C shell

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The C shell (csh) is a Unix shell developed by Bill Joy for the BSD Unix system. It was originally derived from the 6th Edition Unix /bin/sh, the predecessor of the Bourne shell. Its syntax is modeled after the C programming language. The C shell added many feature improvements over the Bourne shell, such as aliases and command history. Today, the C shell is not in wide use; it has been superseded by other shells such as the Tenex C shell (tcsh), the Korn shell (ksh), and the GNU Bourne-Again shell (bash).

Contents 1 Features 1.1 New features 1.2 Syntax differences 2 Criticism 3 External links

Features

The C shell has the typical Unix shell structure: each line of input (or line of a script file) is interpreted as a separate command to execute, with backslashes "escaping" newlines where needed (so that multiple input lines can comprise a single command to be executed.

New features

These features were first introduced in C shell, though most are now common to many Unix and Unix-like shells.

Job control 

The ability to suspend an interactive job and resume it at a later time, or send it into the "background".

History substitution 

The !-substitution feature allowed previous commands to be re-executed or edited in a variety of ways.

Arrays 

Variables could contain multiple elements, which could be numerically indexed.

~ expansion 

Home directory by username lookup, usng ~ was popular enough that most modern, Unix-based Web servers now support it, in addition to most shells.

Aliases 

A command could be aliased to another command or command sequence.

Math operations 

Provided arithmetical operations which sh does not provide except by calling external programs.

Syntax differences

The most obvious difference between C shell and Bourne shell syntax was the use of parentheses for built-in conditional operations that Borne shell called out to test for using square brackets (later versions of Bourne shell would rectify this by using a built-in test operator).

Variable assignment was also performed slightly differently with "set a = b" rather than Bourne shell's "a=b".

Criticism

While the C shell introduced many innovative features in interactive mode, the C shell's scripting capabilities came under criticism in the early-to-mid 1990s for their ambiguity, especially with respect to quoting and redirection. Combined with the fact that Bourne shell is guaranteed to be present on all Single UNIX Specification-compliant systems, sh is usually used for scripting purposes over C shell.

An example of possibly unwelcome behaviour of csh scripts is the following:

 if ( ! -e foo ) echo bar > foo

It would appear to say "if file 'foo' does not exist, create it with contents 'bar'". But it will in fact create an empty file, as the line is parsed such that the output redirect is set up before the file existence is tested.

If scripts are written in csh, it is often useful to run the script with the -f flag, (e.g. by starting with the line #!/bin/csh -f<tt>) in order to avoid running the user's setup file as this may affect the script's behaviour.

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External links

• Csh Programming Considered Harmful by Tom Christiansen
• A comparison of features of various Unix shellsTemplate:Unix-stub

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