# Bash (Unix shell)

In computing, Bash (short for "Bourne Again SHell,") is an interactive command interpreter and command programming language developed for UNIX-like operating systems. Created in 1989 by Brian Fox for the GNU Project, it is supported by the Free Software Foundation and designed as a 100% free alternative for the Bourne shell (sh) and other proprietary Unix shells.
Since its inception, Bash has gained widespread adoption and is commonly used as the default login shell for numerous Linux distributions. It holds historical significance as one of the earliest programs ported to Linux by Linus Torvalds, alongside the GNU Compiler (GCC). It is available on nearly all modern operating systems, making it a versatile tool in various computing environments.
As a command-line interface (CLI), Bash operates within a terminal emulator, or text window, where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features. The keywords, syntax, dynamically scoped variables, and other basic features of the language are all copied from the Bourne shell, (sh). Other features, e.g., history, are copied from the C shell, (csh), and the Korn Shell, (ksh). It is a POSIX-compliant shell with extensions.

## History

While Bash was developed for UNIX and UNIX-like operating systems, such as GNU/Linux, it is also available on Android, macOS, Windows, and numerous other current and historical operating systems. "Although there have been attempts to create specialized shells, the Bourne shell derivatives continue to be the primary shells in use."

### Timeline
#### 1963 (1963)

    While at MIT developing for the Compatible Time-Sharing System operating system, Louis Pouzin wrote 'RUNCOM', "a sort of shell driving the execution of command scripts, with argument substitution." He "felt that commands should be usable as building blocks for writing more commands, just like subroutine libraries."
   * "There was a facility that would execute a bunch of commands stored in a file; it was called runcom for "run commands", and the file began to be called "a runcom". rc in Unix is a fossil from that usage."

#### 1964 (1964)

    Development began on MULTICS at MIT.

#### 1965 (1965)

    April 2, 1965 (1965-04-02): MIT publishes "The SHELL, A Global Tool for Calling and Chaining Procedures in the System," by Louis Pouzin. The paper describes many features found in Bash. "Christopher Strachey, a British scientist, had visited MIT about that time, and his macro-generator design appeared to me a very solid base for a command language, in particular the techniques for quoting and passing arguments. Without being invited on the subject, I wrote a paper explaining how the Multics command language could be designed with this objective. And I coined the word "shell" to name it."
   * "It (i.e., the Thompson Shell) was modeled after the Multics shell, developed in 1965 by American software engineer Glenda Schroeder. Schroeder's Multics shell was itself modeled after the RUNCOM program Louis Pouzin showed to the Multics Team.

#### 1968 (1968)

    ALGOL68 was released. The Bourne shell drew from it the concepts of program flow, closed forms (if...fi, case...esac), and completion of substitutions anywhere. These closed forms and the for...done and while...done loop were defined as "Guarded Commands" in 1976 by Dijkstra.

#### 1969 (1969)

    The MULTICS operating system was introduced as a collaboration between MIT, GE and Bell Labs. "It is also the first to have a command processor implemented as ordinary user code – an idea later used in the Unix shell."Multics Wikipage It was called exec_com or ec.

#### 1971 (1971)

    "Ken Thompson (of Bell Labs) developed the first shell for UNIX called the 'V6 shell'." Its sole purpose was to serve as an interactive shell. It supported external commands called glob, for pattern matching, and if, for conditional expressions. "The shell introduced a compact syntax for redirection (< > and >>) and piping (| or ^) that has survived into modern shells. You can also find support for invoking sequential commands (with ;) and asynchronous commands (with &)." It also incorporated a goto command. "However, the Thompson shell was still distributed with the system as osh."

#### 1972 (1972)

    Pipelines were introduced in Unix.

#### 1973 (1973)

    UNIX, originally written by Ken Thompson, Dennis Ritchie, and Brian Kernighan among others, was released by AT&T (wikipedia UNIX page).

#### 1975 (1975)

   * "PWB Mashey shell started mid 1975."
   * September 1975 (1975-09): Digital Equipment Corporation released the VT52, which "provided a screen of 24 rows and 80 columns of text and supported all 95 ASCII characters, as well as... bi-directional scrolling" and, unlike its predecessor the VT50, lower case support (VT52).

#### 1977 (1977)

   * The American National Standard Code for Information Interchange, or "ASCII," is published by the National Institute of Science and Technology, or "NIST,"as FIPS PUB 1-2. The ASCII character set is still with us today as a core component of writing portable shell scripts.
   * "The Bourne shell was introduced.... Created by Stephen Bourne at AT&T Bell Labs for V7 UNIX, [it] remains a useful shell today (in some cases, as the default root shell)." "Bourne introduced control flows, loops, and variables into scripts, providing a more functional language to interact with the operating system (both interactively and noninteractively). The shell also permitted you to use shell scripts as filters, providing integrated support for handling signals, but lacked the ability to define functions. Finally, it incorporated a number of features we use today, including command substitution (using back quotes) and HERE documents to embed preserved string literals within a script." It used the -eq, -ne, -lt style of test operators
   * The Korn Shell was introduced at around the same time as was the Bourne shell.
   * OpenVMS - Initial Release. It included Bash at some later point.

