Shell Programming - Variables

In this tutorial we will learn about variables and cover some of the Shell Programming keywords.

What are variables?

These are named memory location to hold some value.

Variables inside a shell script dies as soon as the script ends executing.

How to create variables in shell script?

To create a variable in shell script we have to keep the following points in mind.

• Variable name can contain alphabets a-z A-Z, digits 0-9 and underscore _.
• The first character of a variable name must either be an alphabets or underscore.
• Variable names are case-sensitive meaning uppercase and lowercase are treated differently. For example, name and Name are treated as two different variables even though they are same word.

Assign value to a variable

To assign value to a variable we use the = sign also called the assignment operator.

In the following example we are creating message variable and assigning it a string value "Hello World".

message="Hello World"

Take user input and assigning it to a variable

For this we use the read keyword followed by the name of the variable.

In the following example we are taking user input and saving it in variable gameScore.

read gameScore

Printing value of a variable

We use the $ dollar sign to print the value stored in the variable.

In the following example we will print the value stored in the variable gameScore.

echo $gameScore

Null variable

These are the variables without any value. We can create one in the following manner.

x=""
y=''
z=

In the above code all three will create null variable having no value.

If we echo a null varibale then a blank line in shown in the terminal.

Readonly variable

If we want to fix the value stored in a variable i.e., to make it readonly then we use the readonly keyword.

readonly a=10

The value of a readonly variable can't be changed later in the script.

Unset variable

To unset or erase the value and the variable from the shell memory we use the unset keyword.

In the following example we are unsetting the value and the variable tempResult.

unset tempResult

Comments

To create comments in shell script we start the line with a # hash sign.

Comments are ignored when the script file is executed.

Write a shell script to take name of the user as input and print a greetings message

In this example we will create a username variable that will hold the name entered by the user.

Script: username.sh

#!/bin/sh
# take username
echo "Enter username: "
read username

# greetings message
message="Hello $username"

# display greetings
echo $message

Explanation:

Line 1: Shebang line.

Line 2: This is a comment line for us programmers and is ignored by shell.

Line 3: We are printing the string "Enter username:" using the echo keyword.

Line 4: Using the read keyword to read user input and save it in a variable username.

Line 5: Empty line.

Line 6: Comment line.

Line 7: At this line we are creating another variable message and assigning a string "Hello $username".

Note! The $username gives us the value stored in the variable username.

Line 8: Empty line.

Line 9: Comment line.

Line 10: At this line we are printing the value stored in the variable message.

Output:

$ sh username.sh 
Enter username: 
yusufshakeel
Hello yusufshakeel