Linux Bash If-Else Conditionals Tutorial with Examples

If-Elif operators are used to designing the flow of the bash script. We can change the flow according to given conditions with If-Elif . If-Else is one of the most used keywords or structures in Bash in order to make a different execution path according to the situation. For example, if given process ID or programs executes kill this process or rerun the process etc. logic can be implemented with the If-Else conditionals.

If-Else syntax is dynamic where a single condition or multiple conditions can be checked with it. Below we will provide a generic If-Else syntax for single or multiple conditions.

Single Condition If-Else

The most basic and easy implementation of the If-Else is single condition If-Else. We will just use the if , then and fi keywords by provides the CONDITION and CODE_BLOCK. Below you can see the syntax. If the CONDITION_1 returns true or 1 the CODE_BLOCK_1 will be executed.

if CONDITION_1; then
   CODE_BLOCK_1
fi

Below we will make a very simple example related to the single condition if-else. We will set the age variable value and check if the age is above the 20 and print some message to the screen.

age=25

if [ $age -gt 20 ]; then
   echo "You age is above 20"
fi

Two Condition If-Else

Two conditions if-else is another popular use case. We can specify two conditions where the second condition is reverse or not of the first condition. For example, if your age is above 20 bla bla bla, if not bla bla bla. In this syntax we do not have to specify the second condition explicitly but if we need we can use the Multiple Condition If-Else.

if CONDITION_1; then
   CODE_BLOCK_1
else
   CODE_BLOCK_2
fi

Below example we will check given age and print messages for above the 20 and equal to or below 20.

age=25

if [ $age -gt 20 ]; then
   echo "You age is above 20"
else 
   echo "You age is equal or below 20"
fi

Multiple Condition If-Else

The most dynamic and usefull type of if-else is multiple condition if-else. We can specify two or more conditions to meet. There is not limit the count of condition like we can specify 10 or 100 conditions by using the elif keyword.

if CONDITION_1; then
   CODE_BLOCK_1
elif CONDITION_2; then
   CODE_BLOCK_2
elif CONDITION_3; then
   CODE_BLOCK_3
...
else
   CODE_BLOCK_4
fi

In the following examle we will check if the given age is above 20, below 5 or non of them and print message accordingly.

age=25

if [ $age -gt 20 ]; then
   echo "You age is above 20"
elif [ $age -lt 5 ]; then
   echo "Your age is lower than 5"
else 
   echo "You age is between 5 and 20"
fi

If Conditional

When writing scripts we have a lot of situations to make decisions. We solve the questions marks of the scripts with conditionals. For example, the simplest one is we want to run a script if a directory or file exists.

if [ Clean.sh ] 
    then echo "Clean.sh exist" 
fi

If  Clean.sh exists then echo

The most used and simple conditional is if/then/else statement. The statement starts with if and lists conditions and closed with fi. Here [ … ] brackets look for the file and provide logic for if. Do not be afraid there is a lot of functions to test a lot of things. We will touch some of them.

if grep -q tmp Clean.sh
    then echo "File have temp string"
fi

File Test Operators

While scripting there will be a lot of file interactions. Bash provides a lot of options for file testing. For example to check if a file exists to use the following statement

file="Clean.sh" 

if [ -e $file ] 
    then echo "Clean.sh exists"  
fi

Comparison Operators

In order to compare two or more variables or values comparison operators are used. In this example is the count is greater than 5 below example is used. The greater than operator is -gt .

count=6 

if [ $count -gt 5 ] 
    then echo "count is greater then 5"                                                
fi

Condition Check Operators

Like and programming language Bash if-else provides different condition check operators which can be used for different cases like check file existence, number comparison, file type etc. These condition check operators can be categorized as File Conditions , Number Conditions , String Conditions.

File Conditions

File conditions provide following checks.

Operator	Description
-e	Checks if the file exists or not
-d	Checks if the file is a directory
-b	Checks if the file is a block device
-c	Checks if the file is a character device
file1 -nt file2	If file file1 is newer than file file2
file1 -ot file2	If file file1 is older than file file2
-r	File can be read (read permission)
-w	File can be modified (write permission)
-x	File can be executed (execute permission)

Number Conditions

Number conditions provide following checks.

Operator	Description
num1 -eq num2	Check if numbers are equal
num1 -ne num2	Check if numbers are not equal
num1 -lt num2	Checking if num1 is lower than num2
num1 -le num2	Lower or equal than num2
num1 -gt num2	Greater than num2
num1 -ge num2	Greater or equal than num2

String Conditions

String conditions provide following checks.

Operator	Description
str1 = str2	Checks if strings are equal or not
str1 != str2	Checks if strings are different
-z str1	Checks if str1 is empty
-n str1	Checks if str1 is not empty