Bash if else usage guide for absolute beginners

I am quite sure in almost all the scripts we always end up with one or more conditional checks which requires if and else statement. So bash is not different and even in shell scripting we get the requirement to use similar decision making logic.

You can use bash if else in any section of your shell script such as for loops, while loops, functions etc. This can be further enhanced to use if..elif..else and nested if so we will cover all such possible scenarios in this tutorial guide.

I will try to be as descriptive as possible from my side so that even an absolute beginner can understand and start using this in their script. But you must be familiar with below two topics

• How to perform comparison between two or multiple strings
• How to perform comparison between two or multiple integers or numbers

As most of the time you will end us using these conditional operators with if statements

 

if..fi statement

The very first and basic if statement would be with single condition.

Syntax

The syntax to use this statement would be:

if [CONDITION]
then
	COMMAND
fi

The one liner syntax would be:

if [CONDITION]; then COMMAND1; COMMAND2; fi

• You can replace CONDITION with any kind of check which requires a decision
• If the CONDITION returns TRUE i.e. SUCCESS then COMMAND will be executed
• if the CONDITION returns FALSE i.e. FAILED then this will do nothing as we don't have any else statement
• CONDITION is sort of mandatory here, now this CONDITION can be a lot of things which basically requires a DECISION

 

Flowchart

 

Shell script Example

Let us look at some shell script examples to understand this properly:

• In this script I have defined two variables and assigned a number to both the variables
• We want to check if VAR1 is greater than or equal to VAR2 so we are using comparison operator for numbers

# cat /tmp/shell_script.sh
#!/bin/bash

VAR1=10
VAR2=5

if [[ $VAR1 -ge $VAR2 ]]
then
   echo "$VAR2 is less than or equal to $VAR1"
fi

Make sure you give executable permission to your script

# chomd u+x /tmp/shell_script.sh

Execute the script and verify the output:

# /tmp/shell_script.sh
5 is less than or equal to 10

The one liner version of this script would be:

# VAR1=10; VAR2=5; if [[ $VAR1 -ge $VAR2 ]]; then echo "$VAR2 is less than or equal to $VAR1"; fi
5 is less than or equal to 10

 

if..else..fi statement

I would recommend to always use "else" statement wherever possible because ELSE condition denotes FAILED scenario.

Syntax

I will explain this with some examples but first let us check the syntax for using if..else..fi in bash

if [CONDITION]
then
   SUCCESS_COMMAND
else
   FAILED_COMMAND
fi

The one liner syntax for this would be:

if [CONDITION]; then SUCCESS_COMMAND ; else FAILED_COMMAND ; fi

• The syntax is similar to if..fi statement, with a single difference that now we also have an else statement
• So if CONDITION is TRUE then SUCCESS_COMMAND would be executed
• if CONDITION is FALSE then FAILED_COMMAND would be executed
• When we say TRUE and FALSE, we mean the exit code of the condition should be 0 to be TRUE and non-zero to be FALSE

 

Flowchart

 

Shell script Example-1

Let us enhance our last example with comparison operator but this time we will modify the variable values:

#!/bin/bash

VAR1=10
VAR2=50

if [[ $VAR1 -ge $VAR2 ]]
then
   echo "exit_status is: $?"
   echo "$VAR2 is less than or equal to $VAR1"
else
   echo "exit_status is: $?"
   echo "$VAR2 is greater than or equal to $VAR1"
fi

The output from this script:

exit_status is: 1
50 is greater than or equal to 10

• The CONDITION was checking if $VAR1 is greater than equal to $VAR2
• Since VAR1 is NOT greater than VAR2 in this script, the condition becomes FALSE i.e. non-zero exit code
• So for FALSE condition, the else statement is executed

 

Shell script Example-2

Let us take one practical example, in this script we are trying to locate a file by the name data_file inside /opt

#!/bin/bash

FILE=`find /opt -name data_file -type f`

# Check if FILE variable is empty or not
if [[ ! -z $FILE ]]
then
   echo "exit_status is: $?"
   echo "\$FILE variable is not empty, which means file found"
   echo "File path: $FILE"
else
   echo "exit_status is: $?"
   echo "\$FILE variable is empty, so file not found"
fi

In this script we will

1. Find for a file named data_file under /opt
2. Store the path of the found file in FILE variable
3. IF the file is found the the length of FILE variable will be non-zero so we will verify the same using -z. If you are not familiar with this I would recommend reading about comparison operators
4. Check the status of find command using if..else..fi statement

The output from the script for success scenario i.e. when the file is found under /opt

exit_status is: 0
$FILE variable is not empty, which means file found
File path: /opt/data_file

 

Shell script Example-3

Now in this example we had store the path into variable, this is required only if you want to process the files which are found. If you just want to check if the files are there or not, you can use the find command in the if condition:

if find /opt -name data_file -type f >& /dev/null
then
   echo "exit_status is: $?"
   echo "\$FILE variable is not empty, which means file found"
   echo "File path: $FILE"
else
   echo "exit_status is: $?"
   echo "\$FILE variable is empty, so file not found"
fi

So I am running the complete find command in the if statement instead of storing it in variable. Make sure you send the output of such commands to NULL (>& /dev/null) or else you will get output on the screen which will not look good

 

if..elif..else..fi statement

You can add multiple conditions between if..else statement by using elif.

