This article covers the fundamentals of Bash shell scripting and describes the most widely used logical constructs including single if statement, if-else statement, and elif statement. In addition to this, we’ll cover the correct way of comparing strings and numbers. Improve your Linux shell experience in 5 minutes!

Whenever you’re an experienced system administrator or beginner Linux user, it is worth to know the fundamentals of shell scripting. This skill gives you the ability to automate repeatable tasks within a single or across multiple servers. Usually, those tasks include software installation and configuration of any system services.

If statement

if statement serves as a fundamental core for all programming languages. This statement helps to implement decision making logic within your program or a script. That’s why it’s important to understand how to use various if statements.

Here’s the basic syntax:

if [ condition ]
then
  COMMANDS
fi

Where:

• condition – is the logical operator of the if statement.
• COMMANDS – commands, that are executed when the condition is true.
• if, then, fi – are syntax keywords.

Here’s a list of most often used test condition operators:

Operator	Description
! EXPRESSION	The EXPRESSION is false.
-n STRING	The length of STRING is greater than zero.Text
-z STRING	The length of STRING is zero (the STRING it is empty)
STRING1 = STRING2	STRING1 is equal to STRING2
STRING1 != STRING2	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is numerically equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is numerically greater than INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is numerically less than INTEGER2
-e FILE	FILE exists
-d FILE	FILE exists and it is a directory
-s FILE	FILE exists and it’s size is greater than zero (FILE is not empty.
-w FILE	FILE exists and the write permission is granted.
-x FILE	FILE exists and the execute permission is granted.
-r FILE	FILE exists and the read permission is granted.

The most interesting thing is that during Bash script execution everything in [ ] is passed to the test utility, which returns either true or false. So, if you forget the syntax of the operator, just use its man page:

man test

Let’s take a look at the simplest if statement example:

#!/bin/bash
var=2
if [ $var -eq 2 ]
then
  echo "var equals 2"
fi

Expected output:

var equals 2

In the example above, we set the variable var value to 2, then we tested if the var value equals (-eq) 2 and printed the message.

In real life, we’re usually using if statements in combination with Bash looping constructs (for-loops, while-loops, until-loops). Here’s an example of using if statement with for-loop:

#!/bin/bash
for var in 1 2 3 4 5
do
  if [ $var -eq 3 ]
  then
    echo "Value 3 found"
    break
  fi
  echo var=$var
done

Expected output:

var=1
var=2
Value 3 found

In this example, we’re walking through the list of numbers from 1 to 5 in the for-loop and printing the value of the variable, which we just processed. When value 3 is found, we’re breaking the script execution.

In addition to the above examples, you may use result of mathematical calculation in the if statement:

#!/bin/bash
for var in {1..10}
do
  if (( $var % 2 == 0 ))
  then
    echo "$var is even number"
  fi
done

In the example above we’re checking even numbers in the range between 1 and 10.

Expected output:

2 is even number
4 is even number
6 is even number
8 is even number
10 is even number

Now, you may check a Bash command output or execution result. Let’s try to check that the user exists in the /etc/passwd file.

#!/bin/bash
user='root'
check_result=$( grep -ic "^$user" /etc/passwd )
if [ $check_result -eq 1 ]
then
  echo "$user user found"
fi

In the example above we’re using grep command to search through /etc/passwd file:

grep -ic "^$user" /etc/passwd

Where:

• -i – case-insensitive search.
• -c – return the number of lines found (0 – nothing found; 1 or more when something was found).
• “^$user” – search the string which starts (^) from the value of $user variable.

Everything else within the example should be clear.

Expected output:

root user found

Next, let’s check that /etc/passwd file exists:

#!/bin/bash
file='/etc/passwd'
if [ -s $file ]
then
  echo "$file file found"
fi

Expected output:

/etc/passwd file found

If-else statement

After getting familiar with the basic if statement, let’s take a look how to use Bash if-else statement.

Here’s the general syntax:

if [ condition ]
then
  TRUE_COMMANDS
else
  FALSE_COMMANDS
fi

Where:

• condition – is the logical operator of the if statement.
• TRUE_COMMANDS – commands, that are executed if the condition is true.
• FALSE_COMMANDS – commands, that are executed if the condition is false.
• if, then, else, fi – are syntax keywords.

