Comparison of command shells

A command shell is a command line interface computer program to an operating system.

General characteristics

Shell	Usual environment	Usually invoked	Introduced	Platform-independent	Default login shell in	Default script shell in	License	Source code availability	User interface	Mouse support	Unicode support	ISO 8601 support	Console redirection	Stream redirection	Configurability	Startup/shutdown scripts	Batch scripts	Logging	available as statically linked, independent single file executable
Thompson shell	UNIX	sh	1971	N/A	UNIX	UNIX,	N/A	Yes	Text-based CLI	No	No	N/A	Yes	N/A	N/A	N/A	N/A	N/A	N/A
Bourne shell 1977 version	7th Ed. UNIX	sh	1977	Yes[1]	7th Ed. UNIX	7th Ed. UNIX,	Proprietary[2]	Yes	Text-based CLI	No	No	N/A	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Yes (.profile)	Yes (Unix feature)	No	Yes
Bourne shell current version	Various UNIX	sh	1977	Yes[3]	SunOS-5.x, FreeBSD (non-root user)[4]	SunOS-5.x	CDDL[5]	Yes	Text-based CLI	No	Yes[6]	N/A	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Yes (.profile)	Yes (Unix feature)	Yes[7]	Yes
POSIX shell[8]	POSIX	sh	1992[9]	N/A	N/A	POSIX	N/A	N/A	Text-based CLI	No	Yes, if used by configured locale	N/A	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Unspecified (.profile given as an example)	Yes (Unix feature)	Yes	N/A
bash (v4)	POSIX	bash, sh	1989[10]	Yes	GNU, Linux (default for root), Mac OS X 10.3+	GNU, Linux, Haiku, Mac OS X	GPL	Yes	Text-based CLI	No	Yes[11]	Yes (printf builtin)	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Yes (/etc/profile, .bash_profile, .bash_login, .profile, .bashrc)	Yes (Unix feature)	Yes	Yes
csh	POSIX	csh	1978	Yes	SunOS	?	BSD	Yes	Text-based CLI	No	No	?	Yes	Yes (stdin, stdout, stdout+stderr)	Yes (via variables and options)	Yes (~/.cshrc, ~/.login, ~/.logout)	Yes (Unix feature)	Yes	Yes
tcsh	POSIX	tcsh, csh	1983[12]	Yes	FreeBSD (default for root), formerly Mac OS X	?	BSD	Yes	Text-based CLI	No	Yes	?	Yes	Yes (stdin, stdout, stdout+stderr)	Yes (via variables and options)	Yes (/etc/csh.cshrc, /etc/csh.login, ~/.tcshrc, ~/.cshrc, ~/.history, ~/.login, ~/.cshdirs)	Yes (Unix feature)	Yes	Yes
Hamilton C shell	Win32, OS/2	csh	1988[13]	Yes (OS/2 version no longer maintained)	Optional	Optional	Proprietary	No	Text-based CLI	No	No	Yes (-t timestamp operator)	Yes	Yes (stdin, stdout, stdout+stderr)	Yes (via variables and options)	Yes (via login.csh, startup.csh and logout.csh)	Yes (command line option)	Yes	Yes
Scsh	POSIX	scsh	1994	Yes	?	?	BSD-style	Yes	?	?	?	?	?	Yes	?	?	?	?	Yes
ksh (ksh93t+)	POSIX	ksh	1983[14][15]	Yes	AIX, HP-UX	OpenSolaris	Common Public License	Yes	Text-based CLI	No	Yes	Yes (printf builtin with %(%F)T[16])	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Yes (system and user's profile and kshrc)	Yes (Unix feature)	Yes	Yes
pdksh	POSIX	ksh, sh	1989?	Yes	OpenBSD[17]	OpenBSD[17]	Public Domain	Yes	Text-based CLI	No	No	N/A	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Yes (/etc/profile, .profile)	Yes (Unix feature)	Yes	Yes
zsh	POSIX	zsh	1990	Yes	Deepin, Gobolinux, Grml	Grml	MIT-style	Yes	Text-based CLI	via additional code[18]	Yes	Yes (various internal features involving the date, by using the %F strftime format[19] and the -i option for the fc builtin[20])	Yes	Yes (arbitrary fds)	Yes (via variables, options, functions, styles, etc.)	Yes (system and user's zshenv, zprofile, zshrc, zlogin, zlogout)	Yes (Unix feature)	Yes	Yes
ash	POSIX	sh	1989	Yes	Minix, BusyBox based systems	NetBSD, Minix, BusyBox based systems	BSD-style	Yes	Text-based CLI	No	Partial (for BusyBox, supported in command-line editing, but not in string handling[21])	N/A	Yes	Yes (arbitrary fds)	Yes (via variables and options)	Yes (/etc/profile, .profile)	Yes (Unix feature)	Yes	Yes
CCP	CP/M, MP/M	(CCP)	1975 (1973)	No	CP/M (no login), MP/M	CP/M, MP/M	Freeware (originally proprietary)	Yes (originally closed-source)	Text-based CLI	No	No	No	?	No	No	Yes (automatic via $$$.SUB)	Partial (only via external SUBMIT command to update $$$.SUB)	No	Yes
