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 (which was the Thompson shell), 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 Today, the original C shell is not in wide use on Unix; it has been superseded by other shells such as the Tenex C shell (tcsh) based on the original C shell code, but adding filename completion and command line editing, comparable with the Korn shell (ksh), and the GNU Bourne-Again shell (bash). An independently-developed and modernized C shell, created by Nicole Hamilton, also survives on Windows in the form of Hamilton C shell.
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).
These features were first introduced in C shell, though most are now common to many Unix and Unix-like shells.
The ability to suspend an interactive job and resume it at a later time, or send it into the “background”.
The !-substitution feature allowed previous commands to be re-executed or edited in a variety of ways.
Variables could contain multiple elements, which could be numerically indexed.
Home directory by username lookup, using ~ was popular enough that most modern, Unix-based Web servers now support it, in addition to most shells.
A command could be aliased to another command or command sequence.
Provided arithmetical operations which sh does not provide except by calling external programs.
The most obvious difference between C shell and Bourne shell syntax was the use of parentheses for built-in conditional operations that Bourne 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”.
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 on Unix.
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. (These and other criticisms do not, however, apply to the more-modern Hamilton C shell variant of the csh language.)
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) in order to avoid running the user’s setup file as this may affect the script’s behaviour.
Another known problem of csh is its message “0 event not found”, which yields no information about what the problem is.