Unnecessary use of
$ / ${} with arithmetic variable SH-2004
Anti-pattern
Critical
a month ago
—
2 months old
$/${} is unnecessary on arithmetic variables
11 echo "0" > $numPath/num
12fi
13num=`cat $numPath/num`
14num=$(($num + 1))15echo $num > $numPath/num
16
17name="$numPath/message_$num.eml"$/${} is unnecessary on arithmetic variables
11 echo "0" > $numPath/num
12fi
13num=`cat $numPath/num`
14num=$(($num + 1))15echo $num > $numPath/num
16
17name="$numPath/message_$num.eml"
Description
Use of $ or ${..} on regular variables in arithmetic context is unnecessary, and can even lead to subtle bugs. This is because the content of $((..)) is first expanded into a string, and then evaluated as an expression:
$ a='1+1'
$ echo $(($a * 5)) # becomes 1+1*5
6
$ echo $((a * 5)) # evaluates as (1+1)*5
10
The $ is unavoidable for special variables like $1 vs 1, $# vs #.
It is also required when adding modifiers to parameters expansions, like ${#var} or ${var%-}.
Problematic code:
echo $(($n + ${arr[i]}))
Preferred code:
echo $((n + arr[i]))