A command shell is a command-line interface to interact with and manipulate a computer's operating system.
| 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 | UNIX | UNIX | Text-based CLI | ||||||||||||||
| Bourne shell 1977 version | 7th Ed. UNIX | sh | 1977 | 7th Ed. UNIX | 7th Ed. UNIX, | Text-based CLI | ||||||||||||||
| Bourne shell current version | Various UNIX | sh | 1977 | SunOS-5.x, FreeBSD | SunOS-5.x | Text-based CLI | [1] | [2] | ||||||||||||
| POSIX shell[3] | POSIX | sh | 1992[4] | POSIX | Text-based CLI | Unspecified (given as an example) | ||||||||||||||
| bash (v4) | POSIX | bash, sh | 1989[5] | GNU, Linux (default for root), macOS 10.3–10.14 | GNU, Linux, Haiku, macOS 10.3–10.14 | Text-based CLI | [6] | |||||||||||||
| csh | POSIX | csh | 1978 | SunOS | Text-based CLI | |||||||||||||||
| tcsh | POSIX | tcsh, csh | 1983[7] | FreeBSD (former default for root),[8] formerly Mac OS X | Text-based CLI | |||||||||||||||
| Hamilton C shell | Win32, OS/2 | csh | 1988[9] | (OS/2 version no longer maintained) | Text-based CLI | |||||||||||||||
| Scsh | POSIX | scsh | 1994 | |||||||||||||||||
| ksh (ksh93t+) | POSIX | ksh | 1983[10] [11] | AIX, HP-UX | OpenSolaris | Text-based CLI | ||||||||||||||
| pdksh | POSIX | ksh, sh | 1989? | OpenBSD[12] | OpenBSD | Text-based CLI | ||||||||||||||
| zsh | POSIX | zsh | 1990 | Deepin, GoboLinux, Grml, macOS 10.15+, Kali 2020.4+ | Grml, macOS 10.15+ | Text-based CLI | ||||||||||||||
| ash | POSIX | sh | 1989 | Minix, BusyBox based systems | NetBSD, Minix, BusyBox based systems | Text-based CLI | (for BusyBox, supported in command-line editing, but not in string handling[13]) | |||||||||||||
| CCP | CP/M, MP/M | (CCP) | 1976 (1974) | CP/M (no login), MP/M | CP/M, MP/M | (originally closed-source) | Text-based CLI | (automatic via) | (only via external command to update) | |||||||||||
| COMMAND.COM | DOS | COMMAND | 1980 | (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,[14] or BSD/GPL (free clones) | (except for OpenDOS, DR-DOS, PTS/DOS and FreeDOS) | Text-based CLI | (except for DR-DOS) | (via or) | (via startup parameters and environment variables, DR-DOS also supports user-default switch command) | (automatic for primary shell, or explicitly via, or startup options) | (via command or and startup options) | ||||||
| OS/2 CMD.EXE | OS/2, eComStation, ArcaOS | CMD | 1987 | OS/2, eComStation, ArcaOS | OS/2, eComStation, ArcaOS | [15] | Text-based CLI | (only via startup option) | (via command or and startup options) | |||||||||||
| Windows CMD.EXE[16] | Win32 | CMD | 1993 | Windows NT, 2000, XP, Server 2003, Vista | Windows NT, 2000, XP, Server 2003, Vista | [17] | Text-based CLI | (for UTF-8, but program arguments are still encoded in local codepage) | (via registry, startup parameters, and environment variables) | (automatic via registry, or explicitly via startup option) | (via command or and startup options) | |||||||||
| 4DOS, NDOS | DOS, Windows 95, 98, SE, ME | 4DOS, NDOS | 1989 (1986) | Text-based CLI with TUI extensions | (popups, help system, internal variable, command) | (via, except for,,,,,,, commands and file / directory coloring) | (via / file, startup parameters, environment variables, command) | (automatic for primary shell and / as well as / for any shell, or explicitly via, or startup options) | (via command or and startup options) | |||||||||||
| 4OS2 | OS/2, eComStation, ArcaOS | 4OS2 | 1992 | (but bundled with ArcaOS) | Text-based CLI | (via file, startup parameters, environment variables, command) | (automatic via / as well as / files, or explicitly via option) | (via command or and startup options) | ||||||||||||
| TCC (formerly 4NT) | Win32 | TCC | 1993 | optional | optional | Text-based CLI (Take Command: GUI) | (console mouse, popups, help system,, internal variables, command) | (via registry, / file, startup parameters, environment variables, command) | (automatic via registry and / as well as /, or explicitly via startup option) | (via command or and startup options) | ||||||||||
| VMS DCL[18] | OpenVMS | Automatically for login/interactive process | 1977? | VMS | VMS | Proprietary, bundled in VMS | Text-based CLI | |||||||||||||