COMMAND.COM	DOS	COMMAND	1980	No (3rd party implementations, not bound to a specific DOS vendor or version, available)	DOS, Windows 95, 98, SE, ME	DOS, Windows 95, 98, SE, ME	vendor specific, f.e. MS-EULA,[22] or BSD/GPL (free clones)	No (except for OpenDOS, DR-DOS, PTS/DOS and FreeDOS)	Text-based CLI	No	No	No (except for DR-DOS)	Yes (via COMMAND con: or CTTY con:)	Yes (stdin, stdout)	Yes (via startup parameters and environment variables, DR-DOS also supports DIR /C /R user-default switch command)	Yes (automatic \AUTOEXEC.BAT for primary shell, or explicitly via /P, /P:filename.bat or /K startup options)	Yes (via CALL command or /C and /K startup options)	No	Yes
OS/2 CMD.EXE	OS/2	CMD	1987	No	OS/2	OS/2	IBM-EULA[23]	No	Text-based CLI	No	No	No	No	Yes (stdin, stdout, stderr)	?	Partial (only via /K startup option)	Yes (via CALL command or /C and /K startup options)	No	?
Windows CMD.EXE[24]	Win32	CMD	1993	No	Windows NT, 2000, XP, Server 2003, Vista	Windows NT, 2000, XP, Server 2003, Vista	MS-EULA[25]	No	Text-based CLI	No	Partial (CHCP 65001 for UTF-8, but program arguments are still encoded in local codepage)	No	No	Yes	Yes (via registry, startup parameters, and environment variables)	Yes (automatic via registry, or explicitly via /K startup option)	Yes (via CALL command or /C and /K startup options)	No	Yes
4DOS, NDOS	DOS, Windows 95, 98, SE, ME	4DOS, NDOS	1989 (1986)	No (not bound to a specific OS vendor or version)	Optional	Optional	MIT License, with restrictions	Yes	Text-based CLI with TUI extensions	Yes (popups, help system, %_MOUSE internal variable, INKEY /M command)	No	Yes	Yes (via CTTY con:, except for DRAWBOX, DRAWLINE, DRAWVLINE, LIST, SCREEN, SCRPUT, SELECT, VSCRPUT commands and file / directory coloring)	Yes (stdin, stdout, stderr, stdout+stderr)	Yes (via 4DOS.INI/NDOS.INI file, startup parameters, environment variables, SETDOS command)	Yes (automatic \AUTOEXEC.BAT for primary shell and 4START.BTM/4START.BAT as well as 4EXIT.BTM/4EXIT.BAT for any shell, or explicitly via /P, /P:dir\filename.ext or /K startup options)	Yes (via CALL command or /C and /K startup options)	Yes	Yes
4OS2	OS/2	4OS2	1992	No (not bound to specific OS/2 versions)	Optional	Optional	Freeware	Yes	Text-based CLI	No	No	No	No	Yes (stdin, stdout, stderr, stdout+stderr)	Yes (via 4OS2.INI file, startup parameters, environment variables, SETDOS command)	Yes (automatic via 4START.CMD/4START.BTM as well as 4EXIT.CMD/4EXIT.BTM files, or explicitly via /K startup.cmd option)	Yes (via CALL command or /C and /K startup options)	Yes	?
TCC (formerly 4NT)	Win32	TCC	1993	No (not bound to specific NT versions)	optional	optional	Shareware	No	Text-based CLI (Take Command: GUI)	Yes (console mouse, popups, help system, %_XMOUSE, %_YMOUSE internal variables, INKEY /M command)	Yes	Yes	No	Yes (stdin, stdout, stderr, stdout+stderr)	Yes (via registry, TCMD.INI/4NT.INI file, startup parameters, environment variables, SETDOS command)	Yes (automatic via registry and TCSTART/4START as well as TCEXIT/4EXIT, or explicitly via /K startup option)	Yes (via CALL command or /C and /K startup options)	Yes	No
VMS DCL[26]	OpenVMS	Automatically for login/interactive process	1977?	Yes	VMS	VMS	Proprietary, bundled in VMS	by special license only	Text-based CLI	with DECwindows/Motif	Yes	Yes, at least to 1988 standard	Yes	Yes (sys$input, sys$output assignment)	Yes (via symbols, logical names, and options)	Yes (SYS$MANAGER:SYLOGIN.COM and user defined LOGIN.COM)	Yes	Yes	No
PowerShell	.NET	PowerShell	2006	Yes	Windows Server 2008, 7, Vista, XP[27]	Windows Server 2008, 7	MIT-style	Yes	Graphical CLI	Yes	Yes	Yes	No	Yes	Yes (via variables and options)	Yes (%USERPROFILE%\Documents \WindowsPowerShell\Microsoft.PowerShell_profile.ps1)	Yes (PowerShell feature)	Yes	No
rc	Plan 9, POSIX	rc	1989	Yes	Plan 9, Version 10 Unix	Plan 9, Version 10 Unix	Lucent Public License	?	?	?	Yes	?	?	Yes	?	?	?	?	Yes
BeanShell	Java	?	2005	Yes	?	?	LGPL	?	?	?	Yes	?	?	Yes	?	?	?	?	No
fish	POSIX	fish	2005[28]	Yes	GhostBSD	?	GPL	Yes	Text-based CLI	?	Yes	?	?	Yes (arbitrary fds)	Yes (through environment variables and via web interface through fish_config)	Yes (/etc/fish/config.fish and ~/.config/fish/config.fish)	Yes (Unix feature)	Yes (~/.config/fish/fish_history*)	?
Shell	Usual environment	Usually invoked	Introduced	Platform-independent	Default login shell in	Default script shell in	License	Source code availability	User interface	Mouse support	Unicode support	ISO 8601 support	Console redirection	Stream redirection	Configurability	Startup/shutdown scripts	Batch scripts	Logging	available as statically linked, independent single file executable

