One of the most popular arithmetic operations when addressing Bash scripts is incrementing and decrementing variables. This is most regularly used in loops as a counter, but it can happen elsewhere in the script.
Incrementing and Decrementing center on adding or subtracting a value (usually 1), respectively, from the value of a numeric variable. The arithmetic augmentation can be performed using the double parentheses ((…)) and $((…)) or with the built-in let command.
In bash, there are various ways to increment/decrement a variable. In this article, some are explained.
- Advanced Bash Scripting Guide
- Writing Scripts on Linux using Bash
- How To Set Environment Variable in Bash
Using + and – Operators
The easiest way to increment/decrement a variable is by using the + and – operators.
i=$((i+1)) ((i=i+1)) let "i=i+1"
i=$((i-1)) ((i=i-1)) let "i=i-1"
This method enables you to increment/decrement the variable by any value you want.
Here is an example of incrementing a variable within an until loop:
i=0 until [ $i -gt 3 ] do echo i: $i ((i=i+1)) done
i: 0 i: 1 i: 2 i: 3
The += and -= Operators
In addition to the basic operators explained above, bash also provides the assignment operators += and -=. These operators are used to increment/decrement the left operand’s value with the value specified after the operator.
((i+=1)) let "i+=1"
((i-=1)) let "i-=1"
In the example below, we will show how to decrement the I variable’s value by 5.
i=20 while [ $i -ge 5 ] do echo Number: $i let "i-=5" done
Number: 20 Number: 15 Number: 10 Number: 5
Using the ++ and — Operators
The ++ and — operators increment and decrement, respectively, its operand by 1 and return the value.
((i++)) ((++i)) let "i++" let "++i"
((i--)) ((--i)) let "i--" let "--i"
The operators can be used before or after the operand. They are also known as:
- prefix increment: ++i
- prefix decrement: –i
- postfix increment: i++
- postfix decrement: i–
The prefix operators first increment/decrement the operators by one and then return the new value to the operator. On the other hand, the postfix operators return the operators` value before incrementing/decremented the operator’s value.
If you only want to increment/decrement the variable, then there is no difference if you use the prefix or postfix operator. It only makes a difference if the operators’ result is accepted in some other operation or assigned to another variable.
The example below will show how the ++ operator works when is used before and after its operant:
x=5 y=$((x++)) echo x: $x echo y: $y
x: 6 y: 5
x=5 y=$((++x)) echo x: $x echo y: $y
Copy x: 6 y: 6
The example below shows us how to use the postfix incrementor in a bash script:
#!/bin/bash i=0 while true; do if [[ "$i" -gt 3 ]]; then exit 1 fi echo i: $i ((i++)) done
The downside of using these operators is that the variable can only be incremented or decremented by one only.
In bash, incrementing and decrementing variables can be performed in many different ways. So whatever method you use, the result is the same. If you have any query related to this, please write us.