#### 1978 (1978)

   * C shell - Initial release. Created by Bill Joy while he was at Berkeley, and released with 2BSD. "A useful feature introduced by Bill Joy in the C shell was command history. This feature maintained a history of the previously executed commands and allowed the user to review and easily select previous commands to execute. For example, typing the command history would show the previously executed commands. The up and down arrow keys could be used to select a command, or the previous command could be executed using !!. It's also possible to refer to arguments of the prior command; for example, !* refers to all arguments of the prior command, where !$ refers to the last argument of the prior command." "csh, tcsh, zsh, ash, and scsh are all released under the BSD or a BSD-like license."
   * August 1978 (1978-08): Digital Equipment Corporation introduced the VT100.

#### 1983 (1983)

   * The TENEX C shell "introduced file name and command completion in addition to command-line editing features. The tcsh was developed by Ken Greer at Carnegie Mellon University." The shebang, or hashbang symbol was available in tcsh. Also, positional parameters as the argv array including argv[1], the $0 shell variable as argv[0], the Count of Indices parameter expansion $#var, the -d and -x operators of a testing syntax regarding directory and executability tests, respectively, the ! negate symbol, a looping construct in the foreach command, the set, echo and exit commands, variable assignment and arithmetic contexts "@ count = $count + 1", and familiar double quoting rules.
   * Novell NetWare - Initial release. It included Bash at some later point.

#### 1984 (1984)

    "The IEEE POSIX work started in 1984 as an outgrowth of work initiated in the /usr/group organization to build a standard on the two variations of UNIX at the time - version 6 and BSD (Berkeley)."

#### 1986 (1986)

    The documents which would become known as "POSIX" started out as a trial use standard by the IEEE. By some it is considered a de facto standard.

#### 1988 (1988)

   * January 10, 1988 (1988-01-10): Brian Fox began coding Bash after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer. Stallman and the FSF considered a free shell that could run existing shell scripts so strategic to a completely free system built from BSD and GNU code that this was one of the few projects they funded themselves. Fox undertook the work as an employee of FSF.
   * "The first version of the IEEE Std 1003 was published in 1988 with the reference 'IEEE IX'," but these standards would not specify requirements for a "command interpreter," or "Shell and Utilities," until later. "At the suggestion of Richard Stallman, this was changed to POSIX for 'portable operating system interface'." The POSIX "interface enables application writers to write portable applications — it was developed with that goal in mind."

#### 1989 (1989)

   * May 30, 1989 (1989-05-30): Kenneth Almquist's ash shell - Initial release.
   * June 8, 1989 (1989-06-08): Fox released Bash as a beta, version 0.99. The license was GPL-1.0-or-later. "In addition to supporting backward-compatibility for scripting, Bash has incorporated features from the Korn and C shells. You'll find command history, command-line editing, a directory stack (pushd and popd), many useful environment variables, command completion, and more." Eventually it supported "regular expressions (similar to Perl), and associative arrays."
   * Microsoft/IBM DOS via the DJGPP project - Initial release.

#### 1992 (1992)

   * POSIX.2: Shell and Utilities (IEEE Std 1003.2-1992) was published.
   * Brian Fox remained the primary maintainer until sometime between mid-1992 and mid-1994. His responsibility was then transitioned to another early contributor, Chet Ramey. Since then, Bash has become the most popular default interactive shell among the major GNU/Linux distributions, such as Fedora, Debian, and openSUSE, as well as among their derivatives and competitors.

#### 1992 (1992)

    "The first eight entries in (POSIX) Portable Character Set are defined in the ISO/IEC 6429:1992 standard." They are null, alert, backspace, tab, carriage-return, newline, vertical-tab and form-feed.

#### 1994 (1994)

   * January 26, 1994 (1994-01-26): Debian - initial release. Bash is the default interactive and non-interactive shell.

#### 1995 (1995)

   * Cygwin - Initial release.
   * BeOS - Initial release.

#### 1996 (1996)

   * December 31, 1996 (1996-12-31): Chet Ramey released bash 2.0. The license was GPL-2.0-or-later.

#### 1997 (1997)

   * June 5, 1997 (1997-06-05): Bash 2.01 is released.
   * Almquist shell, ash, ported from NetBSD to Debian by Herbert Xu.

