Unix shell

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File:TcshAndShScreenCaptureCropped.png
tcsh and sh shell windows on an OS X desktop

A Unix shell is a command-line interpreter or shell that provides a traditional Unix-like command line user interface. 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. Users typically interact with a Unix shell using a terminal emulator, however, direct operation via serial hardware connections, or networking session, are common for server systems. All Unix shells provide filename wildcarding, piping, here documents, command substitution, variables and control structures for condition-testing and iteration.

Concept

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 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 interactively, a shell program is automatically executed for the duration of the session. The type of shell, which may be customized for each user, is typically stored in the user's profile, for example in the local passwd file or in a distributed configuration system such as NIS or LDAP; however, the user may execute any other available shell interactively.

The Unix shell 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.[1] Shells created for other operating systems often provide similar functionality.

On hosts with a windowing system, like OS X, some users may never use the shell directly. On Unix systems, the shell has historically been the implementation language of system startup scripts, including the program that starts a windowing system, configures networking, and many other essential functions. However, some system vendors have replaced the traditional shell-based startup system (init) with different approaches, such as systemd.

Graphical user interfaces for Unix, such as GNOME, KDE, and Xfce are sometimes called visual or graphical shells.[citation needed]

Early shells

The first Unix shell was the Thompson shell, sh, written by Ken Thompson at Bell Labs and distributed with Versions 1 through 6 of Unix, from 1971 to 1975. Though rudimentary by modern standards, it introduced many of the basic features common to all later Unix shells, including piping, simple control structures using if and goto, and filename wildcarding. Though not in current use, it is still available as part of some Ancient UNIX Systems.

The PWB shell or Mashey shell, sh, was an upward-compatible version of the Thompson shell, augmented by John Mashey and others and distributed with the Programmer's Workbench UNIX, circa 1975-1977. It focused on making shell programming practical, especially in large shared computing centers. It added shell variables (precursors of environment variables, including the search path mechanism that evolved into $PATH), user-executable shell scripts, and interrupt-handling. Control structures were extended from if/goto to if/then/else/endif, switch/breaksw/endsw, and while/end/break/continue. As shell programming became widespread, these external commands were incorporated into the shell itself for performance.

But the most widely-distributed and influential of the early Unix shells were the Bourne shell and the C shell. Both shells have been used as the coding base and model for many derivative and work-alike shells with extended feature sets.

Bourne shell

The Bourne shell, sh, was a complete rewrite by Stephen Bourne at Bell Labs. Distributed as the shell for UNIX Version 7 in 1979, it introduced the rest of the basic features considered common to all the Unix shells, including here documents, command substitution, more generic variables and more extensive builtin control structures. The language, including the use of a reversed keyword to mark the end of a block, was influenced by ALGOL 68.[2] Traditionally, the Bourne shell program name is sh and its path in the Unix file system hierarchy is /bin/sh. But a number of compatible work-alikes are also available with various improvements and additional features. On many systems, sh may be a symbolic link or hard link to one of these alternatives:

The POSIX standard specifies its standard shell as a strict subset of the Korn shell, an enhanced version of the Bourne 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 ($).

C shell

The C shell, csh, was written by Bill Joy while a graduate student at University of California, Berkeley and widely distributed with BSD Unix.[4] 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. On many systems, csh may be a symbolic link or hard link to TENEX C shell (tcsh), an improved version of Joy's original csh. Though the C shell's interactive features have been copied in most other current shells, the language itself has not been widely copied. The only work-alike is Hamilton C shell, written by Nicole Hamilton, first distributed on OS/2 in 1988 and on Windows since 1992.[5]

Exotic shells

Other, more exotic variations on the Unix shell concept include the following:[6]

Configuration files

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[lower-alpha 1]
~/etc/ksh.kshrc int.
/etc/sh.shrc int.[lower-alpha 2]
$ENV (typically ~/.kshrc)[8] int.[lower-alpha 3][lower-alpha 4] int. int.[lower-alpha 5]
~/.login login login
~/.logout login login
/etc/profile login login login login[lower-alpha 6]
~/.profile login login login[lower-alpha 7] login[lower-alpha 6]
~/.bash_profile login[lower-alpha 7]
~/.bash_login login[lower-alpha 7]
~/.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

Shells read configuration files on multiple circumstances that differ depending on the shell. These files usually contain commands for the particular shell and are executed when loaded; they are usually used to set important variables used to find executables, like $PATH, and others that control the behavior and appearance of the shell. The table in this section shows the configuration files for popular shells.

Explanation:

  • blank means a file is not read by a shell at all.
  • "yes" means a file is always read by a shell upon startup.
  • "login" means a file is read if the shell is a login shell.
  • "n/login" means a file is read if the shell is not a login shell.
  • "int." means a file is read if the shell is interactive.

See also

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Notes

  1. ↑ only if ~/.tcshrc not found
  2. ↑ Newer versions of the Bourne Shell only
  3. ↑ Available on systems that support the "User Portability Utilities option"; value of the variable must be an absolute path, and it is ignored "if the user's real and effective user IDs or real and effective group IDs are different."[9]
  4. ↑ $ENV is $HOME/.shrc in newer versions of the Bourne Shell
  5. ↑ Same behavior as sh, but only if invoked as sh (bash 2+) or, since bash 4.2, also if invoked explicitly in POSIX compatibility mode (with options --posix or -o posix).[10]
  6. ↑ 6.0 6.1 Only in sh/ksh compatibility mode (when invoked as bash, sh, ksh)
  7. ↑ 7.0 7.1 7.2 in fact, the first readable of ~/.bash_profile, ~/.bash_login and ~/.profile; and only ~/.profile if invoked as sh or, as of at least Bash 4.2, if invoked explicitly in POSIX compatibility mode (with options --posix or -o posix)

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Harley Hahn, Harley Hahn's Guide to Unix and Linux.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  7. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ SCO Unix Group, SCO Unixware 7 documentation, 22 Apr 2004, retrieved 18 Oct 2012.
  9. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  10. ↑ Lua error in package.lua at line 80: module 'strict' not found.

Further reading

  • Lua error in package.lua at line 80: module 'strict' not found. — a history of the various shells, and the uses of and responsibilities of a shell on Unix

External links

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