Interactive features

Shell	Command name completion	Path completion	Command argument completion	Wildcard completion	Command history	Mandatory argument prompt	Automatic suggestions	Syntax highlighting	Directory history, stack or similar features	Implicit directory change	Auto­correction	Integrated environment	Snippets	Value prompt	Menu/options prompt	Progress indicator	Context sensitive help	Command builder
Thompson shell	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
Bourne shell 1977 version	No	No	No	No	No	No	No	No	No	No	No	No	No	Yes	No	No	No	No
Bourne shell current version	No	Yes[29]	No	No	Yes[29]	No	No	No	Yes(CDPATH, pushd, popd, dirs), CDPATH since SVr4	No	No	No	No	Yes	No	No	No	No
POSIX shell	No	No	No	No	Yes	No	No	No	Yes (CDPATH)	No	No	No	No	Yes	No	No	No	No
bash (v4.0)	Yes	Yes	when defined	Yes[30]	Yes	No	No	No	Yes (CDPATH, pushd, popd)	optional	No	No	No	Yes	Yes	No	No	No
csh	Yes	Yes	No	No	Yes	No	No	No	Yes (cdpath, pushd, popd)	optional	No	No	No	Yes	No	No	No	No
tcsh	Yes	Yes	when defined	No	Yes	No	No	No	Yes (cdpath, pushd, popd)	optional	Yes	No	No	Yes	No	No	No	No
Hamilton C shell	Yes	Yes	No	Yes	Yes	No	No	No	Yes (cdpath, pushd, popd)	No	No	No	No	Yes	No	No	No	No
Scsh	No	No	No	No	No	No	No	No	No	No	No	No	No	Yes	No	No	No	No
ksh (ksh93t+)	Yes (extendable)	Yes (extendable)	No	No	Yes	No	No	No	Yes (cdpath builtin, pushd, popd implemented as functions)	No	No	No	No	Yes	Yes	No	No	No
pdksh	Yes	Yes	No	No	Yes	No	No	No	No	No	No	No	No	Yes	Yes	No	No	No
zsh	Yes	Yes	when defined	Yes[31]	Yes	No	Yes (via predict-on or user-defined[32])	Third-party extension[33]	Yes	optional	Yes	No	when defined (as ZLE widgets)	Yes	Yes	No	Yes	No
ash	No	No	No	No	Yes	No	No	No	No	No	No	No	No	Yes	Yes	No	No	No
CCP	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
COMMAND.COM	No	No	No	No	No[34][35]	No	No	No	No	No	No	No	No	No	No (only via external CHOICE command, in DR-DOS also via SWITCH / DRSWITCH internal commands)	No	No	No
OS/2 CMD.EXE	Yes	Yes	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No	No	No
Windows CMD.EXE	partial	partial	No	No	Yes (F8)	No	No	No	Yes (PUSHD, POPD)	No	No	No	No	Yes (via SET /P command)	No	No	No	No
4DOS	Yes	Yes	Yes	Yes	Yes[36][37]	No	No	No	(via popup, extended directory searches, CDPATH, PUSHD, POPD, DIRHISTORY, DIRS, CDD, CD - commands and %@DIRSTACK[] function)	Yes	No	Yes	No	Yes (via INPUT, INKEY and ESET commands)	Yes (via @SELECT[] function, and indirectly via a combination of INKEY, INPUT, SWITCH commands)	No	Yes	No (except for OPTION command for INI file directives)
4OS2	?	?	?	?	Yes	No	No	No	Yes	Yes	No	?	No	?	?	No	Yes	No
TCC (formerly 4NT)	Yes	Yes	Yes	Yes	Yes	No	No	Yes	(via popup, extended directory searches, CDPATH, PUSHD, POPD, DIRHISTORY, DIRS, CDD, CD - commands and %@DIRSTACK[] function)	Yes	No	Yes	No	Yes (via INPUT, INKEY, ESET and SET /P commands)	Yes (via @SELECT[] function, and indirectly via a combination of INKEY, INPUT, SWITCH commands)[38]	No	Yes	No
PowerShell	Yes	Yes	Yes	Yes	Yes (F8)	Yes	Yes, in ISE[39]	Yes, in ISE[39] and PSReadLine[40] module(included in v5.0[41])	Yes; multiple stacks; multiple location types[42]	No	Yes, in PSReadLine[40] module	Yes, in ISE[39]	Yes, in ISE[39]	Yes	Yes[43]	Yes[44]	Yes, in ISE[39]	popup window[45]
rc	Yes[46]	Yes[46]	No	No	Yes[46]	No	No	No	No	No	No	No	No	?	No	No	No	No
BeanShell	Yes	Yes	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No	No
VMS DCL	Minimum uniqueness scheme	No	No	No	Yes	Yes	No	No	No	No	No	No	No	Yes	No	No	No	No
fish	Yes	Yes	when defined or parseable in man pages[47]	Yes[47]	Yes	No	Yes	Yes	Yes	Yes	Yes	Yes[48]	No	Yes	(via fish_config command)	No	No	No
Shell	Command name completion	Path completion	Command argument completion	Wildcard completion	Command history	Mandatory argument prompt	Automatic suggestions	Syntax highlighting	Directory history, stack or similar features	Implicit directory change	Auto­correction	Integrated environment	Snippets	Value prompt	Menu/options prompt	Progress indicator	Context sensitive help	Command builder

Completions

Completion features assist the user in typing commands at the command line, by looking for and suggesting matching words for incomplete ones. Completion is generally requested by pressing the completion key (often the Tab ↹ key).

