Increment or Decrement Counters in Bash Scripting

Incrementing/decrementing is mostly used in a loop where the user has instructed the script/program to increase/decrease value based on the given condition.

And bash allows you to use various ways to increment or decrement the value.

So in this tutorial, I will explain three ways to do so:

• Using + and - operators
• Using += and -= operators
• Using ++ and  -- operators

Each works the same so you may choose the best possible method for your workflow.

How to increment or decrement variable in bash

Incrementing and decrementing are arithmetic operations that are performed using double parentheses ((...)) and $((...)) or you can also use the built-in let command.

I know it sounds too complex so I will be sharing syntax for both so you can choose the best for you.

1. Using + and – operators

In my opinion, this is the most straightforward way to increment and decrement the variable in bash.

So let’s start with the syntax. If you want to use double parentheses, here’s how you do it.

For increment:

((i=i+1))

For decrement:

((i=i-1))

Using the let command

If you want to use the let command instead of double parentheses, here’s the syntax to do so.

For increment:

let "i=i+1"

For decrement:

let "i=i-1"

Example of using + and – operators

In this section, I will be sharing examples of both incrementing and decrementing a variable.

Here, I will be using double parentheses syntax for the increment example whereas let command for decrement.

Incrementing a variable:

In this example, I have used an until loop which will increment the value of the variable till gets 3 and print the value at each iteration:

i=1 until [ $i -gt 3 ]
do echo i: $i ((i=i+1))
done

And the output should look like this:

Decrementing a variable:

Here, I will be using the let command instead of the double parentheses so you can have a rough idea of how you use both.

In this example, I have used them until the loop which will iterate till the value of the i variable gets 5 or less and till then, the value of i will be decremented by two in each variable:

i=20 until [ $i -ge 5 ]
do echo i: $i let "i=i-2" done

And here’s the output:

2. Using += and -= operators

So if you want to use += and -= operators to increment and decrement the values of a variable, in this section, I will show you syntax and examples for both.

Let’s start with the syntax for the double parentheses.

For increment:

((i+=1))

For decrement:

((i-=1))

Using the let command

If you prefer to use the let instead of the double parentheses, here’s the syntax for both increment and decrement:

For increment:

let "i+=1"

For decrement:

let "i-=1" 

Now, let’s jump to the examples part.

Examples of using += and -= operators

In this section, I will be sharing an example of how you can use the += and -= operators.

Also, I will be using double parentheses for the decrement and let command for the increment example.

Incrementing a variable:

In this example, I have used a while loop which will iterate will the value of the i the variable is less or equal to 5:

i=1 while [ $i -lt 5 ]
do echo i: $i let "i+=1"
done

And here’s the output:

Decrementing a variable:

In the example of decrement, I have used the while loop which will reduce 5 and run till the value is less than 2:

i=20 while [ $i -ge 2 ]
do echo i: $i ((i-=5))
done

Here’s the output after running the loop as a script:

3. Using ++ and — operators

You can use the operators in prefix and postfix manner so let’s have a look at the difference between them:

• Prefix: If you use the operator as a prefix, like ++var, the value of the variable will be increased and then it returns the value.
• Postfix: If you use the operator as a postfix, like var++, the value of the variable will be returned first and then incremented.

So let’s start with how these operators can be evaluated in double parentheses syntax.

For increment:

((i++)) #For postfix
((++i)) #For prefix

For decrement:

((i--))	#For postfix
((--i))	#For prefix

For let command syntax

If you’re someone like me who prefers using the let instead of double parenthesis, here’s the syntax for increment and decrement.

For increment:

let "i++"	#For postfix let "++i"	#For prefix

For decrement:

let "i--"	#For postfix let "--i"	#For prefix

Now, let’s jump to the examples part.

Examples of using ++ and — operators

Here, I will be using the double parentheses syntax for the increment and let command syntax for the decrement example.

Incrementing a variable:

Here, I will be using until the loop which will increase the value till the value reaches 5:

i=1 until [ $i -gt 5 ]
do echo i: $i ((i++))
done

And here’s the output:

Decrementing a variable:

Here, I will be using the let command with the while loop which will iterate till the value of the variable becomes 5:

i=10 while [ $i -ge 5 ]
do echo i: $i let "--i" done

And here’s the output:

Want to learn bash from the scratch?

If you want to learn the bash scripting from scratch, we have a dedicated series of articles that will clear all the basics and give you a strong foundation for advancement:

Bash Tutorial Series for Beginners: Start Learning Bash ScriptingThe 10 chapter series will teach the bash scripting essentials.

I hope you will find this guide helpful. And if you have any doubts, feel free to ask in the comments.