Bash, short for Bourne-Again SHell, is a shell program and command language supported by the Free Software Foundation[2] and first developed for the GNU Project[3] by Brian Fox.[4] Designed as a 100%[5] free software alternative for the Bourne shell,[6][7][8] it was initially released in 1989.[9] Its moniker is a play on words, referencing both its predecessor, the Bourne shell,[10] and the concept of renewal.[11][12]
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).[13] It is available on nearly all modern operating systems, making it a versatile tool in various computing environments.
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Brian Fox began coding Bash on January 10, 1988,[18] after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer.[7] 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, with Fox undertaking the work as an employee of FSF.[7][19] Fox released Bash as a beta, version .99, on June 8, 1989,[9] and remained the primary maintainer until sometime between mid-1992[20] and mid-1994,[21] when he was laid off from FSF[22] and his responsibility was transitioned to another early contributor, Chet Ramey.[23][24][25][26][27]
Since then, Bash has become by far the most popular shell among users of Linux, becoming the default interactive shell on that operating system's various distributions[28][29] and on Apple's macOS releases before Catalina in October 2019.[30][31][32] Bash has also been ported to Microsoft Windows[33][34] and distributed with Cygwin and MinGW, to DOS by the DJGPP project, to Novell NetWare, to OpenVMS by the GNU project,[35] to ArcaOS,[36] and to Android via various terminal emulation applications.
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:
The keywords, syntax, dynamically scoped variables and other basic features of the language are all copied from sh. Other features, e.g., history, are copied from csh and ksh. Bash is a POSIX-compliant shell, but with a number of extensions.
The Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion,[40] command line completion (Programmable Completion),[41] basic debugging[42][43] and signal handling (using trap) since bash 2.05a[44][45] 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),[46] 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,[47] 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)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.[48]
In February 2009,[50] Bash 4.0 introduced support for associative arrays.[51] Associative array indices are strings, in a manner similar to AWK or Tcl.[52] They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPL-3.0-or-later; some users suspect this licensing change is why MacOS continues to use older versions.[53] Apple finally stopped using Bash in its operating systems as default shell with the release of MacOS Catalina in 2019.[54]
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:
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:
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.
Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely to the POSIX 1003.2 standard.[55] Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:[55][56]
A background execution of command1 can occur using (symbol &) at the end of an execution command, and process will be executed in background while returning immediately control to the shell and allowing continued execution of commands.
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:
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).[63][64]
Bash supports programmable completion via built-in complete, compopt, and compgen commands.[65] The feature has been available since the beta version of 2.04 released in 2000.[66][67] These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames.[68]
The program's name is a figure of speech or witticism which begins with an homage to Stephen Bourne, the creator of one of the shell programs which have sometimes been considered superseded by the bash shell. His name is used as a pun on the image of childbirth. With that pun, it would seem, is added an allusion: possibly to the Hindu or Buddhist idea of reincarnation; possibly to the Christian idiom known as "being born again;" or quite possibly just to the more abstract idea of renewal. While numerous English translations of the Christian New Testament, Book of John, chapter 3 do contain the words "born again," Merriam-Webster's dictionary has "born-again" defined as a "...person who has made a renewed or confirmed commitment....[69]" Whatever the original touchstone may have been, in the end the program received the name, "the Bourne Again SHell."[70][71]
The acronym of that name then is "bash," a word meaning "to strike violently.[72]" In the context of computer programming, to "violently hit something," such as a computer keyboard, could be considered a hyperbolic image of some frustration. Such imagery of negative emotionality could be seen as standing in direct juxtaposition to the idea of becoming "born again."
The naming could be considered an instance of verbal irony[73] or accidental innuendo.[74] Bash grammar was initially based on the grammars of the most popular Unix shell programs then currently in use, some of which were considered particularly difficult to use or frustrating at that time. As the years progressed, bash development has made its grammar more user-friendly,[75][76][77][78][79][80] so much so that it seems likely that the bash project has been committed to improving its usablilty. Since then, bash has become the de facto default shell program in most Linux and Unix operating systems. 152ee80cbc
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