Capture result of ImageMagick compare to a variable

Question

I am using a software called ImageMagick, that has command line inputs to do photo comparisons and output a value. I want to save that output value to a specific variable for further analysis. How would I do that? Here is the command I would run in command line:

compare -metric MSE Picture1.jpg Picture2.jpg difference.png

This will compare the pixel differences between picture 1 and picture 2. Save it as difference and output a number of the difference. If identical it will be 0. I want to be able to capture that value in terms of a variable.

This is a really basic operation in shell scripting. You obviously need to start by reading a tutorial. — Barmar, Apr 15 '16 at 23:16
See: [How to set a variable equal to the output from a command in Bash?](http://stackoverflow.com/q/4651437/3776858) — Cyrus, Apr 15 '16 at 23:18
In shell scripting it's important to distinguish between a command's [stdout] _output_ and its _exit code_, which are separate aspects. With the benefit of hindsight: what you're looking for is the exit code. — mklement0, Apr 16 '16 at 03:23

score 7 · Answer 1 · answered Jul 25 '16 at 07:05

7

Worked for me:

variable=$(compare -metric MSE Picture1.jpg Picture2.jpg difference.png 2>&1)

or

variable=$(compare -metric MSE Picture1.jpg Picture2.jpg null: 2>&1)

answered Jul 25 '16 at 07:05

user42

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score 3 · Answer 2 · edited May 23 '17 at 12:08

As noted in a comment by Ignacio Vazquez-Abrams, compare actually reports the similarity status via its exit code, not via its stdout output.

Ignacio, in his own answer, also mentions that

the most recent command's exit code is reported in special variable $?
it's worth saving that exit code in a variable right away, so that subsequent commands don't overwrite it.

Also, exit code 0 indicates that images are similar, not identical.

Here's a code snippet that shows how to save the exit code and handle it subsequently:

compare -metric MSE Picture1.jpg Picture2.jpg difference.png # add >/dev/null for no output
ec=$?  # save exit code
case $ec in  # evaluate exit code
  0)
    echo "images are similar"
    ;;
  1)
    echo "images are dissimilar"
    ;;
  *)
    echo "an unexpected error occured"
    ;;
esac

score 1 · Answer 3 · answered Apr 15 '16 at 23:15

1

Use command substitution.

variable=$(compare -metric MSE Picture1.jpg Picture2.jpg difference.png)

answered Apr 15 '16 at 23:15

Barmar

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2

That's what I thought they were asking as well, but then ["The compare program returns 2 on error otherwise 0 if the images are similar or 1 if they are dissimilar."](http://imagemagick.org/script/compare.php) – Ignacio Vazquez-Abrams Apr 15 '16 at 23:17

score 1 · Answer 4 · answered Apr 15 '16 at 23:16

1

The result of a command will always be in $?. Make sure to save that value in another variable before executing another command or it will be overwritten.

answered Apr 15 '16 at 23:16

Ignacio Vazquez-Abrams

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1,358

score 0 · Answer 5 · answered Apr 15 '16 at 23:40

0

Or use backticks . AKA , command substitution

variable=`compare -metric MSE Picture1.jpg Picture2.jpg difference.png`

echo "$variable"

answered Apr 15 '16 at 23:40

z atef

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This should be comment on [Barmar's answer](http://stackoverflow.com/a/36658287/45375). While the backtick syntax works fine in this simple case, you should mentions its drawbacks compared to the modern `$(...)` syntax. – mklement0 Apr 16 '16 at 23:10

Capture result of ImageMagick compare to a variable

5 Answers5