Incrementing And Decrementing In Bash With Prefix And Postfix Operators

The aim of this page📝 is to show the 3 ways of incrementing and decrementing a number in bash. Basically notes from How to Increment and Decrement Variable in Bash (Counter) | Linuxize.

I used the third option in the C-style loop needed for writing a shell script implementing a wrapper around Snowplow Analytics CLI (called via track_script a function that looked something like) that fired as many events as passed from the shell.

request_count=$1
for ((i = 1; i <= request_count; i++)); do
    echo "request #${i}:"
    track_script "$job_tag" "STARTING" "$cluster_type" "$band" "$size_kb"
done

1. + and -

• The most simple 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 allows 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

2. += and -=

• In addition to the basic operators explained above, bash also provides the assignment operators += and -=
• These operators are used to increment/decrement the value of the left operand with the value specified after the operator.

((i+=1))
let "i+=1" 
((i-=1))
let "i-=1"

• In the following while loop, we are decrementing the value of the i variable by 5.

i=20

while [ $i -ge 5 ]
do
  echo Number: $i
  let "i-=5" 
done
Number: 20
Number: 15
Number: 10
Number: 5

3. ++ and —

• 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:

MEANING           | SYNTAX
------------------|-------
prefix increment  | ++i
prefix decrement  | --i
postfix increment | i++
postfix decrement | i--

• Usually no difference; matters only if the result is bound elsewhere

x=5
y=$((x++))
echo x: $x
echo y: $y
x: 6
y: 5
x=5
y=$((++x))
echo x: $x
echo y: $y
x: 6
y: 6

• Below is an example of 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

4. links

• https://linuxize.com/post/bash-increment-decrement-variable/#using-the--and----operators