Calculate modulo in sh script

Question

If your sh really is sh and not just bash being run as sh then this will work just fine

if [ `expr $n % 5` -eq 0 ]
then
    # do something
fi

If your sh is really bash then put your test in (( )) like so

if (( $n % 5 == 0 ))
then
     # do something
fi

You should use bc when doing math in shell

if [ `echo "3 % 2" | bc` -eq 0 ]

Notes on the above answers

• Backticks are considered deprecated by now (many further notes), so I advise on switching to $(( ... )) or $( ... ) constructs.

• Both expr (man page), and bc (man page) are external commands, and as such would cause some measurable slowdown in more complex examples than mine or those above. Furthermore, there already is a way to easily avoid them (both). Also, they might not be available on all systems by default for instance.

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Suggested new solution

(Might be imperfect under certain conditions, I did test just basic scenarios.)

The simplest possible while portable and POSIX-ly correct code (no Bashisms (Greg's Wiki on how to re-write Bash snippets to POSIX); (Wikipedia overview on POSIX)) to test oddity (as an example) of a number would be as follows:

#!/bin/sh
n=10
if [ $(( n % 2 )) -eq 0 ]; then
    printf '%s\n' "Number $n is Even"
else
    printf '%s\n' "Number $n is Odd"
fi

You can, of course, test any modulo there, not just if a number is even / odd.