Command name completion is the completion of the name of a command. In most shells, a command can be a program in the command path (usually $PATH), a builtin command, a function or alias.

Path completion is the completion of the path to a file, relative or absolute.

Wildcard completion is a generalization of path completion, where an expression matches any number of files, using any supported syntax for file matching.

Variable completion is the completion of the name of a variable name (environment variable or shell variable). Bash, zsh, and fish have completion for all variable names. PowerShell has completions for environment variable names, shell variable names and — from within user-defined functions — parameter names.

Command argument completion is the completion of a specific command's arguments. There are two types of arguments, named and positional: Named arguments, often called options, are identified by their name or letter preceding a value, whereas positional arguments consist only of the value. Some shells allow completion of argument names, but few support completing values.

Bash, zsh and fish offer parameter name completion through a definition external to the command, distributed in a separate completion definition file. For command parameter name/value completions, these shells assume path/filename completion if no completion is defined for the command. Completion can be set up to suggest completions by calling a shell function.^[49] The fish shell additionally supports parsing of man pages to extract parameter information that can be used to improve completions/suggestions. In PowerShell, all types of commands (cmdlets, functions, script files) inherently expose data about the names, types and valid value ranges/lists for each argument. This metadata is used by PowerShell to automatically support argument name and value completion for built-in commands/functions, user-defined commands/functions as well as for script files. Individual cmdlets can also define dynamic completion of argument values where the completion values are computed dynamically on the running system.

Command history

A user of a shell may find that he/she is typing something similar to what the user typed before. If the shell supports command history the user can call the previous command into the line editor and edit it before issuing it again.

Shells that support completion may also be able to directly complete the command from the command history given a partial/initial part of the previous command.

Most modern shells support command history. Shells which support command history in general also supports completion from history rather than just recalling commands from the history. In addition to the plain command text, PowerShell also records execution start- and end time and execution status in the command history.

Mandatory argument prompt

Mandatory arguments/parameters are arguments/parameters which must be assigned a value upon invocation the command, function or script file. A shell that can determine ahead of invocation that there are missing mandatory values, can assist the interactive user by prompting for those values instead of letting the command fail. Having the shell prompt for missing values will allow the author of a script, command or function to mark a parameter as mandatory instead of creating script code to either prompt for the missing values (after determining that it is being run interactively) or fail with a message.

PowerShell allows commands, functions and scripts to define arguments/parameters as mandatory. The shell determines prior to invocation if there is any mandatory arguments/parameters which have not been bound, and will then prompt the user for the value(s) before actual invocation. ^[50]

Automatic suggestions

With automatic suggestions the shell monitors while the interactive user is typing and displays context-relevant suggestions without interrupting the typing instead of the user explicitly requesting completion.

The PowerShell Integrated Scripting Environment use the discoverable metadata to provide "intellisense" - i.e. suggestions that automatically pops up as the user types, in addition to when the user explicitly requests completion lists by pressing e.g. Tab ↹

Directory history, stack or similar features

A shell may record the locations the user has used as current locations and allow fast switching to any location/directory in the history.

One of the uses of the zsh directory stack is to record a directory history. In particular, the AUTO_PUSHD option and advanced cd arguments and completion are used for this purpose.

PowerShell allows multiple named stacks to be used. Locations (directories) can be pushed onto/popped from the current stack or a named stack. Any stack can become the current (default) stack. Unlike most other shells, PowerShell's location concept allow location stacks to hold file system locations as well as other location types like e.g. Active Directory organizational units/groups, SQL Server databases/tables/objects, Internet Information Server applications/sites/virtual directories.

4DOS and Take Command Console record history of current directories and allows the user to switch to a directory in the history using a popup a window.

Implicit directory change

A directory name can be used directly as a command which implicitly changes the current location to the directory.

Autocorrection

When a command line does not match a command or arguments directly, spell checking can automatically correct common typing mistakes (such as case sensitivity, missing letters). There are two approaches to this; the shell can either suggest probable corrections upon command invocation, or this can happen earlier as part of a completion or autosuggestion.

The tcsh and zsh shells feature optional spell checking/correction, upon command invocation.

Fish does the autocorrection upon completion and autosuggestion. The feature is therefore not in the way when typing out the whole command and pressing enter, whereas extensive use of the tab and right-arrow keys makes the shell mostly case insensitive.

The PSReadLine^[40] PowerShell module(which is shipped with version 5.0) provides the option to specify a CommandValidationHandler ScriptBlock which runs before submitting the command. This allows for custom correcting of commonly mistyped commands, and verification before actually running the command.

Integrated environment

An integrated environment is the integration of the command-line interface with editors (typically multiple documents), help system and possibly debugging and other tools.