#### 1998 (1998)

   * April 18, 1998 (1998-04-18): Bash 2.02 is released.
   * MinGW (32-bit) - Initial release.
   * A group of Standards Committees from the IEEE, the Open Group and the ISO/IEC began working on the Single UNIX Specification v 3; they together became known as the Austin Group. A Technical Standard for Shell and Utilities was created in The Open Group Base Specifications, Issue 6, which is itself a part of the Single UNIX Specification v 3. A focus of their work was to "collect into a single document" practices which had been proven to work in industry and academia while making "minimal changes to existing application code."

#### 1999 (1999)

   * February 19, 1999 (1999-02-19): Bash 2.03 is released.

#### 2000 (2000)

   * March 21, 2000 (2000-03-21): Bash 2.04 is released.
   * The Korn shell is released as open source under the Common Public License.
   * "The rest of the characters" in the POSIX Portable Character Set "are defined in the ISO/IEC 10646-1:2000 standard." They comprise the remainder of the characters on a U.S. qwerty keyboard from <space> to <tilde>.
   * BeOS - Final release
   * Darwin OS - Initial release. Bash is one of the available shells.
   * September 14, 2000 (2000-09-14): Bug-bash mailing list exists.

#### 2001 (2001)

   * April 9, 2001 (2001-04-09): Bash 2.05 released.
   * A POSIX-2001 Standard is published which includes a "Commands and Utilities" section. The Austin Group and the Single Unix Specification is involved.

#### 2002 (2002)

   * September 2002 (2002-09): In Debian, version 0.4.1 of ash was renamed to dash.

#### 2003 (2003)

   * Bash became the default shell on Apple's operating systems (i.e., MacOS) starting with OS X 10.3 Panther. It was available on OS X 10.2 Jaguar as well where the default shell was tcsh.
   * A POSIX-2003 Shell and Utilities standard is published by the IEEE. "This is a component product standard for the mandatory shell and utilities related functionality;" a POSIX Certification Program exists.

#### 2004 (2004)

   * July 27, 2004 (2004-07-27): Bash 3.0 is released.
   * POSIX-2004 is published.

#### 2005 (2005)

   * December 9, 2005 (2005-12-09): Bash 3.1 is released.
   * Mingw-w64 — Initial release.

#### 2006 (2006)

   * October 12, 2006 (2006-10-12): Bash 3.2 released. The license was GPL-2.0-or-later.
   * Ubuntu adopts dash as its default shell.

#### 2008 (2008)

    POSIX-2008 is released by the IEEE. "This standard defines a standard source level interface to the shell and utility functionality required by application programs, including shell scripts."

#### 2009 (2009)

   * February 20, 2009 (2009-02-20): Bash 4.0 is released Its license is GPL-3.0-or-later.
   * Novell Netware - final release
   * September 14, 2009 (2009-09-14): Haiku (operating system) - Initial release. "Haiku's interface to the shell, by default," is bash. As of September 13, 2024, Haiku remains in beta.

#### 2010 (2010)

   * January 2, 2010 (2010-01-02): Bash 4.1 is released.

#### 2011 (2011)

   * February 6, 2011 (2011-02-06): Debian Lenny is released with Bash as its default interactive shell, and dash as its default non-interactive shell.
   * February 14, 2011 (2011-02-14): Bash 4.2 is released.

#### 2012 (2012)

    On Solaris 11, "the default user shell is the Bourne-again (bash) shell."

#### 2013 (2013)

    POSIX-2013 is released by the IEEE.

#### 2014 (2014)

   * February 27, 2014 (2014-02-27): Bash 4.3 is released.
   * 8 September 2014 (2014-09-08): Shellshock (software bug). Patches to fix the bugs were made available soon after the bugs were identified.

#### 2015 (2015)

    Termux and other terminal emulation applications provide availability of Bash on Android.

#### 2016 (2016)

   * April 26, 2016 (2016-04-26): Windows 10 Insider Preview Build 14332 supports installation of Bash on Ubuntu in the newly released Windows Subsystem for Linux.
   * POSIX-2016 is released by the IEEE.
   * September 15, 2016 (2016-09-15): Bash 4.4 is released.

#### 2017 (2017)

   * A new version of the POSIX Standard is released.
   * Some other current operating systems which carry Bash include and ArcaOS.

#### 2018 (2018)

   * January 31, 2018 (2018-01-31): The IEEE published the POSIX 2017 Standard.
   * Apple packaged Bash as their default interactive shell through until macOS Mojave (c.2018). However, for explicit licensing reasons with Catalina (c.2019), Apple replaced its default shell, Bash version 3.2 (c.2006), with Z Shell version 5.7 (c.2019). "The bash binary bundled with macOS has been stuck on version 3.2 for a long time now. bash v4 was released in 2009 and bash v5 in January 2019. The reason Apple has not switched to these newer versions is that they are licensed with GPL v3. bash v3 is still GPL v2." Zsh uses a more permissive license.

#### 2019 (2019)

   * January 7, 2019 (2019-01-07): Bash 5.0 is released.

