if return="nothing"; then... else...

[Intrepid Ibex]

Hey!

So, my 'task' is to create a Bash script that reports when the previous command returns nothing (say ls) and when it does return something.

e.g.

if [ -quiet-return ls ]; then
There's no output!
else
There's the following output!; print from memory
fi

[Bachstelze]

ls_output=`ls 2> /dev/null`
if [ "x$ls_output" = "x" ]; then
echo "No output."
else
echo $ls_output
fi

[Intrepid Ibex]

Thanks :)!!

[Intrepid Ibex]

Except of course you wouldn't need the extra "x"s there:


ls_output=`ls 2> /dev/null`
if [ "$ls_output" = "" ]; then
echo "No output."
else
echo $ls_output
fi

[Bachstelze]

Except of course you wouldn't need the extra "x"s there

I put them for a reason. Some shells do funky things when you use the empty string like that.

[Intrepid Ibex]

Oh okay. Figured you knew better but had to be sure =).

[Bachstelze]

Oh okay. Figured you knew better but had to be sure =).

To be honest, you are very unlikely to encounter such a shell today. It is stil a good idea to do it, though. As Henry Spencer wrote in his annotation to the Ninth Commandment of C:



Though some hasty zealots cry ``not so; the Millenium is come, and this saying is obsolete and no longer need be supported'', verily there be many, many ancient systems in the world, and it is the decree of the dreaded god Murphy that thy next employment just might be on one.


http://www.lysator.liu.se/c/ten-commandments.html

[geirha]

To be honest, you are very unlikely to encounter such a shell today. It is stil a good idea to do it, though. As Henry Spencer wrote in his annotation to the Ninth Commandment of C:



http://www.lysator.liu.se/c/ten-commandments.html

I can guarantee you though, that it is not needed in Bash (which the OP stated he/she was using) nor in POSIX sh. It's just ancient shells like the bourne shell that may get confused if the LHS of = starts with a -.

In bash, though, you shouldn't use the [ command, instead use [[ for testing strings and files, and (( for arithmetics.

#bash
output=$(some command)
if [[ $output ]]; then
echo "output"
else
echo "no output"
fi


If you actually want to check if a directory is empty or not, do not use ls. In fact, never use ls in a script ever, it's not designed for it. See http://mywiki.wooledge.org/BashFAQ/004 on how to do that check properly.

[Intrepid Ibex]

Bash (which the OP stated he/she was using)
Actually I'm _using_ Zsh.


do not use ls. In fact, never use ls in a script ever, it's not designed for it.
Sure? Lots of things aren't (designed for lots of things) but thanks for the heads up.