Bash if elif else Statement: A Comprehensive Tutorial

Introduction

Bash scripts help automate tasks on your machine. The if elif else statement in bash scripts allows creating conditional cases and responses to specific code results. The if conditional helps automate a decision-making process during a program.

This article explains what the if elif else statement is and shows the syntax through various examples.

Prerequisites

• A machine with Linux OS.
• Access to the command line/terminal.
• Access to a text editor like Vi/Vim.

What is the Bash if Statement?

In programming, the if statement is a conditional expression. However, the command tested in the if statement evaluates based on the exit status. Therefore:

• If the command completes successfully, the exit status is 0.
• If the statement throws an error, the exit status is any number between 1 and 255.

The zero for success and any non-zero value for failure seems counterintuitive. In most other programming languages, zero represents false, and one (or greater) represents true. However, in bash scripting, the UNIX convention returns the exit status instead of a truth value, and the two should not be confused.

Test a sample error command (1 greater than 100) in the terminal by running:

test 1 -gt 100

Check the exit status using the echo command:

echo $?

The test returns an exit code 1, indicating the expression failed.

Similarly, check a sample command that evaluates successfully (1000 greater than 100) in the terminal with:

test 1000 -gt 100

Print the exit status to confirm the command was successful:

echo $?

The test returns an exit code 0, showing the command completed without an error.

Bash if Statement Example

Follow the instructions below to create an example bash script using an if statement.

1. Open the terminal (CTRL+ALT+T) and create an example script to test how the bash if statement works:

vi test_script.sh

2. In the script, add the following lines:

echo -n "Please enter a whole number: "
read VAR
echo Your number is $VAR
if test $VAR -gt 100
then
        echo "It's greater than 100"
fi
echo Bye!

Each line in the script does the following:

• Lines 1-3 provide instructions to enter a number through the console. The number is read into a variable called VAR and printed.
• Line 4 starts the if statement and checks the exit status for the command right after ($VAR -gt 100).
• Lines 5-6 signals the start of commands to execute only if the statement in line 4 completes successfully (with an exit status 0), meaning we entered a number greater than 100.
• Line 7 signals the end of the if statement.
• Line 8 is outside of the statement and runs as expected, regardless of the if outcome.

3. Save and close Vim:

:wq

4. Next, make the file executable:

chmod +x test_script.sh

5. Lastly, run the script with:

./test_script.sh

The script outputs a different message based on the entered number. Run the script multiple times and test for other numbers to confirm the behavior.

Bash if Statement Syntax

The basic syntax for a bash if statement is:

if <commands>
then
        <commands>
fi

Each keyword has a specific function:

• if signals the statement's beginning. The command right after is the one in which the exit status check applies.
• then executes the commands only if the previous review completes successfully.
• fi closes the if statement.

Enclosing the test command in different brackets results in different execution methods for the if statement. The table below provides a short description as well as a use case for each bracket type.

Below are example bash scripts that use each bracket type with a more in-depth explanation.

Single-Parentheses Syntax

Using single parentheses in bash scripting creates a subshell. When combined with the if statement, the subprocess finishes before continuing the program. The if analyzes the exit status and acts accordingly.

The bash if statement with single parentheses syntax looks like the following:

if ( <commands> )
then
        <commands>
fi

Try the example below to see how the sub-process behaves together with the if statement:

1. Create the script using Vim:

vi single_parentheses.sh

2. Add the following lines of code to the script:

outer_variable=Defined
echo Before if:
echo inner_variable = $inner_variable
echo outer_variable = $outer_variable
if (
        echo Inside subshell:
        inner_variable=Defined
        echo inner_variable = $inner_variable
        outer_variable=Changed
        echo outer_variable = $outer_variable
)
then
        echo After then:
        echo inner_variable = $iner_variable
        echo outer_variable = $outer_variable
fi
echo After fi:
echo inner_variable = $inner_variable
echo outer_variable = $outer_variable

The program does the following:

• Line 1 creates a variable called outer_variable in which we store a string Defined.
• Lines 2-4 print the variables to the console. At this moment, outer_variable has a string stored in it, while inner_variable is blank.
• Line 5 starts the if statement and a sub-process, delimited by single parentheses.
• Line 6-11 store a string inside the inner_variable and change the outer_variable to a different string. Both values print to the console, and the sub-process ends with an exit code. In this case, the sub-process ends successfully with an exit code 0.
• Line 12-16 execute after the sub-process and print the variable values. However, the values change back to what they were before the if statement. The sub-process only stores the values locally and not globally.
• Lines 16-19 run after the commands in the then clause. The values remain unchanged outside the statement.

3. Save the script and close the editor:

:wq

4. Make the script executable:

chmod +x single_parentheses.sh

5. Lastly, run the example to test the results:

./single_parentheses.sh

The output prints the variable states as the program progresses.

Double-Parentheses Syntax

The double-parentheses syntax for a bash if statement is:

if (( <commands> ))
then
        <commands>
fi

The double parentheses construct in bash allows:

• Arithmetic evaluation. Defining a variable as a=$(( 1+1 )) calculates the equation and sets a to 2.
• C-style variable manipulation. For example, incrementing variables with (( a++ )).