Take Command Console (TCC) comes with an integrated environment with command line pane, file explorer, editor, batch debugger and more.^[51]

PowerShell ISE includes a command line pane with support for integrated command line, copy-paste, multiple document editors, source-level debugging, help pane, command explorer pane and scripting interface allowing scripts/modules to manipulate menus, add-ons etc. The ISE (menus, windows, shortcuts, addons) are customizable through scripts.^[52]

Snippets

Snippets are small regions of reusable script code. Snippets are often used to save keystrokes, or to assist the user with common scripting patterns. Many text editors support snippets for scripting either as built-in functionality or through plugins.

PowerShell supports snippets in the Integrated Scripting Environment (ISE) using Ctrl+J.^[53]

Value prompt

A shell script can prompt the interactive user for a value.

Menu/options selector

A shell script can present the interactive user with a list of choices.

Progress indicator

A shell script (or job) can report progress of long running tasks to the interactive user.

Unix/Linux systems may offer other tools support using progress indicators from scripts or as standalone-commands, such as the program "pv".^[54] These are not integrated features of the shells, however.

PowerShell has a built-in command and API functions (to be used when authoring commands) for writing/updating a progress bar. Progress bar messages are sent separates from regular command output and the progress bar is always displayed at the ultimate interactive users console regardless of whether the progress messages originates from an interactive script, from a background job or from a remote session.

Interactive table

Output from a command execution can be displayed in a table/grid which can be interactively sorted and filtered and/or otherwise manipulated after command execution ends.

PowerShell Out-GridView cmdlet displays data in an interactive window with interactive sorting and filtering.

Syntax highlighting

An independent project offers syntax highlighting as an add-on to the Z Shell (zsh).^[55] This is not part of the shell, however.

PowerShell provides customizable syntax highlighting on the command line through the PSReadLine^[40] module. This module can be used with PowerShell v3.0+, and it's included with v5.0. Additionally, it is loaded by default in the command line host "powershell.exe" in v5.0. The PowerShell ISE also includes syntax highlighting on the command line as well as in the script pane.^[52] Take Command Console (TCC) offers syntax highlighting in the integrated environment.

Context sensitive help

4DOS, 4OS2, 4NT / Take Command Console and PowerShell (in PowerShell ISE) looks up context-sensitive help information when F1 is pressed.

Zsh provides various forms of configurable context-sensitive help as part of its run-help widget, _complete_help command, or in the completion of options for some commands.

Command builder

A command builder is a guided dialog which assists the user in filling in a command. PowerShell has a command builder which is available in PowerShell ISE or which can be displayed separately through the Show-Command cmdlet.^[56]

Programming features

Shell	Functions	Exception handling	Search & replace on variable substi­tutions	Arith­metic	Floating point	Math function library	Linear arrays or lists	Assoc­iative arrays	Lambda functions	eval func­tion	Pseudo­random number generation	Bytecode
Bourne shell 1977 version	No	Yes (via trap)	No	No	No	No	No	No	No	Yes	No	No
Bourne shell current version	Yes since SVR2	Yes (via trap)	No	Yes[29]	No	No	No	No	No	Yes	No	No
POSIX shell	Yes	Yes (via trap)	No	Yes	No	No	No	No	No	Yes	No	No
bash (v4.0)	Yes	Yes (via trap)	Yes (via ${//} syntax)	Yes	No	No	Yes	Yes	No	Yes	Yes ($RANDOM)	No
csh	No	No	Yes (via $var:s/// syntax)	Yes	No	No	Yes	No	No	Yes	No	No
tcsh	No	No	Yes (via $var:s/// syntax)	Yes	No	No	Yes	No	No	Yes	No	No
Hamilton C shell	Yes	No	Yes (via $var:s/// syntax)	Yes	Yes	Yes	Yes	No	No	Yes	Yes (random utility)	No
Scsh	Yes	?	Yes (via string functions and regular expressions)	?	?	?	Yes	?	Yes	Yes	Yes (random-integer, random-real)	Yes (compiler is Scheme48 virtual machine, via scshvm)
ksh (ksh93t+)	Yes	Yes (via trap)	Yes (via ${//} syntax and builtin commands)	Yes	Yes	No	Yes	Yes	No	Yes	Yes ($RANDOM)	Yes (compiler is called shcomp)
pdksh	Yes	Yes (via trap)	No	Yes	No	No	Yes	No	No	Yes	Yes ($RANDOM)	No
zsh	Yes	Yes	Yes (via ${:s//} and ${//} syntax)	Yes	Yes	Yes (zsh/mathfunc module)	Yes	Yes	No	Yes	Yes ($RANDOM)	Yes (built-in zcompile command)
ash	Yes	Yes (via trap)	No	Yes (since 1992)[57]	No	No	No	No	No	Yes	No	No
CCP	No	?	No	No	?	?	No	No	No	No	No	No
COMMAND.COM	No	Partial (only Auto-fail (via COMMAND /F (or /N in some versions of DR-DOS))	No	No	No	No	No	No	No	No	No	No
OS/2 CMD.EXE	No	No	No	?	No	No	?	No	No	No	No	No
Windows CMD.EXE	Yes (via CALL :label)	No	Yes (via SET %varname:expression syntax)	Yes (via SET /A)[58]	No	No	Yes (via SET[59])	No	No	No	Yes (%random%)	No
4DOS	Yes	Yes (via ON command, optional Auto-fail via 4DOS /F)	Yes (via %@Replace[…] function)	Yes (via SET /A)	?	?	Yes (via ranges, include lists, @file lists and FOR command)	No	No	Yes	Yes (%@Random[…] function)	Yes (via BATCOMP command)
4OS2	?	?	?	?	?	?	?	?	No	Yes	Yes (%@Random[…] function)	?
TCC (formerly 4NT)	Yes	Yes (via ON and various …MONITOR commands)	Yes (via %@Replace[…] function)	Yes (via SET /A)	?	?	Yes (via ranges, include lists, @file lists and FOR command)	?	No	Yes	Yes (%@Random[…] function)	Yes (via BATCOMP command)
PowerShell	Yes	Yes	Yes (-replace operator)	Yes	Yes	[Math] class[60]	Yes	Yes	Yes	Yes	Yes	Yes, automatic
rc	Yes	Yes	No	?	?	?	?	?	No	Yes	No	No
BeanShell	Yes	Yes	?	Yes	?	?	Yes	Yes	No	Yes	Yes	Yes
VMS DCL	Yes	Yes	No	Yes	No	yes, for compiled programs	Yes	No	No	No	No	No
fish	Yes	Yes (via trap)	No	Optional fish math	Optional fish math	Optional fish math	Yes	No	No	Yes	Yes (random)	No