To demonstrate how it works, let’s change one example, where we detected even numbers. Now, let’s print which number is even and which is odd:

#!/bin/bash
for var in {1..10}
do
  if (( $var % 2 == 0 ))
  then
    echo "$var is even number"
  else
    echo "$var is odd number"
  fi
done

Here’s an expected output:

1 is odd number
2 is even number
3 is odd number
4 is even number
5 is odd number
6 is even number
7 is odd number
8 is even number
9 is odd number
10 is even number

Elif statement

Now, what if we need to check several different conditions during our script execution? That is also possible using the elif statement, which is a shortcut of “else if”.

Here’s how the syntax looks like:

if [ condition_1 ]
then
  CONDITION_1_COMMANDS
elif [ condition_2 ]
then
  CONDITION_2_COMMANDS
else
  ALL_OTHER_COMMANDS
fi

Where:

• condition_1 – is the logical condition to execute CONDITION_1_COMMANDS.
• CONDITION_1_COMMANDS – commands, that are executed if the condition_1 is true.
• CONDITION_2_COMMANDS – commands, that are executed if the condition_2 is true.
• ALL_OTHER_COMMANDS – commands, that are executed when condition_1 and condition_2 are false.
• if, then, elif, else, fi – are syntax keywords.

Let’s take a look at the following example:

#/bin/bash

time=8
if [ $time -lt 10 ]
then
  echo "Good morning"
elif [ $time -lt 20 ]
then
  echo "Good day"
else
  echo "Good evening"
fi

In this example we set up a time variable to 8 AM and printed a message based on the time value.

Expected output:

Good morning

Nesting if statements

Now, when we get familiar with the basics, we can finish this topic by learning how to nest if statements.

Here’s an example:

#/bin/bash

value=5
if [ $value -lt 10 ]
then
  if [ $value -eq 5 ]
  then
    echo "Value equals 5"
  else
    echo "Value not equals 5"
  fi
else
  echo "value is greater than 10"
fi

Here’s an expected output:

Value equals 5

Boolean operators

It might be useful to check several conditions in a single if statement. You may do it by using the following boolean operators:

• && – logical AND.
• || – logical OR.

Here’s a quick example:

#!/bin/bash
username="andrei"
birthday="12/16"
today=$( date +"%m/%d")
if [ $USER = $username ] && [ $today = $birthday ]
then
  echo "Happy birthday\!"
fi

In this simple example we’re checking two conditions to be true:

• Username of logged in user should be equal to andrei.
• Today’s date should be 16-th of December.

If both conditions are true, we’re printing “‘Happy birthday!”

Case statement

In some situations you may need to have more than 3 elif statements to check in your script. In this case using an elif statement becomes complicated. Bash provides you a better way of handling such situations by using a case statement.

Here’s the syntax:

case VARIABLE in
  VALUE_1)
    COMMANDS_FOR_VALUE_1
    ;;
  VALUE_2)
    COMMANDS_FOR_VALUE_2
    ;;
  *)
    DEFAULT_COMMANDS
    ;;
esac

Where:

• VARIABLE – the variable which value will be compared with VALUE_1 or VALUE_2.
• COMMANDS_FOR_VALUE_1 – commands to execute when VARIABLE value equals VALUE_1.
• COMMANDS_FOR_VALUE_2 – commands to execute when VARIABLE value equals VALUE_2.
• DEFAULT_COMMANDS – commands to execute when VARIABLE value was not equal to one of the upper conditions.
• case, in, esac – syntax keywords.

Here’s the very common pattern to process input to the script and start, stop or restart the service based on the user input:

#!/bin/bash

case $1 in
  start)
    echo "Starting the service"
    ;;
  stop)
    echo "Stopping the service"
    ;;
  restart)
    echo "Restarting the service"
    ;;
  *)
    echo "Unknown action"
    ;;
esac

In this example we’re checking the value of the special $1 Bash variable. It contains the first argument, provided to a script. For example, if you save the above example in a file service.sh and execute it like this:

bash service.sh start

$1 will contain the word “start” as its value.

The rest is handled by the case statement logic.

Summary

In this article we covered the fundamentals of Bash shell scripting and described the most widely used logical constructs including single if statement, if-else statement, and elif statement.

We hope you found this article useful. If yes, please, please, help us to spread it to the world!

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