Using csh & tcsh

Using Variables

Variables control shell behavior, including the prompt display, where to look for commands, and whether to maintain a history list. The shell understands two kinds of variables: shell variables and environment variables. In general, shell variables are used internally by the shell and affect its own operation, whereas environment variables are accessible to programs started from the shell. Appendix B, csh and tcsh Quick Reference, lists several useful variables of each type.

Variables can be set directly from the command line, but you'll usually set them in your startup files, either to override default values that the shell supplies, or to define variables that the shell leaves undefined.

Shell Variables

Use the set command to define shell variables. Some variables need no explicit value; they have an effect merely by existing:

set notify                       Turn on immediate notification of job completion
set ignoreeof                    Disable logging out via CTRL-D

Other variables require a value:

set history = 20                 Tell shell to remember the last 20 commands
set prompt = "% "                Define command line prompt
set fignore = (.o .bak)          Specify suffixes to ignore during filename completion
set term = vt100                 Specify terminal type

Values that contain spaces or other special characters should be put in quotes, as in the set prompt command above. Values consisting of multiple strings must be surrounded with parentheses, as in the set fignore command.

You don't need to have spaces around the = separating the variable name from the value. If you do, there must be spaces on both sides:

set history= 20                  Illegal
set history =20                  Illegal
set history=20                   Legal
set history = 20                 Legal

I prefer to use spaces because I find set commands more readable that way.

Environment Variables

Use setenv to define environment variables:

setenv TERM vt100                Specify terminal type
setenv PAGER /usr/ucb/more       Specify preferred output pager
setenv EDITOR /usr/bin/emacs     Specify preferred editor

Notice that, unlike set, setenv has no = between the variable name and its value.

Environment variables differ from shell variables in that, although their values are known by the shell, those values are also accessible to programs you run from the shell. (For example, if your mailer allows you to drop into an editor to edit a message, it might determine which editor to use by consulting the EDITOR environment variable.) Environment variable names are usually in uppercase type. Uppercase is not a requirement, but is a useful convention that helps distinguish environment variables from shell variables.

Examining and Using Variable Values

Use set or setenv with no argument to see the values of all your shell or environment variables:

% set                              Display all shell variables
% setenv                           Display all environment variables

Some systems have env or printenv commands that act like setenv.

A reference to a variable name is written $name or ${name}. You can examine individual variable values using echo:

% echo $TERM $path                 Display terminal type and command search path
% echo ${TERM} ${path}             This is equivalent to the above command

If you try to use a variable that doesn't exist (i.e., has not been defined), the shell complains:

% echo $junk
junk: Undefined variable.

If a variable refers to a filename, you can apply one of the modifiers shown in Table 4-1 to extract different parts of the value. Modifiers are applied using the syntax $name:m or ${name:m}:

% set xyz = /etc/csh.cshrc

% echo root $xyz:r extension $xyz:e head $xyz:h tail $xyz:t
root /etc/csh extension cshrc head /etc tail csh.cshrc

% echo root ${xyz:r} extension ${xyz:e} head ${xyz:h} tail ${xyz:t}
root /etc/csh extension cshrc head /etc tail csh.cshrc

Table 4-1. Variable Modifiers

Modifier	Description
r	Root of value (everything but extension following dot)
e	Extension of value (suffix following dot)
h	Head of value (all but last pathname component)
t	Tail of value (last pathname component)

Turning Off Variables

If you want to turn off a shell or environment variable, use unset or unsetenv:

unset mail              Turn off notification of new mail
unsetenv DISPLAY        Turn off X display specification

Making tcsh Shell Variables Read-Only

In tcsh, a shell variable can be made read-only using set –r. A read-only variable cannot be modified with set, or removed with unset:

% set -r xyz
% set xyz
set: $xyz is read-only.
% unset xyz
unset: $xyz is read-only.

A system administrator can use read-only variables to place a non-modifiable variable in the working environment of all tcsh users. At a busy site where terminal lines and other resources are at a premium, the following line can be placed in a system-wide startup file to log out idle users after an hour of inactivity:

set -r autologout = 60            Log out idle users after 60 minutes

Paired Variables

Some shell variables have a corresponding environment variable, so that setting one member of the pair implicitly sets the other. For instance, you can use either of the following commands to specify your terminal type:

set term = vt100                This also sets the TERM environment variable
setenv TERM vt100               This also sets the term shell variable

path and PATH are another such pair, although their values are specified using different formats:

set path = (˜/bin /usr/bin /bin /usr/ucb)
setenv PATH /usr/staff/dubois/bin:/usr/bin:/bin:/usr/ucb

When you set either path or PATH, the shell automatically converts the value to the format required by the other.