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.
Standard Linux 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 and shell scripts you run within that shell inherit the value of the environment variable set. By convention, environment variables have UPPER CASE and shell variables have lower case names.
An example of an environment variable is the
OSTYPE variable. The value of this is the
current operating system you are using. Type
You will get the result
More examples of environment variables are
USER(your login name)
HOME(the path name of your home directory)
HOSTNAME(the name of the computer you are using)
DISPLAY(the name of the computer screen to display X windows)
PATH(the directories the shell should search to find a command)
The value of a variable can be displayed at the prompt using
For example, to show the hostname:
To show all values of these variables, type
env | less
Each time you login to a Linux host, the system looks in your home
directory for initialisation files. Information in these files is used to
set up your working environment. The shell uses two files called
.bashrc (note that
both file names begin with a dot).
At login the shell first reads
.bashrc followed by
.login is to set conditions which will apply to the
whole session and to perform actions that are relevant only at login.
.bashrc 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
WARNING: NEVER put commands that run graphical displays (e.g. a web browser) in your .bashrc or .login 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.
Check this has worked by typing
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
First open the
.bashrc file in a text editor.
An easy, user-friendly editor to use is gedit (but note it needs X
windows so if you are logged onto a system via ssh you may not be able
to use this).
Add the following line AFTER the list of other commands.
Save the file and force the shell to reread its .cshrc file buy using the
Check this has worked by typing
When you type a command, your 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
~/units174/bin/units), or you need to have
~/units174/bin in your path.
You can add it to the end of your existing path (the
$PATH represents this) by issuing the
Not that here we append the existing path the new definition so we can
still find commands like
Test that this worked by trying to run units in any directory other that where units is actually located.
HINT: You can run multiple commands on one line by separating them with a semicolon.
To add this path PERMANENTLY, add the following line to your
.bashrc AFTER the list of other commands.