Let's learn about creating variables in shell.

Declaring variables in Linux is very easy. We just need to use the variable name and initialize it with the required content.

$ person="Ganesh Naik"

To get the content of the variable we need to prefix $ before the variable.

For example:

$ echo person
person
$ echo $person
Ganesh Naik

The unset command can be used to delete a variable:

$ a=20
$ echo $a
$ unset a

The unset command will clear or remove the variable from shell environment as well.

$ person="Ganesh Naik"
$ echo $person
$ set

Here, the set command will show all variables declared in shell.

$ declare  -x variable=value

Here, the declare command with the –x option will make it an environmental or global variable. We will understand more about environmental variables in the next sessions.

$ set

Again here, the set command will display all variables as well as functions that have been declared.

$ env

Here, the env command will display all environmental variables.

variable=value

Whenever we declare a variable, that variable will be available in the current terminal or shell. This variable will not be available to any other processes, terminal, or shell.

Let's write a Shell script as follows:

#!/bin/bash
# This script clears the window, greets the user,
# and displays the current date and time.

clear                                   # Clear the window
echo "SCRIPT BEGINS"
echo "Hello $LOGNAME!"                  # Greet the user
echo

echo "Today's date and time:"
date                           # Display current date and time
echo          # Will print empty line

my_num=50
my_day="Sunday"

echo "The value of my_num is $my_num"
echo "The value of my_day is $my_day"
echo

echo "SCRIPT FINISHED!!"
echo

Let's see the effect of $, "", '' and \ on variable behavior:

#!/bin/bash

planet="Earth"

echo $planet
echo "$planet"
echo '$planet'
echo \$planet

echo Enter some text
read planet
echo '$planet' now equals $planet

exit 0

Output:

You will learn about the read command in the next chapters. Using read, we can ask the user to enter data, which can be stored in a variable.

From the preceding script execution, we can observe that $variable and "$ variable" can be used for displaying the content of the variable. But if we use '$variable' or \$variable, then special functionality of the $ symbol is not available. The $ symbol is used as a simple text character instead of utilizing its special functionality of getting variable content.