#### 2020 (2020)

   * With release 2020.4, Kali Linux switched to zsh as its default shell for desktop images, but its ARM, containers, NetHunter and WSL images still use Bash.
   * December 7, 2020 (2020-12-07): Bash 5.1 is released.

#### 2022 (2022)

   * September 26, 2022 (2022-09-26): Bash 5.2 is released.
   * "At this point, 2022, a significant number of POSIX compatible or influenced systems exist in servers, cloud computing centers, high performance computers, Apple systems (initially BSD variations), and many cell phone systems. The number of devices measuring into the billions."

#### 2023 (2023)

   * September 18, 2023 (2023-09-18): Windows Subsystem for Linux v 2.0.0 is released for Windows 11.

#### 2024 (2024)

   * June 14, 2024 (2024-06-14): POSIX-2024 is published by the IEEE.

## Features
### List of Short Descriptions

As a command processor, Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features, including:
* A User Manual provided by the GNU Project, also available at info bash, and a technical manual available at man bash.
* Invocation as a...
** Interactive shell,
** Non-interactive shell, or
** Login shell;
* A command-line interface;
* Exit status codes;
* Control structures for
** Condition testing,
*** if, case, select,
*** logical AND (&&) and OR (||), and
** Iteration:
*** for, while, until loops, and
*** Arithmetic C-style loop: for ((;
* Syntaxes for Boolean testing of file attributes, string and integer values, etc.:
** Traditional test command,
** Traditional single bracket test: [,
** Modern double bracket test: [[ ... ]], which includes advanced features:
*** Extended regular expression and extglob matching
*** Lexicographic sorting with < and >;
* UNIX-style pipelines: |;
* Subshells: ( ... );
* Signaling as a means of inter-process communication using the trap builtin;
* Asynchronous execution, i.e., Jobs and job control:
** job_spec & where job_spec can be one of:
*** A full commandline: <command_name> <options> <operands> <arguments> &, or
*** A job control identifier as denoted by a leading percent symbol: %1 &;
* A shell portability mode where command lines can be interpreted in conformance with the POSIX standard;
* Command parsing:
** Comments are ignored:
*** Bourne-style # hashtag comments, and
*** Thompson-style : colon comments;
** Commands are parsed one line at a time:
*** Control structures are honored, and
*** Backslash \ escapes are also honored at the ends of lines;
** Split into words (i.e., word splitting) according to quoting rules,
*** Including ANSI-C quoting $'...';
** Seven kinds of expansions are performed in the following order on the resulting string:
*** (Step 1) Brace expansion kernel{-headers},
*** (Step 2) Tilde expansion ~,
*** (Step 3) In a left-to-right fashion:
**** Parameter and variable expansion $foo or ${bar}, including
***** Dynamically scoped variables,
***** Indexed arrays of unlimited size,
***** Associative arrays via declare -A, and
***** Expansion syntaxes which can perform some tasks more quickly than external utilities, including, among others:
****** Pattern Substitution
******* ${foo//x/y} for sed 's/x/y/g',
****** Remove Matching Prefix or Suffix Pattern
******* ${bar##[a-zA-Z0-9]*} for cut -c8-,
****** Print Array Keys
******* ${!array[@]}, and
****** Display Error if Null or Unset
******* ${var:?error message},
**** Command substitution: $( ... ),
**** Process substitution, < or >, when a system supports it:
**** Arithmetic expansion, (( ... )) or $(( ... )), including
***** Integer arithmetic in any base from two to sixty-four, although
***** Floating-point arithmetic is not available from within the shell itself (for this functionality, see current versions of bc and awk, among others),
*** (Step 4) Word splitting (again),
*** (Step 5) Pathname expansion, i.e., shell-style globbing and pattern matching using *, ?, [...], and
**** (Although they can be used in conjunction, the use of brackets in pattern matching, [...], and the use of brackets in the testing commands, [ and [[ ... ]], are each one different things.)
*** Quote removal;
** Redirections of Standard Input, Standard Output and Standard Error data streams are performed, including
*** File writing, >, and appending, >>,
*** Here documents, <<,
*** Here strings, <<<, which allow parameters to be used as input, and
*** A redirection operator, >|, which can force overwriting of a file when a shell's "noclobber" setting is enabled;
** Command name lookup is performed, in the following order:
*** Commands internal to the shell:
**** Shell aliases,
**** Shell reserved words,
**** Shell functions, and
**** Shell built-in commands;
*** Commands external to the shell:
**** Separate UNIX-style programs such as ls or ln, and
**** Shell scripts, which are files containing executable commands. (Shell scripts do not require compilation before execution and, when certain requirements are met, can be invoked as commands by using their filename.)
** The resulting string is executed as a command.
Bash also offers...
* Configurable execution environment(s):
** Shell and session startup files such as ~/.bashrc and ~/.profile (i.e., dotfiles);
** Settings (set built-in) and shell options (shopt built-in) which alter shell behavior;
* Support for Unicode;
* With interactive invocation only,
** Unlimited size command history,
** A directory stack (see pushd and popd built-ins),
** Tab completion,
** Configurable prompts, and
** Command line editing with GNU readline;
* Lightweight logging for debugging purposes (xtrace), and other lightweight debugging options (errexit, noexec, nounset, pipefail, etc.);
* Shell compatibility modes: bash 5.1 can operate as if it were bash 4.2, etc.;
* Various Built-In Commands:
** cd
** pwd
* Documentation:
** A built-in help command.
** A man page, and
** An info page which is the same as the GNU manual;
* Informal avenues of support via:
** IRC at libera.chat #bash
** Mailing lists at https://www.gnu.org/software/bash/