Notes

^fish math :The bundled math function requires bc.

String processing and filename matching

Shell	String processing	Alternation (Brace expansion)	Pattern matching (regular expressions built-in)	Pattern matching (filename globbing)	Globbing qualifiers (filename generation based on file attributes)	Recursive globbing (generating files from any level of subdirectories)
Bourne shell 1977 version	?	No	No	Yes (*, ?, […])	No	No
Bourne shell recent version	Partial (prefix and suffix stripping in variable expansion)	No	No	Yes (*, ?, […])	No	No
POSIX shell	Partial (prefix and suffix stripping in variable expansion)	No	No	Yes (*, ?, […])	No	No
bash (v4.0)	Partial (prefix and suffix stripping in variable expansion)	Yes	Yes	Yes (*, ?, […], {…})	No	Yes (**/...)
csh	Yes (:s and other editing operators)	Yes	No	Yes	No	No
tcsh	Yes (:s and other editing operators)	Yes	Yes	Yes	No	No
Hamilton C shell	Yes (:s and other editing operators + substr, strlen, strindex, printf, reverse, upper, lower, concat and other builtin functions)	Yes	No	Yes	No	Yes (via indefinite directory "..." wildcard[61])
Scsh	?	?	Yes	Yes	No	No
ksh (ksh93t+)	Partial (prefix, suffix stripping and string replacement in variable expansion)	Yes[62]	Yes	Yes (*, ?, […])	No	Yes (with set -G, no following of symlinks)
pdksh	?	Yes[62]	No	Yes	No	No
zsh	Yes (through variable processing: e.g. substring extraction, various transformations via parameter expansion)	Yes	Yes	Yes (*, ?, […], extended globbing[63])	Yes	Yes (**/... or ***/... to follow symlinks)
ash	?	?	No	Yes	No	No
CCP	No	No	No	No	No	No
COMMAND.COM	No	No	No	Yes (*, ?)	No	No
OS/2 CMD.EXE	No	No	No	Yes (*, ?)	Partial (only in DIR /A:… command)	No
Windows CMD.EXE	Partial (only through FOR /F and SET /A)	No	No[64]	Yes (*, ?)	Partial (only in DIR /A:… command)	Yes (via FOR /R command, or, where available, indirectly via /S subdir option)
4DOS	Yes (through variable functions %@…[], extended environment variable processing, various string commands and FOR /F and SET /A)	No	No	Yes (*, ?, […], extended wildcards, SELECT popup command)	Yes (via /A:… attribute and /I"…" description options and /[S…] size, /[T…] time, /[D…] date, and /[!…] file exclusion ranges)	Yes (via FOR /R command, or indirectly via GLOBAL command or, where available, /S subdir option)
4OS2	?	No	No	?	?	?
TCC (formerly 4NT)	Yes (through variable functions %@…[], extended environment variable processing, various string commands and FOR /F and SET /A)	No	Yes	Yes (*, ?, […], extended wildcards, SELECT popup command)	Yes (via /A:… attribute and /I"…" description options and /[S…] size, /[T…] time, /[D…] date, /[O…] owner, and /[!…] file exclusion ranges)	Yes (via FOR /R command, or indirectly via GLOBAL command or, where available, /S subdir option)
PowerShell	Yes (Concat/Substring/Insert/Remove/Replace, ToLower/ToUpper, Trim/TrimStart/TrimEnd, Compare, Contains/StartsWith/EndWith, Format, IndexOf/LastIndexOf, Pad/PadLeft/PadRight, Split/Join, regular expression functions and other .NET string functions)	No	Yes (full regex support)[65]	Yes (*, ?, […])	?	?
rc	?	?	No	Yes	No	No
BeanShell	?	?	Yes	?	?	?
VMS DCL	Yes	No	No	Yes	No	Yes (via [SUBDIR…])
fish	No	Yes	No	Yes (*, ?, {…})	No	Yes (**/...)

