How to assign output of compare command to a variable

Question

i have a problem with compare command.

I use this to output result to screen, but I receive nothing ( echo is empty ) but after executing command I obtain a numeric value

COMP=`compare -metric PSNR 00000003.jpg 00000004.jpg difference.png`<br>
echo "$COMP"

I tried this:

OUTPUT="$(compare -metric PSNR 00000003.jpg 00000004.jpg difference.png)"
echo "${OUTPUT}"

But it doesn't help

@shellter Thank you for your reply, but the problem was that compare outupt to `stderr` , not to `stdout` as Mark Setchell already told. — codeNinja7, Dec 08 '15 at 17:02

score 1 · Accepted Answer · answered Dec 08 '15 at 16:37

1

compare writes on stderr not stdout, so use the following to divert stderr to stdout:

comp=$(compare -metric PSNR 1.png 2.png diff.png 2>&1)
echo $comp

answered Dec 08 '15 at 16:37

Mark Setchell

Than you so much! And for the future, how can i understand to where command is writing stderr or stdout ? – codeNinja7 Dec 08 '15 at 16:43
Glad it worked for you. One way to test is to try it like this `command 1> 1.txt 2> 2.txt` and then look in file `1.txt` and see what went to `stdout` and look in `2.txt` and see what went to `stderr`. – Mark Setchell Dec 08 '15 at 16:45
many thanks for your help! – codeNinja7 Dec 08 '15 at 16:49

1 Answers1