Variables

Overview

Reading: 10 min
Exercises: 0 min

Questions

• How are variables set and accessed in the Unix shell?

Objectives

• Understand how variables are implemented in the shell

• Read the value of an existing variable

• Create new variables and change their values

(from https://www.tutorialspoint.com/unix/unix-using-variables.htm) A variable is a character string to which we assign a value. The value assigned could be a number, text, filename, device, or any other type of data.

A variable is nothing more than a pointer to the actual data. The shell enables you to create, assign, and delete variables.

Showing the Value of a Variable

Some variables are defined by default. Let’s show the value of the variable HOME:

$ echo HOME

HOME

That just prints “HOME”, which isn’t what we wanted (though it is what we actually asked for). Let’s try this instead:

$ echo $HOME

/home/vlad

The dollar sign tells the shell that we want the value of the variable rather than its name. This works just like wildcards: the shell does the replacement before running the program we’ve asked for. Thanks to this expansion, what we actually run is echo /home/vlad, which displays the right thing.

Creating and Changing Variables

Creating a variable is easy—we just assign a value to a name using “=”:

$ SECRET_IDENTITY=Dracula
$ echo $SECRET_IDENTITY

Dracula

To change the value, just assign a new one:

$ SECRET_IDENTITY=Camilla
$ echo $SECRET_IDENTITY

Camilla

If we want to set some variables automatically every time we run a shell, we can put commands to do this in a file called .bashrc in our home directory. (The ‘.’ character at the front prevents ls from listing this file unless we specifically ask it to using -a: we normally don’t want to worry about it. The “rc” at the end is an abbreviation for “run control”, which meant something really important decades ago, and is now just a convention everyone follows without understanding why.)

For example, here are two lines in /home/vlad/.bashrc:

export SECRET_IDENTITY=Dracula
export TEMP_DIR=/tmp
export BACKUP_DIR=$TEMP_DIR/backup

These three lines create the variables SECRET_IDENTITY, TEMP_DIR, and BACKUP_DIR, and export them so that any programs the shell runs can see them as well. Notice that BACKUP_DIR’s definition relies on the value of TEMP_DIR, so that if we change where we put temporary files, our backups will be relocated automatically.

Key Points

• Shell variables are by default treated as strings

• Variables are assigned using “=” and recalled using the variable’s name prefixed by “$”