Inter-process communication

Shell	Pipes	Command substitution	Process substitution	Subshells	TCP/UDP connections as streams	Keystroke stacking
Bourne shell	bytes concurrent	Yes	No	Yes	No	N/A[66]
POSIX shell	bytes concurrent	Yes	No	Yes	No	N/A[66]
bash (v4.0)	bytes concurrent	Yes	Yes (if system supports /dev/fd/<n> or named pipes)	Yes	Yes (client only)	N/A[66]
csh	bytes concurrent	Yes	No	Yes	No	N/A[66]
tcsh	bytes concurrent	Yes	No	Yes	No	N/A[66]
Hamilton C shell	bytes concurrent	Yes	No	Yes	No	?
Scsh	text	?	?	?	Yes	N/A[66]
ksh (ksh93t+)	bytes (may contain serialized objects if print -C is used) concurrent	Yes ($(…) and ${<space>…;})	Yes (if system supports /dev/fd/<n>)	Yes	Yes (and SCTP support, client only)	N/A[66]
pdksh	bytes concurrent	Yes	No	Yes	No	N/A[66]
zsh	bytes concurrent	Yes	Yes	Yes	Yes (client and server, but only TCP)	N/A[66]
ash	bytes concurrent	Yes	No	Yes	No	N/A[66]
CCP	No	No	No	No	No	No
COMMAND.COM	text sequential temporary files	No	No	Partial (only under DR-DOS multitasker via COMMAND.COM /T)	No	No
OS/2 CMD.EXE	text concurrent	No	No	?	No	No
Windows CMD.EXE	text concurrent	Yes (via FOR /F command)	No	Yes (Backtick: ` in FOR /F usebackq)	No	No
4DOS	text sequential temporary files	Yes (via FOR /F command)	?	Partial (via %@EXECSTR[] and %@EXEC[], or via SET /M, ESET /M and UNSET /M and %@MASTER[…])	No	Yes (via KEYSTACK and KSTACK)[67]
4OS2	text concurrent	?	?	?	No	Yes (via KEYSTACK)
TCC (formerly 4NT)	text concurrent	Yes (via FOR /F command)	?	Partial (via %@EXECSTR[] and %@EXEC[])	Yes (via FTP, TFTP, FTPS, SFTP, HTTP, HTTPS and IFTP, client only)	Yes (via KEYSTACK)
PowerShell	objects concurrent	Yes	No	Yes	Yes	?
rc	text concurrent	Yes	Yes (via: <{cmd} if system supports /dev/fd/<n>)	Yes	No	?
BeanShell	not supported	?	?	?	Yes	?
VMS DCL	text (via PIPE command)	Yes	No	Yes (spawn)	Yes (server TCP only)	No
fish	bytes concurrent	Yes (...)	No (broken)[68]	No	No	N/A[66]

Keystroke stacking

In anticipation of what a given running application may accept as keyboard input, the user of the shell instructs the shell to generate a sequence of simulated keystrokes, which the application will interpret as an keyboard input from an interactive user. By sending keystroke sequences the user may be able to direct the application to perform actions that would be impossible to achieve through input redirection or would otherwise require an interactive user. For example, if an application acts on keystrokes, which cannot be redirected, distinguishes between normal and extended keys, flushes the queue before accepting new input on startup or under certain conditions, or because it does not read through standard input at all. Keystroke stacking typically also provides means to control the timing of simulated keys being sent or to delay new keys until the queue was flushed etc. It also allows to simulate keys which are not present on a keyboard (because the corresponding keys do not physically exist or because a different keyboard layout is being used) and therefore would be impossible to type by a user.

Security features

Shell	Secure (password) prompt	Encrypted variables/ parameters	File/directory passwords	Execute permission	Untrusted script blocking	Restricted shell subset	Safe data subset
Bourne shell	via stty[69]	No	?	N/A[70]	No	Yes	No
POSIX shell	via stty[69]	No	?	N/A[70]	No	No	No
bash (v4.0)	read -s	No	?	N/A[70]	No	Yes	No
csh	via stty[69]	No	?	N/A[70]	No	Yes	No
tcsh	via stty[69]	No	?	N/A[70]	No	Yes	No
Hamilton C shell	No	No	No	No	No	No	No
Scsh	via stty[69]	No	?	N/A[70]	No	No	No
ksh (ksh93t+)	via stty[69]	No	?	N/A[70]	No	Yes	No
pdksh	via stty[69]	No	?	N/A[70]	No	Yes	No
zsh	read -s	No	?	N/A[70]	No	Yes	No
ash	via stty[69]	No	?	N/A[70]	No	Yes	No
CCP	No	No	No	No	No	No	No
COMMAND.COM	Partial (only under DR-DOS, prompts for password if file/directory is protected)	No	Partial (only under DR-DOS via \dirname;dirpwd\filename;filepwd syntax)[71]	Partial (only under DR-DOS, if files are password-protected for read and/or execute permission)[72]	No	No	No
OS/2 CMD.EXE	No	No	No	No	No	No	No
Windows CMD.EXE	No	No	No	No	No	No	No
4DOS	Yes (via INPUT /P or INKEY /P)[73]	No	Partial (only under DR-DOS via \dirname;;dirpwd\filename;;filepwd syntax)[71]	Partial (only under DR-DOS, if files are password-protected for read and/or execute permission)[72]	No	No	No
4OS2	?	No	No	No	No	No	No
TCC (formerly 4NT)	Yes (via INPUT /P, INKEY /P or QUERYBOX /P)[73]	No	No	No	No	No	No
PowerShell	Yes[74]	Yes	No	No[75]	Yes[76]	Yes[77]	Yes[78]
rc	via stty[69]	No	?	N/A[70]	No	Yes[79]	No
BeanShell	?	?	?	?	?	?	?
VMS DCL	Yes	No	No	Yes	No	Yes	No
fish	read -s	No	?	N/A[70]	No	Yes (via fish -l)	?

