BASH SHELL

Bash is a Unix shell written by Brian Fox for the GNU Project as a free software replacement for the Bourne shell .Released in 1989, it has been distributed widely as the shell for the GNU operating system and as a default shell on Linux and Mac OS X. It has 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. 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 a description. 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 similar to 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.
Early computers had a teletype machine with a keyboard for I/O. Later, glass terminals became the norm, but the behavior was much the same a keyboard, a screen, a text display. A program was responsible for mediating the transaction between the operator and the machine, and as the years passed this program (the command interpreter or shell) became more sophisticated.
At this stage the command shell has become rather too sophisticated, typically having a dozen ways to do any particular thing. In