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. 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
$ echo $OSTYPE
More examples of environment variables are
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
Each time you login to a Linux host, the system looks in your home directory for initialization files. Information in these files is used to set up your working environment. The bash uses a file called .bashrc (note the file name begins with a dot).
At login the Bourne Again shell reads and executes the .bashrc . The .bashrc file is used to set conditions which will apply to the whole session and to perform actions that are relevant only at login as well as set conditions and perform actions specific to the shell and to each invocation of it.
WARNING: NEVER put commands that run graphical displays (e.g. a web browser) in your .cshrc or .login file.
One of the most common actions to do in the .bashrc file is to set command aliases. The command alias is generally used for two things: setting the "default" for a specific command and making new commands from the set of Linux commands.
For example, if you want your ls command to always do a long listing, you could use alias like this
$ alias ls='ls -l'
Check this has worked by typing
$ ls
Also, if you want a new command, say dir, to do a long listing similar to the old DOS listing, you could use an alias like this
$ alias dir='ls -al'
Check this has worked by typing
$ dir
However, this has only set the variable for the lifetime of the current shell. If you open a new terminal window, it will not have your new aliases set. To PERMANENTLY set the alias values, you will need to add the set commands in the .bashrc file.
First open the .bashrc file in a text editor. An easy, user-friendly editor to use is gedit.
$ gedit ~/.bashrc
Add the following line AFTER the list of other commands at the bottom of the file.
alias dir='ls -al'
Save the file and force the shell to reread the contents of the file buy using the shell source command.
$ source .bashrc
Check this has worked by typing
$ dir
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:
$ PATH=~/units174/bin:"${PATH}"
$ export $PATH
Test that this worked by trying to run units in any directory other that where units is actually located.
$ cd; units
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.
PATH=~/units174/bin:"${PATH}"