Syntax

The syntax to use this would be:

if CONDITION_1
then
   COMMAND
elif CONDITION_2
then
   COMMAND
..
else
   COMMAND
fi

You can add multiple elif in the block between if..else statement depending upon your requirement. The else statement is optional and you can choose to only use if..elif..fi

if CONDITION_1
then
   COMMAND
elif CONDITION_2
then
   COMMAND
..
fi

 

Flowchart

 

Shell script Example-1

• The check would happen in sequential order from top to bottom so you should place your conditions accordingly in the same order.
• Let us evaluate if..elif..else..if statement with some shell script examples:
• In this script I have two variables namely VAR1 and VAR2 for which both I have assigned the same value
• Now in the if condition I will check if $VAR1 is greater than or equal to $VAR2
• In elif block I will check if $VAR1 is equal to $VAR2
• The else block will be executed if both if and elif condition return FALSE

#!/bin/bash

VAR1=10
VAR2=10

if [[ $VAR1 -ge $VAR2 ]]
then
   echo "exit_status is: $?"
   echo "$VAR2 is less than or equal to $VAR1"
elif [[ $VAR1 -eq $VAR2 ]]
then
   echo "exit_status is: $?"
   echo "$VAR2 is equal to $VAR1"
else
   echo "exit_status is: $?"
   echo "$VAR2 is greater than or equal to $VAR1"
fi

The output from this script

exit_status is: 0
10 is less than or equal to 10

So the first condition was TRUE because VAR1 is greater than or equal to VAR2

Did you noticed the mistake I did here?

• I have added duplicate conditions in if and elif block.
• This is something which I was trying to highlight earlier, that bash will execute this in sequential order so you should know how to put the conditions
• The elif block will never be called if both VAR1 and VAR2 are equal as that will match in the if condition
• The proper way to do it would be either use -gt in the if condition to only check for greater than or you create a separate if..fi block only to check if VAR1 and VAR2 are equal

 

Shell script Example-2

This shell script example also uses multiple if and elif blocks

#!/bin/bash

VAR1=10

if [[ $VAR1 -eq 5 ]]
then
   echo "exit_status is: $?"
   echo "$VAR1 is equal to 5, matched in if block"
elif [[ $VAR1 -eq 10 ]]
then
   echo "exit_status is: $?"
   echo "$VAR1 is equal to 10, matched in elif block"
else
   echo "$VAR1 is neither equal to 5 nor 10, no match found"
fi

The condition is trying to match VAR1 is certain value and based on the match, respective block would be executed. The output from this script:

exit_status is: 0
10 is equal to 10, matched in elif block

Please NOTE for if and elif block the CONDITION should be TRUE for the respective block to be executed. If the CONDITION is FALSE then else block will be executed.

 

Nested if statement

If can also put if condition inside another if..elif..else..fi block.

 

Syntax

The syntax to use nested if condition would be same as earlier, we just need to add the if block inside existing section of COMMAND

if CONDITION_1
then
   if [ CHILD_CONDITION_1 ]
   then
       NESTED_COMMAND
   elif [ CHILD_CONDITION_1 ]
   then
       NESTED_COMMAND
   ..
   fi
elif CONDITION_2
then
   if [ CHILD_CONDITION_1 ]
   then
       NESTED_COMMAND
   elif [ CHILD_CONDITION_1 ]
   then
       NESTED_COMMAND
   ..
   fi
..
else
   COMMAND
fi

Here using elif and else is optional, I have only placed it there for better understanding. You can choose to only use nested if without elif and else block

 

Shell script Example

Let us understand this better with some shell script example:

• In this script, I will prompt user to give some numbers as input separated by white space character
• We will store this input into NUM variable
• Then we will check if NUM is empty of not using the first if condition
• If NUM is empty i.e. user has not provided any input then else condition will be executed
• If user gives some input then we have again created nested if block under the main if block
• This nested if condition will count the numbers available in NUM variable
• I have used wc -w to count the number of words in NUM variable
• Then based on this value I will check if number of words in NUM variable is equal to 1 or 2 or 3 or more than 3 in respective nested if..elif..elif..else block

#!/bin/bash

read -r -p "Enter a list of numbers separated by whitespace char: " NUM

if [[ ! -z $NUM ]]
then
   echo "Good, the user has provided some input"
   echo "But how may words has user given as input?, Let me check"
   WORD_COUNT=`echo $NUM | wc -w`
   if [ $WORD_COUNT -eq 1 ];then
      echo "User has given one number as input"
   elif  [ $WORD_COUNT -eq 2 ];then
      echo "User has given two numbers as input"
   elif  [ $WORD_COUNT -eq 3 ];then
      echo "User has given three numbers as input"
   else
      echo "User has given more than 3 numbers as input"
   fi
else
   echo "\$NUM is empty, did you provide any input?"
fi

The output from this script when I give more than 3 input words

Enter a list of numbers separated by whitespace char: 1 2 3 4
Good, the user has provided some input
But how may words has user given as input?, Let me check
User has given more than 3 numbers as input

The output from this script if I don't give any input

Enter a list of numbers separated by whitespace char:
$NUM is empty, did you provide any input?

