Bash is a Unix shell written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell (sh).Released in 1989, it has been distributed widely as the shell for the GNU operating system and as the default shell on Linux, Mac OS X and Darwin. It has been ported to Microsoft Windows and distributed with Cygwin and MinGW, to DOS by the DJGPP project and to Novell NetWare.
Bash is a command processor, typically run in a text window, allowing the user to type commands which cause actions. Bash can also read commands from a file, called a script. Like all Unix shells, it supports filename wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration. The keywords, syntax and other basic features of the language were all copied from sh. Other features, e.g., history, were copied from csh and ksh. Bash is a POSIX shell but with a number of extensions.
The name itself is an acronym, a pun and descriptive. As an acronym, it stands for Bourne-again shell, referring to its objective as a free replacement for the Bourne shell. As a pun, it expressed that objective in a phrase that sounds the same as born again, a term for spiritual rebirth. The name is also descriptive of what it did, bashing together the features of sh, csh and ksh.