| PowerShell | .NET, .NET Framework | PowerShell | 2006 | Windows 10, 8, Server 2008, 7[19] | Windows 10, 8, Server 2008, 7 | Graphical CLI | ||||||||||||||
| rc | Plan 9, POSIX | rc | 1989 | Plan 9, Version 10 Unix | Plan 9, Version 10 Unix | Text-based CLI | ||||||||||||||
| BeanShell | Java | 2005 | ||||||||||||||||||
| fish | POSIX | fish | 2005[20] | GhostBSD | Text-based CLI | |||||||||||||||
| Ion | Redox, Linux | ion | 2015[21] | Redox | Redox | Text-based CLI | (not distributed as a standalone executable, but it can be built as one) | |||||||||||||
| 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 |
| Shell | Command name completion | Path completion | Command argument completion | Wildcard completion | Command history | Mandatory argument prompt | Automatic suggestions | Colored directory listings | Text highlighting | 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 | ? | ? | |||||||||||||||||||
| Bourne shell 1977 version | ? | ? | |||||||||||||||||||
| Bourne shell current version | [22] | (CDPATH, pushd, popd, dirs), CDPATH since SVr4 | |||||||||||||||||||
| POSIX shell | |||||||||||||||||||||
| bash (v4.0) | [23] | ||||||||||||||||||||
| csh | |||||||||||||||||||||
| tcsh | |||||||||||||||||||||
| Hamilton C shell | |||||||||||||||||||||
| Scsh | ? | ? | |||||||||||||||||||
| ksh (ksh93t+) | |||||||||||||||||||||
| pdksh | |||||||||||||||||||||
| zsh | [24] | [25] | [26] | ||||||||||||||||||
| ash | |||||||||||||||||||||
| CCP | |||||||||||||||||||||
| COMMAND.COM | [27] [28] | (only in DR-DOS through %$ON%, %$OFF%, %$HEADER%, %$FOOTER%) | (only single-stepping with COMMAND /Y) | (only via external command, in DR-DOS also via / internal commands) | |||||||||||||||||
| OS/2 CMD.EXE | |||||||||||||||||||||
| Windows CMD.EXE | (via command) | ||||||||||||||||||||
| 4DOS | [29] [30] | (via, and commands) | (via function, and indirectly via a combination of,, commands) | (except for command for INI file directives) | |||||||||||||||||
| 4OS2 | ? | ? | ? | ? | ? | ? | ? | ||||||||||||||
| TCC (formerly 4NT) | (via,, and commands) | (via function, and indirectly via a combination of,, commands)[31] | |||||||||||||||||||
| PowerShell | [32] | [33] | Yes, in PSReadLine module | Yes, in ISE[34] | Yes, in ISE | [35] | [36] | , in ISE | [37] | , in ISE | |||||||||||
| rc | [38] | ? | ? | ||||||||||||||||||
| BeanShell | ? | ? | |||||||||||||||||||
| VMS DCL | ? | ? | |||||||||||||||||||
| fish | [39] | [40] | |||||||||||||||||||
| Shell | Command name completion | Path completion | Command argument completion | Wildcard completion | Command history | Mandatory argument prompt | Automatic suggestions | Colored directory listings | Text highlighting | 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 |
Background execution allows a shell to run a command without user interaction in the terminal, freeing the command line for additional work with the shell. POSIX shells and other Unix shells allow background execution by using the & character at the end of command. In PowerShell, the Start-Process[41] or Start-Job[42] cmdlets can be used.
See main article: article and Command-line completion.
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 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.[43] 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.
See main article: article and Command history. Users of a shell may find themselves typing something similar to what they have typed before. Support for command history means that a user can recall a previous command into the command-line editor and edit it before issuing the potentially modified command.
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 support 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 arguments/parameters are arguments/parameters which must be assigned a value upon invocation of 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 anymandatory arguments/parameters which have not been bound, and will then prompt the user for the value(s) before actual invocation.[44]
See main article: article and Autocomplete. Shells featuring automatic suggestions display optional command-line completions as the user types. The PowerShell and fish shells natively support this feature; pressing the key inserts the completion.