 

if multiple condition

Now above we only had a single condition per if..elif condition. But we can also add multiple conditional operator in single if condition

 

Syntax 1: Using square braces

There are different syntax you can use for putting multiple condition in single if condition

if [[ CONDITION_1 || CONDITION_2 ]] || [[ CONDITION_3 && CONDITION_4 ]]
then
     SUCCESS_COMMAND
else
     FAILED_COMMAND
fi

You can use || to put OR condition and && to use AND condition. Replace these condition operator in the above syntax as per your requirement
We will use this syntax in our sample script to compare two numbers

#!/bin/bash

read -r -p "Enter first number: " NUM1
read -r -p "Enter second number: " NUM2

if [[ $NUM1 -eq 1 || $NUM -eq 2 ]] || [[ $NUM1 -eq 3 && $NUM2 -eq 4 ]]
then
   echo "SUCCESS"
else
   echo "FAILED"
fi

 

Syntax 2: Using parenthesis

You can also use double parenthesis instead of square braces with similar conditional operator as used with square braces:

if (( CONDITION_1 || CONDITION_2 )) || (( CONDITION_3 && CONDITION_4 ))
then
     SUCCESS_COMMAND
else
     FAILED_COMMAND
fi

We will use this syntax in our sample script to compare two numbers

#!/bin/bash

read -r -p "Enter first number: " NUM1
read -r -p "Enter second number: " NUM2

if (( $NUM1 -eq 1 || $NUM -eq 2 )) || (( $NUM1 -eq 3 && $NUM2 -eq 4 ))
then
   echo "SUCCESS"
else
   echo "FAILED"
fi

 

Syntax 3: Combine parenthesis with square brace

The third syntax would be combination of parenthesis and square braces:

if [ \( CONDITION_1 -a CONDITION_2 \) -o \( CONDITION_3 -a CONDITION_4 \) ]
then
     SUCCESS_COMMAND
else
     FAILED_COMMAND
fi

Here -a represents AND condition and -o represents OR condition. You can choose to add more blocks or just use single block in these syntax based on your requirement.

In this example we will compare two number's value using different conditional operator with single parenthesis and curly braces

#!/bin/bash

read -r -p "Enter first number: " NUM1
read -r -p "Enter second number: " NUM2

if [ \( "$NUM1" -eq 1 -a "$NUM2" = 2 \) -o \( "$NUM1" -eq 3 -a "$NUM2" = 4 \) ]
then
   echo "SUCCESS"
else
   echo "FAILED"
fi

This is another practical example where we will ask for user confirmation as INPUT with Yes/No: It is possible end user gives Yes or YES or yes/No or NO or no so we need to handle all these answers:

#!/bin/bash

read -r -p "Do you want to confirm: " INPUT

if [ \( "$INPUT" == "YES" -o "$INPUT" == "Yes" -o "$INPUT" == "yes" \) ]
then
   echo "SUCCESS with Yes"
elif [[ $INPUT == "NO" || $INPUT == "No" || $INPUT == "no" ]]
then
    echo "SUCCESS with No"
else
   echo "Unable to identify the answer"
fi

In the if condition we are using -o to check the OR condition across all the answers for YES
and with elif we are using || to check the OR condition across all the answers for NO

The output from this script if user gives "Yes" as INPUT:

Do you want to confirm: Yes
SUCCESS with Yes

The output from this script if user gives "No" as INPUT:

Do you want to confirm: No
SUCCESS with No

The out from this script where there is some un-supported INPUT

Do you want to confirm: None
Unable to identify the answer

 

Conclusion

In this tutorial we learned about different possible syntax which we can use with if..else statement in bash. It is important that you are familiar with different comparison operator used for strings, integers, under parenthesis etc because without proper conditional operator you may get errors like

[: YES: integer expression expected
syntax error near unexpected token
[: too many arguments
syntax error in conditional expression

and similar other errors.

The if condition in bash is very useful but you must check the order in which which you perform the check, as I have also shown in this tutorial, bash will perform sequential check from top to bottom.

If you do not arrange your if..elif..elif..else..fi condition, then most likely the results will not be correct. Lastly I hope the steps from the article to use bash if else statement on Linux was helpful. So, let me know your suggestions and feedback using the comment section.