Unnecessary use of
$ / ${} with arithmetic variable SH-2004
Anti-pattern
Critical
a year ago
—
a year old
$/${} is unnecessary on arithmetic variables
6 local seconds="${1}"
7 local d=$(($(date +%s) + "${seconds}"))
8 while [ "$d" -ge `date +%s` ]; do
9 echo -ne "$(date -u --date @$(($d - `date +%s`)) +%H:%M:%S)\r";10 sleep 0.1
11 done
12}
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]))