Implementations of this feature can differ between shells; for example, PowerShell[45] and zsh[46] use an external module to provide completions, and fish derives its completions from the user's command history.[47]
Shells may record a history of directories the user has been in and allow for fast switching to any recorded location. This is referred to as a "directory stack". The concept had been realized as early as 1978[48] in the release of the C shell (csh).
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.
Command line interpreters 4DOS and its graphical successor Take Command Console also feature a directory stack.
A directory name can be used directly as a command which implicitly changes the current location to the directory.
This must be distinguished from an unrelated load drive feature supported by Concurrent DOS, Multiuser DOS, System Manager and REAL/32, where the drive letter L: will be implicitly updated to point to the load path of a loaded application, thereby allowing applications to refer to files residing in their load directory under a standardized drive letter instead of under an absolute path.[49]
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 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.
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".[50] 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 outputand 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.
JP Software command-line processors provide user-configurable colorization of file and directory names in directory listings based on their file extension and/or attributes through an optionally defined environment variable.
For the Unix/Linux shells, this is a feature of the command and the terminal.
The command line processors in DOS Plus, Multiuser DOS, REAL/32 and in all versions of DR-DOS support a number of optional environment variables to define escape sequences allowing to control text highlighting, reversion or colorization for display or print purposes in commands like TYPE. All mentioned command line processors support [[%$ON%]] and [[%$OFF%]]. If defined, these sequences will be emitted before and after filenames. A typical sequence for would be in conjunction with ANSI.SYS, for an ASCII terminal or for an IBM or ESC/P printer. Likewise, typical sequences for would be,,, respectively. The variables [[%$HEADER%]] and [[%$FOOTER%]] are only supported by COMMAND.COM in DR-DOS 7.02 and higher to define sequences emitted before and after text blocks in order to control text highlighting, pagination or other formatting options.
For the Unix/Linux shells, this is a feature of the terminal.
See main article: article and Syntax highlighting. A defining feature of the fish shell is built-in syntax highlighting, As the user types, text is colored to represent whether the input is a valid command or not (the executable exists and the user has permissions to run it), and valid file paths are underlined.[51]
An independent project offers syntax highlighting as an add-on to the Z Shell (zsh).[52] This is not part of the shell, however.
PowerShell provides customizable syntax highlighting on the command line through the PSReadLine module. This module can be used with PowerShell v3.0+, and is bundled with v5.0 onwards. It is loaded by default in the command line host "powershell.exe" since v5.0.[53]
Take Command Console (TCC) offers syntax highlighting in the integrated environment.
See main article: article and Context-sensitive help.
4DOS, 4OS2, 4NT / Take Command Console and PowerShell (in PowerShell ISE) looks up context-sensitive help information when is pressed.
Zsh provides various forms of configurable context-sensitive help as part of its widget, command, or in the completion of options for some commands.
The fish shell provides brief descriptions of a command's flags during tab completion.
| 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 | |||||||||||||
| Bourne shell current version | |||||||||||||
| POSIX shell | |||||||||||||
| bash (v4.0) | |||||||||||||
| csh | |||||||||||||
| tcsh | |||||||||||||
| Hamilton C shell | |||||||||||||
| Scsh | ? | ? | ? | ? | ? | ||||||||
| ksh (ksh93t+) | |||||||||||||
| pdksh | |||||||||||||
| zsh | |||||||||||||
| ash | (via) | ||||||||||||
| CCP | ? | ? | ? | ||||||||||
| COMMAND.COM | (only Auto-fail (via (or in some versions of DR-DOS)) | ||||||||||||
| OS/2 CMD.EXE | ? | ? | |||||||||||
| Windows CMD.EXE | |||||||||||||
| 4DOS | ? | ? | |||||||||||
| 4OS2 | ? | ? | ? | ? | ? | ? | ? | ? | (function) | ? | |||
| TCC (formerly 4NT) | ? | ? | ? | (function) | (via command) | ||||||||
| PowerShell | (Try-Catch-Finally) | [54] | [55] | ||||||||||
| rc | ? | ? | ? | ||||||||||
| BeanShell | ? | ? | ? | ||||||||||
