I am working on an sh script in which I am in a WHILE loop where a variable gets incremented every iteration, and I'd like to save a file every five iterations.
What I'd normally do (say in C) would be to do an if ( n % 5 == 0) test, but I don't know whether that's possible here..? If it isn't, does anyone have any ideas that would get the job done?
Cheers!
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
-12 mod 10 is 8, bash will calculate it as -2. You can test it with simple echo $((-12 % 10)) (-2) and compare it with python3 python3 -c "print(-12 % 10)" (8).
$(xxxxx).You should use bc when doing math in shell
if [ `echo "3 % 2" | bc` -eq 0 ]
bc
sh includes arithmetic expansion since way back.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.
(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.
