Use variable value in a Math expression in Shell Scripting

Question

I am trying to use the value of the iteration counter in a FOR loop in another mathematical expression inside the loop. Example code:

#! /bin/bash

for i in {100..1}
do
    j=$($i-1)
    echo $i $j
done

However, this does not work. I want to get the output as shown below:

100 99
99 98

and so on.

Matt · Accepted Answer · 2016-03-11 09:50:19Z

3

Try this:

for i in {100..1}
do
    j=$(($i-1))  <- Here is the change.
    echo $i $j
done

answered Mar 11, 2016 at 9:50

Matt

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Comments

SLePort · Accepted Answer · 2016-03-11 10:11:53Z

2

You have to double parenthesis :

j=$(( $i-1 ))

or :

j=$(( i-1 ))

This syntax works :

$[ $i-1 ]

But you shoud avoid it. It's deprecated and will be removed in upcoming versions of Bash.

Moreover, you can decrement with :

j=i
echo $i $(( --j ))

For more about calculation in Bash :

answered Mar 11, 2016 at 9:50

SLePort

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or just not use j at all as it is redundant and is always 1 less than i, just print i-1

You're right, It doesn't make a lot of sense, it's just to cover all syntax.