓	–	–
Exported arrays	–	–	–	–	None of them export arrays

Associative arrays

Feature	dash	bash	zsh	luish	Notes
typeset -A h, h[k]=v, ${h[k]}, ${#h[@]}	–	✓	✓	✓
h=([k]=v [j]=w)	–	✓	✓	✓
h=(k v j w)	–	✓	✓	✓
Keys, ${!h[@]}	–	✓	–	✓
Keys, ${(k)h[@]}	–	–	✓	✓
Order of the keys		Hash order	Hash order	Insertion order

Arithmetic

All four have $((...)) with C’s integer operators, assignments (x+=2), ?:, hexadecimal (0x10) and octal (010) numbers.

Feature	dash	bash	zsh	luish	Notes
let	–	✓	✓	✓
((...)) as a command	–	✓	✓	–	luish: let '...', or [ $((...)) -ne 0 ]
**	–	✓	✓	✓
++, --	–	✓	✓	✓
,	–	✓	✓	✓
Bases, 16#ff	–	✓	✓	–
Floating point	–	–	✓	–
Integer variables, typeset -i	–	✓	✓	✓

Conditionals

Feature	dash	bash	zsh	luish	Notes
[ ... ], test	✓	✓	✓	✓
[ a == b ], the same as =	–	✓	✓	✓	zsh: quote it ('=='), as a word that starts with = is a command’s path (=ls)
[[ ... ]], with patterns ([[ $x == *.c ]])	–	✓	✓	✓
Regular expressions, [[ $x =~ re ]]	–	✓	✓	✓	luish sets both bash’s BASH_REMATCH and zsh’s MATCH and match
[[ -v name ]]	–	✓	✓	✓
[ -v name ]	–	✓	✓	–

Brace expansion

Feature	dash	bash	zsh	luish	Notes
a{b,c}	–	✓	✓	opt	luish: setopt expand.braces; not in zsh’s sh emulation
{1..10}, {a..z}, {01..10..2}	–	✓	✓	opt
{1..$n}	–	–	✓	opt	bash expands braces before variables

Where bash and zsh differ (the sign of a step, {a..e..2}, {1..a}, empty words such as {a,}, > f{1,2}), luish follows bash (see Compatibility).

Globbing

All four have POSIX’s *, ? and [...]. luish sorts matches by bytes, where bash and zsh use the locale’s collation (the same in the C locale).

Feature	dash	bash	zsh	luish	Notes
**/, any number of directories	–	opt	✓	opt	bash: shopt -s globstar; luish: setopt glob.star
Glob qualifiers, *(/), *(.om[0])	–	–	✓	opt	luish: setopt glob.bare_qualifiers; most of zsh’s qualifiers (see Extended globbing)
ksh’s patterns, @(a|b), !(x)	–	opt	opt	–	bash: shopt -s extglob; zsh: setopt ksh_glob
zsh’s patterns, (a|b), ^x, x~y, x#	–	–	✓	–	^, ~ and # need setopt extended_glob
No match gives nothing (nullglob)	–	opt	opt	opt	luish: setopt glob.null; also a qualifier, *(N) (zsh and luish)
No match is an error (failglob)	–	opt	✓	opt	zsh: NOMATCH, on by default (off in its sh emulation); luish: setopt glob.fail
Error only if no pattern matches (CSH_NULL_GLOB)	–	–	opt	opt	luish: setopt glob.fail_all
Matching dot files (dotglob)	–	opt	opt	opt	luish: setopt glob.dots; also a qualifier, *(D) (zsh and luish)

Redirections and pipes

Feature	dash	bash	zsh	luish	Notes
POSIX’s, including >| and here-documents	✓	✓	✓	✓
File descriptors above 9, exec 20>f	–	✓	–	✓	dash and zsh read 20 as a word
Here-strings, <<< word	–	✓	✓	✓
&>, &>> (stdout and stderr)	–	✓	✓	–	luish, as dash and POSIX, reads cmd &> f as cmd & then > f; write > f 2>&1
|& (pipe stdout and stderr)	–	✓	✓	✓
Process substitution, <(cmd), >(cmd)	–	✓	✓	✓
zsh’s =(cmd) (a temporary file)	–	–	✓	–
Allocated descriptors, exec {fd}>f	–	✓	✓	–
$(<file)	–	✓	✓	–	$(cat file)
Command substitution without a subshell, ${ list; }, ${| list; }	–	–	–	✓	As in bash 5.3 and mksh
Coprocesses, coproc	–	✓	✓	–

Control flow and functions

Feature	dash	bash	zsh	luish	Notes
function name { ... }	–	✓	✓	✓
local	✓	✓	✓	✓
case fall-through, ;&	–	✓	✓	✓	In POSIX since 2024
case continue, ;;& (bash), ;| (zsh)	–	✓	✓	✓	luish takes both
for ((i=0; i<n; i++))	–	✓	✓	–	i=0; while [ $i -lt $n ]; do ...; i=$((i+1)); done
select	–	✓	✓	–
time as a keyword (times pipelines and functions)	–	✓	✓	–	luish and dash run the time program
repeat N, foreach, short loops (for i in a b; cmd)	–	–	✓	–
Anonymous functions, () { ...; } args	–	–	✓	–

Errors and debugging

Feature	dash	bash	zsh	luish	Notes
set -e, set -u, set -x	✓	✓	✓	✓	luish’s set -x quotes arguments as bash does
set -o pipefail	–	✓	✓	✓	In POSIX since 2024
$LINENO	–	✓	✓	✓
ERR trap (zsh also ZERR)	–	✓	✓	–
DEBUG trap	–	✓	✓	–
Call stack: BASH_SOURCE, FUNCNAME, BASH_LINENO, caller (bash)	–	✓	–	✓	zsh has funcstack, funcfiletrace and %x instead, outside its sh emulation
The call stack in error messages	–	–	–	✓	bash and zsh name the file (bash) or the function (zsh) of the error, without the calls that led there
Deep recursion	Limit for functions (Debian’s), crashes otherwise	Crashes, unless FUNCNEST is set	Limit (FUNCNEST)	Error	luish stops at 1000 levels of function calls, and makes other nesting that would run out of stack an error

Variables the shell sets

Variable	dash	bash	zsh	luish	Notes
RANDOM, SECONDS	–	✓	✓	✓
EPOCHSECONDS, EPOCHREALTIME	–	✓	opt	✓	zsh: zmodload zsh/datetime
UID, EUID, HISTCMD	–	✓	✓	✓
PIPESTATUS (bash)	–	✓	–	✓
pipestatus (zsh)	–	–	✓	✓
path, dirstack, tied to PATH and the directory stack	–	–	✓	✓
$_	–	✓	✓	–
SHLVL	–	✓	✓	✓

Variable attributes

Feature	dash	bash	zsh	luish	Notes
export, readonly, local	✓	✓	✓	✓
typeset/declare -i, -l, -u, -r, -x, -g	–	✓	✓	✓
typeset -U (unique elements, as for path)	–	–	✓	✓
Namerefs, typeset -n	–	✓	–	–
Floating point, typeset -F, -E	–	–	✓	–
Padding, typeset -L, -R, -Z	–	–	✓	–
Tied variables, typeset -T	–	–	✓	–	luish ties only path and dirstack