Bash is a Unix shell and command language written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell. First released in 1989, it has been distributed widely as the default login shell for most Linux distributions and Apple’s macOS (formerly OS X). A version is also available for Windows 10.
Bash is a command processor that typically runs in a text window, where the user types commands that cause actions. Bash can also read and execute commands from a file, called a script. Like all Unix shells, it supports filename globbing (wildcard matching), piping, here documents, command substitution, variables, and control structures for condition-testing and iteration. The keywords, syntax 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 shell’s name is an acronym for Bourne-again shell, punning on the name of the Bourne shell that it replaces and on the term “born again” that denotes spiritual rebirth in contemporary American Christianity.
A security hole in Bash dating from version 1.03 (August 1989), dubbed Shellshock, was discovered in early September 2014 and quickly led to a range of attacks across the Internet. Patches to fix the bugs were made available soon after the bugs were identified, but not all computers have been updated.
Brian Fox began coding Bash on January 10, 1988 after Richard Stallman became dissatisfied with the lack of progress being made by a prior developer. Stallman and the Free Software Foundation (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. Fox released Bash as a beta, version .99, on June 8, 1989 and remained the primary maintainer until sometime between mid-1992 and mid-1994, when he was laid off from FSF and his responsibility was transitioned to another early contributor, Chet Ramey.
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 (although Almquist shell may be the default scripting shell) and on Apple’s macOS. Bash has also been ported to Microsoft Windows and distributed with Cygwin and MinGW, to DOS by the DJGPP project, to Novell NetWare and to Android via various terminal emulation applications.
In September 2014, Stéphane Chazelas, a Unix/Linux, network and telecom specialist working in the UK, discovered a security bug in the program. The bug, first disclosed on September 24, was named Shellshock and assigned the numbers CVE-2014-6271, CVE-2014-6277 and CVE-2014-7169. The bug was regarded as severe, since CGI scripts using Bash could be vulnerable, enabling arbitrary code execution. The bug was related to how Bash passes function definitions to subshells through environment variables.
The Bash command syntax is a superset of the Bourne shell command syntax. 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, 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 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 Korn shell 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.
Bash 4.0 introduced support for associative arrays. Associative arrays allow a fake support for multi-dimensional (indexed) arrays, in a similar way to AWK:
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