C-shell family (c-shell and tcsh)

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 Thompson shell), the predecessor of the Bourne shell. What is important is that its syntax is modeled after the C programming language and as such it was much closer to scripting language that shell from Borne shell family.

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 Today, the original C shell is not in wide use on Unix; it has been superseded by other shells such as the Tenex C shell (tcsh). Tcsh replaced csh as the default shell on FreeBSD. The letter "t" in tcsh comes from the T in TENEX, an operating system which inspired Ken Greer, the author of tcsh, with its command-completion feature. Ken Greer worked on tcsh in the late 1970s at the Carnegie Mellon University. Paul Placeway from Ohio State University continued work on it in the 1980s, and since then it has been maintained by numerous people. Wilfredo Sanchez, the former lead engineer of Mac OS X, worked on tcsh in the early 1990s at MIT. Early versions of Mac OS X shipped with tcsh as the default shell, but it has since been replaced by bash.

A modernized variant of C shell for Windows was written by Nicole Hamilton and called Hamilton C shell.

The program in C shell (C shell script) has Fortran-style 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.

C-shell has two features that make it closer to traditional Algol-style languages:

Built-in math operations Provided arithmetical operations which sh does not provide except by calling external programs.
Arrays Variables could contain multiple elements, which could be numerically indexed.
It used parentheses for built-in conditional operations that Bourne shell called out to test for using square brackets
Variable assignment was Basic style: "set a = b" rather than Bourne shell's "a=b".

It has some ambiguites due to precenden of operators:

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.)

Webliography

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Last modified: February 28, 2008