How to capture the output of a system command?

[J Luis]

Ok, I've spend about an hour around "run" "open", "run(pipeline(..." but no way.
In Matlab I would do

    [status, cmdout] = system(cmd);

but in Julia the most a can reach is to run the command

com = "C:/programs/GraphicsMagick/gm.exe compare -density 200 ...

   run(`com')

but I need the result of that execution.
How to?

Thanks

[Simon Kornblith]

readall(`cat test`) or similar

[J Luis]

Thanks. It turned out my main error was that I was build the Cmd object by first creating the command as a string and after wrapping it in back ticks.

But I continue in troubles. Right, I can run the command, a GraphicsMagic image comparison command, but when that comparison say the two images are different gm.exe returns an error code != 0 and Julia interprets it a command error. But it isn't, it only means the two images are different. As a consequence the julia function where this happens aborts (an example in REPL bellow).

I tried with a try catch but not even that prevented the function abortion. How can I get out of this one?

julia> readall(cm)
C:/programs/GraphicsMagick/gm.exe compare: image difference exceeds limit (0.33549 > 0.001).
ERROR: failed process: Process(`C:/programs/GraphicsMagick/gm.exe compare -density 200 -maximum-error 0.001 -highlight-color magenta -highlight-style assign -metric rmse -file V:/example_02.png C:/progs_cygw/GMTdev/gmt5/branches/5.2.0/doc/examples/ex02/example_02.ps V:/example_02.ps`, ProcessExited(1)) [1]
 in pipeline_error at process.jl:548

[J Luis]

Found a ignorestatus function. Better, but not yet good enough. It still prints a message even if using a suppression ';'

julia> readall(ignorestatus(cm));
C:/programs/GraphicsMagick/gm.exe compare: image difference exceeds limit (0.336711 > 0.001).

[Miguel Bazdresch]

What could be happening is that `gm.exe` is printing that message to STDERR, which is not captured by `readall()`. In theory, the new pipeline infrastructure should let you capture STDERR, but I haven't had the time to figure out exactly how. Maybe somebody who knows will chime in.

-- mb

[J Luis]

Right, that's indeed what is (very sadly) happening. It turned out that the stderr output is the part that I really need in this case.
The only working solution I found was to do

    run(pipeline(ignorestatus(cmd), stdout=DevNull, stderr="errs.txt"))
    t = readall("errs.txt");

but it's annoying having to write a disk file and read its contents.
 
Had some hope on this one, but no way too

julia> readall(run(pipeline(ignorestatus(cm), STDOUT, STDOUT)))
Image Difference (RootMeanSquaredError):
           Normalized    Absolute
          ============  ==========
     Red: 0.3709618690    24311.0
   Green: 0.3611391197    23667.3
    Blue: 0.2638682470    17292.6
   Total: 0.3354900017    21986.3

C:/programs/GraphicsMagick/gm.exe compare: image difference exceeds limit (0.33549 > 0.001).

ERROR: MethodError: `readall` has no method matching readall(::Void)

and this one completely hangs the REPL giviing me no choice but killing the shell window

    rd,wr=redirect_stderr();
    run(pipeline(ignorestatus(cmd), stdout=DevNull, stderr=rd))

now if try to read from 'rd'

    readall(rd)    # ---> IceAge

[Miguel Bazdresch]

See this PR: https://github.com/JuliaLang/julia/pull/12807/files

This is a change to `open` that would allow capturing STDERR the way you want. This change won't make it into v0.4, but the code in there could point you towards a solution. I hope to have some time soon to dive into this (I need to implement the same thing in Gaston).

-- mb

[J Luis]

Thanks, I'll keep an eye on that PR.
One other thing that is also missing is the possibility of returning the command status like Matlab does. In my workaround above I force it to be ignored (otherwise it may abort the function execution) but we than have no info of its value. Right, one can do a

    success(cm)

but this implies running the command twice ... and `cmd` may do a heavy job