### General Discussion

The Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion, command line completion (Programmable Completion), basic debugging and signal handling (using trap) since bash 2.05a among other features. Bash can execute the vast majority of Bourne shell scripts without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn Shell (ksh) and the C shell (csh) such as command line editing, command history (history command), the directory stack, the $RANDOM and $PPID variables, and POSIX command substitution syntax $(...).
When a user presses the tab key within an interactive command-shell, Bash automatically uses command line completion, since beta version 2.04, to match partly typed program names, filenames and variable names. The Bash command-line completion system is very flexible and customizable, and is often packaged with functions that complete arguments and filenames for specific programs and tasks.
Bash's syntax has many extensions lacking in the Bourne shell. Bash can perform integer calculations ("arithmetic evaluation") without spawning external processes. It uses the ((...)) command and the $((...)) variable syntax for this purpose. Its syntax simplifies I/O redirection. For example, it can redirect standard output (stdout) and standard error (stderr) at the same time using the &> operator. This is simpler to type than the Bourne shell equivalent 'command > file 2>&1'. Bash supports process substitution using the <(command) and >(command)syntax, which substitutes the output of (or input to) a command where a filename is normally used. (This is implemented through /proc/fd/ unnamed pipes on systems that support that, or via temporary named pipes where necessary).
When using the 'function' keyword, Bash function declarations are not compatible with Bourne/Korn/POSIX scripts (the KornShell has the same problem when using 'function'), but Bash accepts the same function declaration syntax as the Bourne and Korn shells, and is POSIX-conformant. Because of these and other differences, Bash shell scripts are rarely runnable under the Bourne or Korn shell interpreters unless deliberately written with that compatibility in mind, which is becoming less common as Linux becomes more widespread. But in POSIX mode, Bash conforms with POSIX more closely.
Bash supports here documents. Since version 2.05b Bash can redirect standard input (stdin) from a "here string" using the <<< operator.
Bash 3.0 supports in-process regular expression matching using a syntax reminiscent of Perl.
In February 2009, Bash 4.0 introduced support for associative arrays. Associative array indices are strings, in a manner similar to AWK or Tcl. They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPL-3.0-or-later.

### Control Structures

Bash supplies "conditional execution" command separators that make execution of a command contingent on the exit code set by a precedent command. For example:
Where ./do_something is only executed if the cd (change directory) command was "successful" (returned an exit status of zero) and the echo command would only be executed if either the cd or the ./do_something command return an "error" (non-zero exit status).
For all commands the exit status is stored in the special variable $?. Bash also supports if ...;then ...;else ...;fi and case $VARIABLE in $pattern)...;;$other_pattern)...;; esac forms of conditional command evaluation.

### Process Management (a.k.a., "Job control")

The Bash shell has two modes of execution for commands: batch (asynchronous), and concurrent (synchronous).
To execute commands in batch mode (i.e., in sequence) they must be separated by the character ";", or on separate lines:
In this example, when command1 is finished, command2 is executed, and when command2 has completed, command3 will execute.
A background execution of command1 can occur using (symbol &) at the end of an execution command, and process will be executed in background while immediately returning control to the shell and allowing continued execution of commands.
Or to have a concurrent execution of command1 and command2, they must be executed in the Bash shell in the following way:
In this case command1 is executed in the background & symbol, returning immediately control to the shell that executes command2 in the foreground.
A process can be stopped and control returned to bash by typing Ctrl+z while the process is running in the foreground.
A list of all processes, both in the background and stopped, can be achieved by running jobs:
In the output, the number in brackets refers to the job id. The plus sign signifies the default process for bg and fg. The text "Running" and "Stopped" refer to the process state. The last string is the command that started the process.
The state of a process can be changed using various commands. The fg command brings a process to the foreground, while bg sets a stopped process running in the background. bg and fg can take a job id as their first argument, to specify the process to act on. Without one, they use the default process, identified by a plus sign in the output of jobs. The kill command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:

