Shell/Scripting

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This page is a reference guide to scripting-related differences between shells. It is not intended to be a general introduction to shell scripting, either in general or for a particular shell (e.g. Bash).

Tip
For automated checks that scripts are not using behavior and/or functionality only found in Bash and not other shells ("bashisms"), install and use the dev-util/checkbashisms package.

The shells referenced in this page are:

Name Package Notes
Bash app-shells/bash The Bourne-Again Shell, first released in 1989; used by Portage.
Dash app-shells/dash Debian Almquist Shell; intended to be POSIX-conformant.
ATT Ksh (ksh93u+m) app-shells/ksh The original KornShell / Ksh, first released in 1983.
OpenBSD Ksh app-shells/loksh A Linux port of OpenBSD's Ksh.
BusyBox sh sys-apps/busybox The sh applet provided by the BusyBox binary.
This page assumes BusyBox has been compiled with the default configuration (e.g. with ASH_BASH_COMPAT enabled).
Zsh app-shells/zsh The Z Shell, first released in 1990.

POSIX

Non-POSIX

The following are non-POSIX, as per Volume 3 of POSIX.1-2024, "Shells and Utilities". Note that something being specified by POSIX does not mean that all shells have necessarily implemented it (although they might plan to).

  • == in test expressions[1] . Instead, = should be used to compare strings, -eq to compare numbers.
  • The function keyword for defining functions.
  • Arrays (e.g. $VAR[1]) and associative arrays (e.g. $VAR['key']).
  • ${<var>:<offset>} and ${<var>:<offset>:<length>}, for subscripting.
  • Process substitution, e.g. diff <$(command one) <$(command two).
  • select, for creating a menu of options selectable by number.
  • rehash to refresh the hash containing the locations of utilities. hash -r should be used instead.
  • Options to the type utility. Note also that POSIX doesn't require shells to provide type as a builtin. If type -P functionality is required, use command -v.
  • local, for creating a variable scoped to a function and its children. However, it is supported by Dash.
  • disown, for specifying that a job should either be removed from the jobs table, or not get sent a SIGHUP if the shell receives a SIGHUP. This builtin is implemented (with varying semantics) by Bash, ATT Ksh, and Zsh, but not by Dash, OpenBSD Ksh, or BusyBox sh.
  • declare and typeset, for setting and getting shell variables and their attributes. Refer to the "declare and typeset" section for shell differences involving these builtins.

Introduced in POSIX-1.2024

  • The pipefail shell option.
  • The -d option for the read builtin.
  • make(1p) changes include, but are not limited to:
    • new operators for macro assignment: ::=, ?=, +=, !=, and :::=;
    • the -j <maxjobs> option and the .NOTPARALLEL and .WAIT special targets;
    • slashes and hyphens in target names.

For a more comprehensive (but non-official) list, refer to this document.

POSIX conformance modes in shells

Warning
POSIX conformance modes do not restrict a shell's functionality to a POSIX-conformant subset; that is, they do not disable a shell's non-POSIX extensions. Instead, they alter a shell's behavior in specific contexts.

Bash

POSIX behavior in Bash can be requested via either the --posix option (when starting the shell) or set -o posix (from within a running shell). Additionally, invoking Bash as sh will enable POSIX mode once startup files are read.

When in POSIX mode, the POSIXLY_CORRECT variable is set.

Details about how Bash behaves in POSIX mode can be found in this document.

ATT Ksh

POSIX behavior can be requested via set -o posix. Refer to the relevant section of the man page for details about behavioral changes in POSIX mode.

OpenBSD Ksh

The man page for app-shells/loksh , the Linux port of OpenBSD's Ksh, states:

The shell is intended to be POSIX compliant; however, in some cases, POSIX behaviour is contrary either to the original Korn shell behaviour or to user convenience.

POSIX behavior can be requested via set -o posix or by setting the POSIXLY_CORRECT variable in the environment from which ksh is started.

Refer to the relevant section of the man page for details about behavioral changes in POSIX mode.

Zsh

  • To enable POSIX-style word splitting, set the SH_WORD_SPLIT option.

Shell comparisons

Behavior of echo

As noted above, echo is not required to be a shell builtin, and the echo utility is not required to support any options. Additionally, however, the behavior of echo varies (as at 2025年03月06日):

Version Behavior of echo '\n' Behavior of echo "\n" Behavior of echo $'\n'
Bash builtin echo $ echo '\n'
\n
$ 		 $ echo "\n"
\n
$ 		 $ echo $'\n'


$ 
ATT Ksh builtin echo $ echo '\n'
\n
$ 		 $ echo "\n"
\n
$ 		 $ echo $'\n'


$ 
OpenBSD Ksh builtin echo $ echo '\n'


$ 		 $ echo "\n"


$ 		 $ echo $'\n'
$

$ 
BusyBox sh builtin echo $ echo '\n'
\n
$ 		 echo "\n"
\n
$ 		 echo $'\n'


$ 
Zsh builtin echo $ echo '\n'


$ 		 $ echo "\n"


$ 		 $ echo $'\n'


$ 
GNU utility echo $ /usr/bin/echo '\n'
\n
$ 		 $ /usr/bin/echo "\n"
\n
$ 		 $ /usr/bin/echo $'\n'


$ 
OpenBSD utility echo $ /bin/echo '\n'
\n
$ 		 $ /bin/echo "\n"
\n
$ 		 $ /bin/echo $'\n'
$\n
$ 

Note that, as shells are not required by POSIX to provide an echo builtin, Dash uses the available echo utility (i.e. by default on Gentoo, the GNU echo utility).

