C Shell

The C shell is an interactive command interpreter and a command programming language. It uses syntax that is similar to the C programming language. The csh command starts the C shell.

When you log in, it first searches the systemwide setup file /etc/csh.cshrc. If it is there, the C shell executes the commands stored in that file. Next, the C shell executes the systemwide setup file /etc/csh.login if it is available. Then, it searches your home directory for the .cshrc and .login files. If they exist, they contain any customized user information pertinent to running the C shell. All variables set in the /etc/csh.cshrc and /etc/csh.login files may be overridden by your .cshrc and .login files in your $HOME directory. Only the root user can modify the /etc/csh.cshrc and /etc/csh.login files.

The /etc/csh.login and $HOME/.login files are executed only once at login time. They are generally used to hold environment variable definitions, commands that you want executed once at login, or commands that set up terminal characteristics.

The /etc/csh.cshrc and $HOME/.cshrc files are executed at login time, and every time the csh command or a C shell script is invoked. They are generally used to define C shell characteristics like aliases and C shell variables (for example, history, noclobber, or ignoreeof). It is recommended that you only use the C shell built-in commands in the /etc/csh.cshrc and $HOME/.cshrc files because using other commands increases the startup time for shell scripts.

This section discusses the following:

• C Shell Limitations
• Signal Handling
• C Shell Commands
  • C Shell Built-In Commands
  • C Shell Expressions and Operators
  • Command Substitution in the C Shell
  • Nonbuilt-in Command Execution
• History Substitution in the C Shell
  • History Lists
  • Event Specification
  • Quoting with Single and Double Quotes
• Alias Substitution in the C Shell
• Variable and File Name Substitution in the C Shell
  • Variable Substitution in the C Shell
  • File Name Substitution in the C Shell
  • File Name Expansion
  • File Name Abbreviation
  • Character Classes
• Environment Variables in the C Shell
• Input and Output Redirection in the C Shell
  • Control Flow
• Job Control in the C Shell
• C Shell Related Information

C Shell Limitations

The following are limitations of the C shell:

• Words can be no longer than 1024 bytes.
• Argument lists are limited to ARG_MAX bytes. Values for the ARG_MAX variable are found in the /usr/include/sys/limits.h file.
• The number of arguments to a command that involves file name expansion is limited to 1/6th the number of bytes allowed in an argument list.
• Command substitutions can substitute no more bytes than are allowed in an argument list.
• To detect looping, the shell restricts the number of alias substitutions on a single line to 20.
• The csh command does not support file name expansion based on equivalence classification of characters.

Signal Handling

The C shell normally ignores quit signals. Jobs running detached are immune to signals generated from the keyboard (INTERRUPT, QUIT, and HANGUP). Other signals have the values the shell inherits from its parent. You can control the shell's handling of INTERRUPT and TERMINATE signals in shell procedures with onintr. Login shells catch or ignore TERMINATE signals depending on how they are set up. Shells other than login shells pass TERMINATE signals on to the child processes. In no cases are INTERRUPT signals allowed when a login shell is reading the .logout file.