When using double-parentheses syntax in an if statement, the evaluation behaves differently. Suppose the expression evaluates to 0, then the if test does not pass.

Try the following example to see how double parentheses work:

1. Create a bash script in the terminal:

vi double_parentheses.sh

2. Add the following code to double_parentheses.sh:

variable=-2
echo Before first if: $variable
if (( variable++ ))
then
        echo Incremented ++ style: $variable
fi
echo After first if: $variable
if (( variable=variable+1 ))
then
        echo Incremented arithmetically $variable
fi
echo After second if: $variable

Each line number in the script does the following:

• Line 1 defines a variable and sets the value to -2.
• Lines 3-5 increments the value C-style inside double parentheses and checks the value. If the variable is not zero, the if prints a message to the console.
• Lines 8-10 increments the variable by one using regular arithmetic notation and prints a message if the variable is not zero.

3. Save the script and close Vim:

:wq

4. Change script permissions to executable:

chmod +x double_parentheses.sh

5. Run the script to see the results:

./double_parentheses.sh

Single-Bracket Syntax

The single bracket is another name for the test command and a standard POSIX utility available for all shells. The basic syntax is:

if [ <commands> ]
then
        <commands>
fi

The first bash if example provided in this tutorial (test_script.sh) works equally well with the alternative syntax:

echo -n "Please enter a whole number: "
read VAR
echo Your number is $VAR
if [ $VAR -gt 100 ]
then
        echo "It's greater than 100"
fi
echo Bye!

Run the script to confirm the output is equivalent. For the complete documentation and details on using bracket syntax, run the man command on the test utility:

man test

Double-Bracket Syntax

The double-bracket syntax in bash if scripts is the best option if portability is not necessary. The double-brackets are superior to single-brackets and include many advanced options. The syntax is:

if [[ <commands> ]]
then
        <commands>
fi

Try the example below to see how wildcard string matching works in an if command:

1. Create a shell script file called double_brackets:

vi double_brackets.sh

2. Add the following code:

if [[ $USER == k* ]]
then
        echo Hello $USER
fi
echo Bye!

3. The script checks if the starting letter of the username is k and sends a hello message if it is. Save and close the script:

:wq

4. Make the file executable with chmod:

chmod +x double_brackets.sh

5. Run the program with:

./double_brackets.sh

Other Types of Bash Conditional Statements

The if statement only performs one conditional check. Modify the if with other types of bash conditionals to create complex assessments.

if else Statement

The if else statement provides one method to define different actions based on the output of the checked conditional. The basic syntax is:

if <command>
then
	<commands>
else
	<commands>
fi

The following example demonstrates how the if else conditional works:

1. Create a new script using Vim:

vi if_else.sh

2. Insert the following code into the script:

echo -n "Please enter a whole number: "
read VAR
echo Your number is $VAR
if [ $VAR -gt 100 ]
then
        echo "It's greater than 100"
else
        echo "It's less than 100"
fi
echo Bye!

The statement checks the command output in line 4 and prints a descriptive message based on the result:

• If the entered number is greater than 100, the program enters line 6 and prints the message.
• If the number is less than 100, the message in the else clause (line 8) prints to the console.

3. Save the script and close Vim:

:wq

4. Make the script executable:

chmod +x if_else.sh

5. Lastly, run the script multiple times and test for various values:

./if_else.sh

if elif Statement

The elif clause combined with the if else statement creates multiple conditional checks. The if elif creates a series of checks with different results. The syntax is:

if <command>
then
        <commands>
elif <command>
then
        <commands>
else
        <commands>
fi

To create a script using elif:

1. Create a shell file named elif:

vi elif.sh

2. In the elif.sh file, add the following example code:

echo -n "Please enter a whole number: "
read VAR
echo Your number is $VAR
if [ $VAR -gt 100 ]
then
        echo "It's greater than 100"
elif [ $VAR -lt 100 ]
then
        echo "It's less than 100"
else
        echo "It's exactly 100"
fi
echo Bye!

The example adds an elif check on line 7 to see if the entered number is less than 100. If the statements in lines 4 and 7 both fail, the program jumps to the else clause.

3. Save and close the file:

:wq

4. Make the elif.sh file executable:

chmod +x elif.sh

5. Run the script multiple times and check the behavior for different numbers:

./elif.sh

Add multiple elif clauses to branch out the statement for further detailed checks. For instances where the if and elif pattern series grows, the better option is to use a case statement.

Nested if Statement

Nested if statements add a branch inside the if. Specifically, when a command passes the first if check, it goes through a new check to filter the result further. The syntax is:

if <commands>
then
        if <commands>
        then
                <commands>
        fi
fi

The nested if is commonly used to search through multi-dimensional arrays. However, try to avoid having more than two or three nested if statements to reduce program complexity. Rethink the code's logic when the nested if keeps growing in depth.

Conclusion

Following this tutorial, you should know how to create an if elif else statement in a bash script and different syntaxes available. Next, check out how to implement the if statement to check if a file or directory exists in bash.