A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems. Users direct the operation of the computer by entering commands as text for a command line interpreter to execute or by creating text scripts of one or more such commands.
The most influential Unix shells have been the Bourne shell and the C shell. The Bourne shell, sh, was written by Stephen Bourne at AT&T as the original Unix command line interpreter; it introduced the basic features common to all the Unix shells, including piping, here documents, command substitution, variables, control structures for condition-testing and looping and filename wildcarding. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68.[1]
The C shell, csh, was written by Bill Joy while a graduate student at University of California, Berkeley. The language, including the control structures and the expression grammar, was modeled on C. The C shell also introduced a large number of features for interactive work, including the history and editing mechanisms, aliases, directory stacks, tilde notation, cdpath, job control and path hashing.
Both shells have been used as coding base and model for many derivative and work-alike shells with extended feature sets.
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The most generic sense of the term shell means any program that users employ to type commands. A shell hides the details of the underlying operating system with the shell interface and manages the technical details of the operating system kernel interface, which is the lowest-level, or 'inner-most' component of most operating systems. In Unix-like operating systems users typically have many choices of command-line interpreters for interactive sessions. When a user logs in to the system, a shell program is automatically executed. The login shell may be customized for each user. In addition, a user is typically allowed to execute another shell program interactively.
The Unix shell was unusual when it was introduced. It is both an interactive command language as well as a scripting programming language, and is used by the operating system as the facility to control (shell script) the execution of the system. Shells created for other operating systems than Unix, often provide similar functionality.
On systems with a windowing system, some users may never use the shell directly. On Unix systems, the shell is still the implementation language of system startup scripts, including the program that starts the windowing system, the programs that facilitate access to the Internet, and many other essential functions.
Graphical user interfaces for Unix, such as GNOME, KDE, and Xfce are often called visual or graphical shells.
The Bourne shell was one of the major shells used in early versions of the Unix operating system and became a de facto standard. It was written by Stephen Bourne at Bell Labs and was first distributed with Version 7 Unix, circa 1977. Every Unix-like system has at least one shell compatible with the Bourne shell. The Bourne shell program name is sh and it is typically located in the Unix file system hierarchy at /bin/sh. On many systems, however, /bin/sh may be a symbolic link or hard link to a compatible, but more feature-rich shell than the Bourne shell. The POSIX standard specifies its standard shell as a strict subset of the Korn shell. From a user's perspective the Bourne shell was immediately recognized when active by its characteristic default command line prompt character, the dollar sign ($).
The C shell was developed by Bill Joy for the Berkeley Software Distribution, a line of Unix operating systems derived from Unix and developed at the University of California, Berkeley. It was originally derived from the 6th Edition Unix shell (Thompson shell). Its syntax is modeled after the C programming language. It is used primarily for interactive terminal use, but less frequently for scripting and operating system control. C shell has many interactive commands.
A list of various shells may be found at www.freebsd.org.
Shells read configuration files on multiple circumstances which differ depending on the shell. These files usually contain commands for that particular shell and are executed when loaded. These files are usually used to set important variables like $PATH used to find executables, and others that control the behavior and appearance of the shell. This table shows the configuration files for popular shells:
sh | ksh | csh | tcsh | bash | zsh | |
---|---|---|---|---|---|---|
/etc/.login |
login | login | ||||
/etc/csh.cshrc |
yes | yes | ||||
/etc/csh.login |
login | login | ||||
~/.tcshrc |
yes | |||||
~/.cshrc |
yes | yes[4] | ||||
~/.login |
login | login | ||||
~/.logout |
login | login | ||||
/etc/profile |
login | login | login | login | ||
~/.profile |
login | login | login[5] | login | ||
~/.bash_profile |
login[5] | |||||
~/.bash_login |
login[5] | |||||
~/.bash_logout |
login | |||||
~/.bashrc |
int.+n/login | |||||
/etc/zshenv |
yes | |||||
/etc/zprofile |
login | |||||
/etc/zshrc |
int. | |||||
/etc/zlogin |
login | |||||
/etc/zlogout |
login | |||||
~/.zshenv |
yes | |||||
~/.zprofile |
login | |||||
~/.zshrc |
int. | |||||
~/.zlogin |
login | |||||
~/.zlogout |
login |
Explanation:
~/.tcshrc
not found~/.bash_profile
, ~/.bash_login
and ~/.profile
; and only ~/.profile
if invoked as sh