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Portability might vary here; these assume ksh on OpenBSD 7.3.
$ FOO=42 ; echo $((FOO+1))
43
$ FOO=42 ; echo $(($FOO+1))
43
So what's the big deal?
$ FOO='42+9'
$ echo $((FOO+1))
52
$ echo $(($FOO+1))
52
Okay maybe if you somehow get arithmetic operators into your supposed-to-be-numeric strings you get some unexpected math. But it's consistent, right?
$ FOO='1+2'
$ echo $((FOO*2))
6
$ echo $(($FOO*2))
5
Whoops, that one had an order-of-operation issue.
$ FOO=0xa ; echo $((FOO-1)) ; echo $(($FOO-1))
9
9
$ FOO=077 ; echo $((FOO-1)) ; echo $(($FOO-1))
62
62
Octals and hexadecimal inputs will be acted on.
Probably the FOO='1+2' and similar forms are undefined behavior, and hopefully never show up unexpectedly in a shell script of yours that is trying to math. (I'd sooner switch to some other language.) `typeset -i` might force integer-ness on a variable, but then you have a case where weird input like '1+2' will be silently accepted. Others might prefer the script to blow up so someone can review where the bad input came from.
$ FOO=1+2
$ echo $((FOO*2)) $(($FOO*2))
6 5
$ typeset -i FOO
$ echo $((FOO*2)) $(($FOO*2))
6 6
This is mostly a problem for scripts that process uncontrolled inputs that come from who knows where and contain who knows what and get handled as arithmetic. So probably pretty rare.
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04
tags #unix #sh