### Portability with POSIX

Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely with the POSIX 1003.2 standard. Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:
* Certain extended invocation options
* Brace expansion
* Arrays and associative arrays
* The double bracket [[...]] extended test construct and its regex matching
* The double-parentheses arithmetic-evaluation construct (only (( ... )); $(( ... )) is POSIX)
* Certain string-manipulation operations in parameter expansion
* local for scoped variables
* Process substitution
* Bash-specific builtins
* Coprocesses
* $EPOCHSECONDS and $EPOCHREALTIME variables
If a piece of code uses such a feature, it is called a "bashism" – a problem for portable use. Debian's checkbashisms and Vidar Holen's shellcheck can be used to make sure that a script does not contain these parts. The list varies depending on the actual target shell: Debian's policy allows some extensions in their scripts (as they are in the dash shell), while a script intending to support pre-POSIX Bourne shells, like autoconf's configure, are even more limited in the features they can use.

### Brace Expansion

Brace expansion, also called alternation, is a feature copied from the C shell. It generates a set of alternative combinations. Generated results need not exist as files. The results of each expanded string are not sorted and left to right order is preserved:
Users should not use brace expansions in portable shell scripts, because the Bourne shell does not produce the same output.
When brace expansion is combined with wildcards, the braces are expanded first, and then the resulting wildcards are substituted normally. Hence, a listing of JPEG and PNG images in the current directory could be obtained using:
In addition to alternation, brace expansion can be used for sequential ranges between two integers or characters separated by double dots. Newer versions of Bash allow a third integer to specify the increment.
When brace expansion is combined with variable expansion (A.K.A. parameter expansion and parameter substitution) the variable expansion is performed after the brace expansion, which in some cases may necessitate the use of the eval built-in, thus:

### Configurable execution environment(s)
#### Shell and Session Startup Files (a.k.a., "Dot Files")

When Bash starts, it executes the commands in a variety of dot files. Unlike Bash shell scripts, dot files do typically have neither the execute permission enabled nor an interpreter directive like #!/bin/bash.

##### Legacy-compatible Bash startup example

The example ~/.bash_profile below is compatible with the Bourne shell and gives semantics similar to csh for the ~/.bashrc and ~/.bash_login. The [ -r filename ] && cmd is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the && if it is not.

##### Operating system issues in Bash startup

Some versions of Unix and Linux contain Bash system startup scripts, generally under the /etc directory. Bash executes these files as part of its standard initialization, but other startup files can read them in a different order than the documented Bash startup sequence. The default content of the root user's files may also have issues, as well as the skeleton files the system provides to new user accounts upon setup. The startup scripts that launch the X window system may also do surprising things with the user's Bash startup scripts in an attempt to set up user-environment variables before launching the window manager. These issues can often be addressed using a ~/.xsession or ~/.xprofile file to read the ~/.profile — which provides the environment variables that Bash shell windows spawned from the window manager need, such as xterm or Gnome Terminal.

#### Settings and Shell Options
##### The set Built-in

* Xtrace: [ set -x | set -o xtrace ]
The shell's primary means of debugging. Both xtrace and verbose can be turned off at the same time with the command set -.
* Verbose: [ set -v | set -o verbose ]
Prints a command to the terminal as Bash reads it. Bash reads constructs all at once, such as compound commands which include if-fi and case-esac blocks. If a set -v is included within a compound command, then "verbose" will be enabled the next time Bash reads code as input, ie, after the end of the currently executing construct. Both xtrace and verbose can be turned off at the same time with the command set -.

##### The shopt Built-in

* expand-aliases
On by default in interactive shells. Some developers discourage its use in scripts.

### Programmable Completion

Bash supports programmable completion via built-in complete, compopt, and compgen commands. The feature has been available since the beta version of 2.04 released in 2000. These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames.
The complete and compopt two commands specify how arguments of some available commands or options are going to be listed in the readline input. As of version 5.1 completion of the command or the option is usually activated by the Tab ↹ keystroke after typing its name.

### Keyboard shortcuts with Readline

Bash uses GNU Readline to provide keyboard shortcuts for command line editing using the default ( Emacs ) key bindings. Vi-bindings can be enabled by running set -o vi.

## Documentation

As the standard upon which bash is based, the POSIX Standard, or IEEE Std 1003.1, et seq, is especially informative.
The Linux "man page" is intended to be the authoritative explanatory technical document for the understanding of how bash operates. It is usually available by running man bash.
The GNU manual is sometimes considered more user-friendly for reading. "You may also find information about Bash by running info bash ... or by looking at /usr/share/doc/bash/, /usr/local/share/doc/bash/, or similar directories on your system. A brief summary is available by running bash --help.
" If a user invoke RUNCOM without any arguments it prints some instructions on how to use it and stops, returning the user to the supervisor's (system's) command line.(RUNCOM)"
On modern Linuxes, information on shell built-in commands can be found by executing help, help [built-in name] or man builtins at a terminal prompt where bash is installed. Some commands, such as echo, false, kill, printf, test or true, depending on your system and on your locally installed version of bash, can refer to either a shell built-in or a system binary executable file. When one of these command name collisions occurs, bash will by default execute a given command line using the shell built-in. Specifying a binary executable's absolute path (i.e., /bin/printf) is one way of ensuring that the shell uses a system binary. This name collision issue also effects any "help summaries" viewed with kill --help and /bin/kill --help. Shell built-ins and system binary executable files of the same name often have differing options.
"The project maintainer also has a Bash page which includes Frequently Asked Questions", this FAQ is current as of bash version 5.1 and is no longer updated.

