8.1 UNIX Variables
Variables are a way of passing information from the shell to programs when
you run them. Programs look "in the environment" for particular variables and if
they are found will use the values stored. Some are set by the system, others by
you, yet others by the shell, or any program that loads another program.
A D V E R T I S E M E N T
Standard UNIX variables are split into two categories, environment variables
and shell variables. In broad terms, shell variables apply only to the current
instance of the shell and are used to set short-term working conditions;
environment variables have a farther reaching significance, and those set at
login are valid for the duration of the session. By convention, environment
variables have UPPER CASE and shell variables have lower case names.
8.2 Environment Variables
An example of an environment variable is the OSTYPE variable. The value of
this is the current operating system you are using. Type
% echo $OSTYPE
More examples of environment variables are
- USER (your login name)
- HOME (the path name of your home directory)
- HOST (the name of the computer you are using)
- ARCH (the architecture of the computers processor)
- DISPLAY (the name of the computer screen to display X windows)
- PRINTER (the default printer to send print jobs)
- PATH (the directories the shell should search to find a command)
Finding out the current values of these variables.
ENVIRONMENT variables are set using the setenv
command, displayed using the printenv or
env commands, and unset using the
unsetenv command.
To show all values of these variables, type
% printenv | less
8.3 Shell Variables
An example of a shell variable is the history variable. The value of this is
how many shell commands to save, allow the user to scroll back through all the
commands they have previously entered. Type
% echo $history
More examples of shell variables are
- cwd (your current working directory)
- home (the path name of your home directory)
- path (the directories the shell should search to find a command)
- prompt (the text string used to prompt for interactive commands shell
your login shell)
Finding out the current values of these variables.
SHELL variables are both set and displayed using the
set command. They can be unset by using the unset command.
To show all values of these variables, type
% set | less
So what is the difference between PATH and path ?
In general, environment and shell variables that have the same name (apart
from the case) are distinct and independent, except for possibly having the same
initial values. There are, however, exceptions.
Each time the shell variables home, user and term are changed, the
corresponding environment variables HOME, USER and TERM receive the same values.
However, altering the environment variables has no effect on the corresponding
shell variables.
PATH and path specify directories to search for commands and programs. Both
variables always represent the same directory list, and altering either
automatically causes the other to be changed.
8.4 Using and setting variables
Each time you login to a UNIX host, the system looks in your home directory
for initialisation files. Information in these files is used to set up your
working environment. The C and TC shells uses two files called .login and .cshrc
(note that both file names begin with a dot).
At login the C shell first reads .cshrc followed by
.login
.login is to set conditions which will apply to the whole
session and to perform actions that are relevant only at login.
.cshrc is used to set conditions and perform actions
specific to the shell and to each invocation of it.
The guidelines are to set ENVIRONMENT variables in the .login
file and SHELL variables in the .cshrc file.
WARNING: NEVER put commands that run graphical
displays (e.g. a web browser) in your .cshrc or .login file.
8.5 Setting shell variables in the .cshrc file
For example, to change the number of shell commands saved in the history
list, you need to set the shell variable history. It is set to 100 by default,
but you can increase this if you wish.
% set history = 200
Check this has worked by typing
% echo $history
However, this has only set the variable for the lifetime of the current
shell. If you open a new xterm window, it will only have the default history
value set. To PERMANENTLY set the value of history, you will need to add the set
command to the .cshrc file.
First open the .cshrc file in a text editor. An easy,
user-friendly editor to use is nedit.
% nedit ~/.cshrc
Add the following line AFTER the list of other commands.
set history = 200
Save the file and force the shell to reread its .cshrc file buy using the
shell source command.
% source .cshrc
Check this has worked by typing
% echo $history
8.6 Setting the path
When you type a command, your path (or PATH) variable defines in which
directories the shell will look to find the command you typed. If the system
returns a message saying "command: Command not found", this indicates that
either the command doesn't exist at all on the system or it is simply not in
your path.
For example, to run units, you either need to directly specify the units path
(~/units174/bin/units), or you need to have the directory
~/units174/bin in your path.
You can add it to the end of your existing path (the $path
represents this) by issuing the command:
% set path = ($path ~/units174/bin)
Test that this worked by trying to run units in any directory other that
where units is actually located.
% cd
% units
To add this path PERMANENTLY, add the following line to your .cshrc AFTER the
list of other commands.
set path = ($path ~/units174/bin)
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