Secure prompt

Some shell scripts need to query the user for sensitive information such as passwords, private digital keys, PIN codes or other confidential information. Sensitive input should not be echoed back to the screen/input device where it could be gleaned by unauthorized persons. Plaintext memory representation of sensitive information should also be avoided as it could allow the information to be compromised, e.g., through swap files, core dumps etc.^[80]

The shells bash, zsh and PowerShell offer this as a specific feature.^[81][82] Shells which do not offer this as a specific feature may still be able to turn off echoing through some other means. Shells executing on a Unix/Linux operating system can use the stty external command to switch off/on echoing of input characters.^[83] In addition to not echoing back the characters, PowerShell's -AsSecureString option also encrypts the input character-by-character during the input process, ensuring that the string is never represented unencrypted in memory where it could be compromised through memory dumps, scanning, transcription etc.

Encrypted variables/parameters

If a script reads a password into an environment variable it is in memory in plain text, and thus may be accessed via a core dump. It is also in the process environment, which may be accessible by other processes started by the script.^[84]

PowerShell can work with encrypted string variables/parameters.^[85] Encrypted variables ensure that values are not inadvertently disclosed through e.g. transcripts, echo'ing, logfiles, memory or crash dumps or even malicious memory scanning. PowerShell also supports saving of such encrypted strings in text files, protected by a key owned by the current user.

Execute permission

Some operating systems define an execute permission which can be granted to users/groups for a file.

On Unix systems, the execute permission controls access to invoking the file as a program, and applies both to executables and scripts. As the permission is enforced in the program loader, no obligation is needed from the invoking program, nor the invoked program, in enforcing the execute permission — this also goes for shells and other interpreter programs. The behaviour is mandated by the POSIX C library that is used for interfacing with the kernel: POSIX specifies that the exec family of functions shall fail with EACCESS (permission denied) if the file denies execution permission (see execve – System Interfaces Reference, The Single UNIX Specification, Issue 7 from The Open Group).

The execute permission only applies when the script is run directly. If a script is invoked as an argument to the interpreting shell, it will be executed regardless of whether the user holds the execute permission for that script.

Although Windows also specifies an execute permission, none of the Windows specific shells block script execution if the permission has not been granted.

Untrusted script blocking

Some shells will block scripts determined to be untrustworthy, or refuse to run scripts if mandated by a system administrator.

Script origin execution restriction

PowerShell can be set to block execution of scripts which has been marked as obtained from an unknown/untrusted origin (e.g. the Internet).^[86] Internet facing applications such as web browsers, IM clients, mail readers etc. mark files downloaded from the internet with the origin zone in an alternate data stream which is understood by PowerShell.

Signed script restriction

Script/code signing policies can be used to ensure that an operations department only run approved scripts/code which have been reviewed and signed by a trusted reviewer/approver. Signing regimes also protects against tampering. If a script is sent from vendor to a client, the client can use signing to ensure that the script has not been tampered with during transit and that the script indeed originates from the vendor and not an attacker trying to social engineer an operator into running an attack script.

PowerShell can be set to allow execution of otherwise blocked scripts (e.g. originating from an untrusted zone) if the script has been digitally signed using a trusted digital certificate.^[87][88][89]

Multilevel execution policies

A company may want to enforce execution restriction globally within the company and/or certain parts of the company. It may want to set a policy for running signed scripts but allow certain parts of the company to set their own policies for zoned restrictions.

PowerShell allows script blocking policies to be enforced at multiple levels: Local machine, current user etc. A higher level policy overrides a lower level policy, e.g. if a policy is defined for the local machine it is in place for all users of the local machine, only if it is left undefined at the higher level can it be defined for the lower levels.

Restricted shell subset

Several shells can be started or be configured to start in a mode where only a limited set of commands and actions is available to the user. While not a security boundary (the command accessing a resource is blocked rather than the resource) this is nevertheless typically used to restrict users' actions before logging in.

A restricted mode is part of the POSIX specification for shells, and most of the Linux/Unix shells support such a mode where several of the built-in commands are disabled and only external commands from a certain directory can be invoked.^[90][91]

PowerShell supports restricted modes through session configuration files or session configurations. A session configuration file can define visible (available) cmdlets, aliases, functions, path providers and more.^[92]

Safe data subset

Scripts that invoke other scripts can be a security risk as they can potentially execute foreign code in the context of the user who launched the initial script. Scripts will usually be designed to exclusively include scripts from known safe locations; but in some instances, e.g. when offering the user a way to configure the environment or loading localized messages, the script may need to include other scripts/files.^[93] One way to address this risk is for the shell to offer a safe subset of commands which can be executed by an included script.

PowerShell data sections can contain constants and expressions using a restricted subset of operators and commands.^[94] PowerShell data sections are used when e.g. localized strings needs to be read from an external source while protecting against unwanted side effects.

References

External links

• Linux Magazine: Bash vs. Vista PowerShell
• IEEE. 1003.1 Standard for Information Technology – Portable Operating System Interface (POSIX): Shell and Utilities, Issue 7