All of Bash, OpenBSD Ksh and Zsh provide the -e, -E and -n options to their echo builtin (although OpenBSD Ksh only treats -e and -E as options in non-POSIX mode). BusyBox sh only provides the -e and -n options. However, the escape sequences affected by the -e and -E options differ between shells:

Sequence Description Bash OpenBSD Ksh BusyBox sh Zsh
\a Alert (bell) Yes Yes Yes Yes
\b Backspace Yes Yes Yes Yes
\c Suppress further output Yes Yes Yes Yes
\e Escape character Yes No Yes Yes
\E Escape character Yes No No No
\f Form feed Yes Yes Yes Yes
\n New line Yes Yes Yes Yes
\r Carriage return Yes Yes Yes Yes
\t Horizontal tab Yes Yes Yes Yes
\v Vertical tab Yes Yes Yes Yes
\\ Backslash Yes Yes Yes Yes
\0nnn The eight-bit character whose value is the octal value nnn (zero to three octal digits) Yes Yes Yes Yes
\xHH The eight-bit character whose value is the hexadecimal value HH (one or two hex digits) Yes No Yes Yes
\uHHHH The Unicode character whose value is the hexadecimal value HHHH (one to four hex digits) Yes No No Yes
\uHHHHHHHH The Unicode character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits) Yes No No Yes

Behavior of arrays

  • Dash doesn't support arrays, as arrays are not specified by POSIX.
  • BusyBox sh doesn't appear to support arrays (as at 2025年03月06日).

Declaration syntax

Shell Syntax
Bash x=(1 2 3), declare -a x
ATT Ksh x=(1 2 3)
OpenBSD Ksh set -A x 1 2 3
Zsh x=(1 2 3), set -A x 1 2 3

Subscripting syntax

By default, and after having defined an array a with the elements 1, 2, and 3:

Syntax Bash ATT Ksh OpenBSD Ksh Zsh
$a 1 1 1 1 2 3
$a[1] 1[1] 1[1] 1[1] 1
${a[0]} 1 1 1 Empty string
${a[1]} 2 2 2 1
${a[-1]} 3 3 Error 3
${a[1,-1]} 3 3 Error 1 2 3
${a[1..-1]} Error 2 3 Error Error

Zsh can be configured to use 0-based array indexing via the KSH_ZERO_SUBSCRIPT and KSH_ARRAYS options.

Additionally, when Zsh's KSH_ARRAYS option is set, braces are required when subscripting.

Behavior of pipelines

In general, each command in a pipeline is run in a subshell. However, there are differences in how shells handle a pipeline's final command: Bash, Dash, OpenBSD Ksh, and BusyBox sh run the final command in a subshell, but ATT Ksh and Zsh run the final command in the current shell. Bash's behavior can be changed to run the final command in the current shell by disabling job control (set +m) and then setting the lastpipe option (shopt -s lastpipe).

Expansion precedence

In ATT Ksh, OpenBSD Ksh, and Zsh, parameter expansion is done before brace expansion; in Bash, the reverse is true.

declare and typeset

In Bash and Zsh, declare is a synonym for typeset. declare is not available in ATT Ksh or OpenBSD Ksh, only typeset. Neither declare nor typeset are available in BusyBox sh or Dash.

Bash vs Ksh

ATT Ksh

  • Bash supports the local builtin, ATT Ksh does not.
  • ATT Ksh allows subscript ranges (e.g. ${a[1..-1]}), Bash does not.

OpenBSD Ksh

Bash vs Zsh

  • Bash's builtin to set options is shopt; Zsh's analogous builtin is setopt. The list of available Zsh options is not a strict superset of Bash options; for example, cdspell is a Bash option but not a Zsh option.
  • Zsh's alias builtin has options unavailable in Bash's alias builtin. In particular, Bash's alias only has the -p option, equivalent to providing no option at all, which prints defined aliases on standard output.
  • The ** syntax for 'recursive globbing' is available in Zsh by default, but is not enabled by default in Bash; it can be enabled via the globstar option.
  • In Bash, by default, if the * glob has no matches, no error will be produced (option nofailglob). However, in Zsh, by default, an error will be produced (option NOMATCH).
  • The available options for the read builtin differ between Bash and Zsh.

Bash: 

read [ -ers ] [ -a aname ] [ -d delim ] [ -i text ] [ -n nchars ] [ -N nchars ] [-p prompt ] [ -t timeout ] [ -u fd ] [ name ... ]

Zsh: 

read [ -rszpqAclneE ] [ -t [ num ] ] [ -k [ num ] ] [ -d delim ] [ -u n ] [ [name][?prompt] ] [ name ... ]
  • Bash supports format specifications for the printf builtin which aren't supported by Zsh: %Q and %(<datefmt>)T.

Arrays

  • Bash uses 0-based indexing, Zsh uses 1-based indexing unless the Zsh KSH_ZERO_SUBSCRIPT option or the KSH_ARRAYS option is set.
  • Bash requires curly braces around subscripted array references (e.g. ${a[1]}), Zsh does not, unless the Zsh KSH_ARRAYS option is set.
  • Zsh allows specifying subscript ranges (e.g. ${a[1,-2]}), Bash does not.

References