| VMS DCL | |||||||||||||
| fish |
| 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 | ? | ||||||
| Bourne shell recent version | (prefix and suffix stripping in variable expansion) | ||||||
| POSIX shell | (prefix and suffix stripping in variable expansion) | ||||||
| bash (v4.0) | (prefix and suffix stripping in variable expansion) | ||||||
| csh | |||||||
| tcsh | |||||||
| Hamilton C shell | |||||||
| Scsh | ? | ? | |||||
| ksh (ksh93t+) | (prefix, suffix stripping and string replacement in variable expansion) | [56] | (with, no following of symlinks) | ||||
| pdksh | ? | ||||||
| zsh | (through variable processing: e.g. substring extraction, various transformations via parameter expansion) | (or to follow symlinks) | |||||
| ash | ? | ? | |||||
| CCP | |||||||
| COMMAND.COM | |||||||
| OS/2 CMD.EXE | (only in command) | ||||||
| Windows CMD.EXE | (only through and) | (only in command) | (via command, or, where available, indirectly via subdir option) | ||||
| 4DOS | (through variable functions, extended environment variable processing, various string commands and and) | (via attribute and description options and size, time, date, and file exclusion ranges) | (via command, or indirectly via command or, where available, subdir option) | ||||
| 4OS2 | ? | ? | ? | ? | |||
| TCC (formerly 4NT) | (through variable functions, extended environment variable processing, various string commands and and) | (via attribute and description options and size, time, date, owner, and file exclusion ranges) | (via command, or indirectly via command or, where available, subdir option) | ||||
| PowerShell | ? | ? | |||||
| rc | ? | ? | |||||
| BeanShell | ? | ? | ? | ? | ? | ||
| VMS DCL | (via) | ||||||
| fish |
| Shell | Pipes | Command substitution | Process substitution | Subshells | TCP/UDP connections as streams | Keystroke stacking | |
|---|---|---|---|---|---|---|---|
| Bourne shell | [57] | ||||||
| POSIX shell | |||||||
| bash (v4.0) | |||||||
| csh | |||||||
| tcsh | |||||||
| Hamilton C shell | ? | ||||||
| Scsh | ? | ? | ? | ||||
| ksh (ksh93t+) | |||||||
| pdksh | |||||||
| zsh | |||||||
| ash | |||||||
| CCP | |||||||
| COMMAND.COM | (only under DR-DOS multitasker via) | ||||||
| OS/2 CMD.EXE | ? | ||||||
| Windows CMD.EXE | |||||||
| 4DOS | ? | (via and, or via, and and) | (via and)[58] | ||||
| 4OS2 | ? | ? | ? | (via) | |||
| TCC (formerly 4NT) | ? | (via and) | (via,,,,, and, client only) | (via) | |||
| PowerShell | ? | ||||||
| rc | ? | ||||||
| BeanShell | ? | ? | ? | ? | |||
| VMS DCL | |||||||
| fish |
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 a 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.
| Shell | Secure (password) prompt | File/directory passwords | Execute permission | Restricted shell subset | Safe data subset | |
|---|---|---|---|---|---|---|
| Bourne shell | [59] | ? | [60] | |||
| POSIX shell | ? | |||||
| bash (v4.0) | ? | |||||
| csh | ? | |||||
| tcsh | ? | |||||
| Hamilton C shell | ||||||
| Scsh | ? | |||||
| ksh (ksh93t+) | ? | |||||
| pdksh | ? | |||||
| zsh | ? | [61] | ||||
| ash | ? | |||||
| CCP | ||||||
| COMMAND.COM | (only under DR-DOS, prompts for password if file/directory is protected) | (only under DR-DOS via syntax)[62] | (only under DR-DOS, if files are password-protected for read and/or execute permission)[63] | |||
| OS/2 CMD.EXE | ||||||
| Windows CMD.EXE | ||||||
| 4DOS | (via or)[64] | (only under DR-DOS via syntax) | (only under DR-DOS, if files are password-protected for read and/or execute permission) | |||
| 4OS2 | ? | |||||
| TCC (formerly 4NT) | (via, or) | |||||
| PowerShell | [65] | [66] | [67] | [68] | ||
| rc | ? | [69] | ||||
| BeanShell | ? | ? | ? | ? | ? | |
| VMS DCL | ||||||
| fish | ? | ? |
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.[70]
The shells bash, zsh and PowerShell offer this as a specific feature.[71] [72] 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 external command to switch off/on echoing of input characters.[73] In addition to not echoing back the characters, PowerShell's 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.
Some operating systems define an execute permission which can be granted to users/groups for a file when the file system itself supports it.
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).
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.
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.[74] [75]
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.[76]
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.[77] 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.[78] PowerShell data sections are used when e.g. localized strings needs to be read from an external source while protecting against unwanted side effects.