## Security and Vulnerabilities
### Root Scripts

Running any shell scripts as the root user has, for years, been widely criticized as poor security practice. One commonly given reason is that, when a script is executed as root, the negative effects of any bugs in a script would be magnified by root's elevated privileges.
One common example: a script contains the command, rm -rf ${dir}/, but the variable $dir is left undefined. In Linux, if the script was executed by a regular user, the shell would attempt to execute the command rm -rf / as a regular user, and the command would fail. However, if the script was executed by the root user, then the command would likely succeed and the filesystem would be erased.
It is recommended to use sudo on a per-command basis instead.

### Debugging

Bash features which can be useful during debugging. :
| Feature | Feature | Feature | POSIX 2024 | Description | Bash ver. |
| Grammar type | Formal name | Syntax | POSIX 2024 | Description | Bash ver. |
| Parameter Expansions | Indicate Null or Unset | "${parameter:?[word]}" | Yes | "Where the expansion of [word], perhaps an error message or a line number, is written to STDERR and the shell exits with a non-zero exit code." | ? |
| Special Parameters | Exit Status | "$?" | Yes | "Expands to the shortest representation of the decimal exit status." | ? |
| Special Parameters | PID of Invoked Shell | "$$" | Yes | "Expands to the shortest representation of the decimal process ID of the invoked shell." | ? |
| Special Built-In Utility | set :: xtrace | set -x | Yes | The shell's primary means of debugging. It "writes to standard error a trace for each command after it expands the command and before it executes it." | ? |
| Special Built-In Utility | set :: verbose | set -v | Yes | "Writes its input to standard error as it is read." | ? |
| Special Built-In Utility | set :: pipefail | set -o pipefail | Yes | "Derive the exit status of a pipeline from the exit statuses of all of the commands in the pipeline, not just the last (rightmost) command." | ? |
| Special Built-In Utility | set :: nounset | set -u | Yes | When enabled, will cause the shell to exit with an error message when it encounters an unset variable expansion. Its use has a number of counter-intuitive pitfalls. | ? |
| Special Built-In Utility | set :: errexit | set -e | Yes | ErrExit, is a setting that, when enabled, will, under certain very specific conditions, cause the shell to exit without an error message whenever the shell receives a non-zero exit code. Its use is somewhat controversial, to the extent that any somewhat obscure computer program can be considered controversial. Adherents claim that ErrExit provides an assurance of verifiability in situations where shell scripts "must not fail." However, opponents claim that its use is unreliable, deceptively simple, highly counter-intuitive, rife with gotchas and pitfalls, and in essence "security theater." Numerous developers of Bash have strongly discouraged the use of this particular setting. | ? |
| Special Built-In Utility | trap :: EXIT | trap '[arg]' EXIT | Yes | "If a [sigspec] (signal specifier) is 0 or EXIT, [arg] is executed when the shell exits." If [arg] contains expansions, then [arg] should be in single quotes. | ? |
| Utility | printf | printf '<%s>\n' "${var}" | Yes | A means of reliably printing the contents of a variable. | ? |
| Bash Variables | BASHPID | "${BASHPID}" | No | "Expands to the process ID of the current bash process." | ? |
| Bash Variables | BASH_ARGC | "${BASH_ARGC[@]}" | No | "An array variable whose values are the number of parameters in each frame of the current bash execution call stack." | ? |
| Bash Variables | BASH_ARGV | "${BASH_ARGV[@]}" | No | "An array variable containing all of the parameters in the current bash execution call stack." | ? |
| Bash Variables | BASH_LINENO | "${BASH_LINENO[@]}" | No | "An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked." | ? |
| Bash Variables | BASH_REMATCH | "${BASH_REMATCH[@]}" | No | "An array variable whose members are assigned by the =~ binary operator to the [[ conditional command." | ? |
| Bash Variables | BASH_SOURCE | "${BASH_SOURCE}" | No | "An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined." | ? |
| Bash Variables | BASH_XTRACEFD | "${BASH_XTRACEFD}" | No | "If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when ‘set -x’ is enabled to that file descriptor." | ? |
| Bash Variables | EPOCHREALTIME | "${EPOCHREALTIME}" | No | "Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)) as a floating point value with micro-second granularity." | ? |
| Bash Variables | FUNCNAME | "${FUNCNAME[@]}" | No | "An array variable containing the names of all shell functions currently in the execution call stack." | ? |
| Bash Variables | LINENO | "${LINENO}" | No | "Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function." | ? |
| Bash Variables | PIPESTATUS | "${PIPESTATUS[@]}" | No | "An array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command)." | ? |
| Bash Variables | PPID | "${PPID}" | No | "The process ID of the shell's parent." | ? |
| Bash Variables | PS4 | "${PS4}" | No | "The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays during an execution trace." | ? |
| Shell Builtin | set :: restricted | set -r | No | Restricted mode is intended to improve the security of an individual shell instance from a malicious human with physical access to a machine. As threat models have changed, it has become less commonly used now than it once was. | ? |
| Shell Builtin | shopt :: extdebug | shopt -s extdebug | No | "Behavior intended for use by debuggers." | ? |
| Shell Builtin | trap :: DEBUG | trap '[arg]' DEBUG | No | "If a sigspec is DEBUG, the command arg is executed before" certain kinds of commands. | ? |
| Shell Builtin | trap :: ERR | trap '[arg]' ERR | No | "If a sigspec is ERR, the command arg is executed whenever..." certain kinds of commands "return a non-zero exit status," subject to similar restrictions as with ErrExit. | ? |
| Shell Builtin | trap :: RETURN | trap '[arg]' RETURN | No | "If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the . or source builtins finishes executing." | ? |
* Shell features specified by POSIX:
** Parameter Expansions:
** Special Parameters:
** Special Built-In Utility set:
** Special Built-In Utility trap [-lp] [arg] [sigspec …]:
** Utility printf: a means of reliably printing the contents of a variable:
* Bash features not specified by POSIX:
** Bash Variables:
** Shell Builtin set:
** Shell Builtin shopt:
** Shell Builtin trap [-lp] [arg] [sigspec …]: While POSIX does specify certain uses of the trap builtin, the following signal specs are Bash extensions.
* Third party debugging utilities:
** ShellCheck: Shell script analysis tool;
** devscripts-checkbashisms: Check whether a /bin/sh script contains any common bash-specific constructs;
** kcov: Code coverage tool without special compilation options;
** Bashdb: The Bash symbolic debugger.

#### Examples

With the :? parameter expansion, an unset or null variable can halt a script.
* ex.sh
    #!/bin/bash bar="foo is not defined" echo "${foo:?$bar}" echo this message doesn't print
    $ ./ex.sh ./ex.sh: line 3: foo: foo is not defined
Reliably printing the contents of an array that contains spaces and newlines first in a portable syntax, and then the same thing in Bash. Note that in Bash, the number of spaces before the newline is made clear.
    $ # In POSIX shell: $ array=( "a " " b" " > c " ) $ printf ',%s,\n' "${array[@]}" ,a , , b, , c ,
    # In Bash: declare -p array declare -a array=([0]="a " [1]=" b" [2]=$' \n c ')
Printing an error message when there's a problem.
* error.sh
    if ! lsblk | grep sdb then echo Error, line $LINENO fi
    $ ./error.sh Error, line 130
Using xtrace. If errexit had been enabled, then echo quux would not have been executed.
* test.sh
    #!/bin/bash set -x foo=bar; echo $foo false echo quux
    $ ./test.sh + foo=bar + echo bar bar + false + echo quux quux

### Deprecated syntax

* Back-tick style command substitutions: `...` is deprecated in favor of $(...);
* Use of -a or -o in test/[/[[ commands,
** for example, [ -r ./file -a ! -l ./file ] is deprecated in favor of [ -r ./file ] && ! [ -l ./file ];
* Use of the arithmetic syntax $[...] is deprecated in favor of $((...)) or ((...)), as appropriate;
* Use of ^ as a pipeline is deprecated in favor of |;
* Any uses of expr or let.

### Shellshock

In September 2014, a security bug was discovered in the program. It was dubbed "Shellshock." Public disclosure quickly led to a range of attacks across the Internet.
Exploitation of the vulnerability could enable arbitrary code execution in CGI scripts executable by certain versions of Bash. The bug involved how Bash passed function definitions to subshells through environment variables. The bug had been present in the source code since August 1989 (version 1.03) and was patched in September 2014 (version 4.3).
Patches to fix the bugs were made available soon after the bugs were identified. Upgrading to a current version is strongly advised.
It was assigned the Common Vulnerability identifiers CVE-2014-6271, CVE-2014-6277 and CVE-2014-7169, among others. Under CVSS Metrics 2.x and 3.x, the bug is regarded as "high" and "critical," respectively.

### Bug reporting

An external command called bashbug reports Bash shell bugs. When the command is invoked, it brings up the user's default editor with a form to fill in. The form is mailed to the Bash maintainers (or optionally to other email addresses).