Mirror of forum.nim-lang.org

4997 :: Choosing the right `osproc` function

• sschwarzer (orginal) [2019-07-08T19:03:05+02:00] view original

  First some background about me: I started to use Nim only recently. It looks very interesting. :-) My programming experience of the last 20 years is mostly Python (wrote a book and several articles, and gave about 15 talks), but I also programmed in compiled languages, e. g. Pascal, C++ and Fortran. I'm working almost exclusively on Linux.

  My current problem is this:

  I want to run an external process, with the following conditions:

  • There may be untrusted arguments, so I want them escaped or better not use the shell at all.
  • I don't want to show the output (neither stdout nor stderr) on the screen, i. e. I don't want the option poParentStreams.
  • I want to wait until the program finishes.
  • After the program run, I want to check the return code and fail if it's not 0.
  • (Although I don't need it right now, in other cases I might want the content of stdout and/or stderr as well.)

  Now, looking through the documentation https://nim-lang.org/docs/osproc.html my understanding is

  • execProcess takes a list of arguments and doesn't need a shell, but the function only returns a string and not the return code.
  • execCmd and execCmdEx take just a string and thus don't seem to deal with untrusted arguments.
  • startProcess looks flexible, but to wait for the program to exit, I need to call waitForExit. However, in this case the documentation says it may deadlock unless you use poParentStreams (which I don't want).

  I saw the source code of execCmdEx at https://github.com/nim-lang/Nim/blob/master/lib/pure/osproc.nim#L1314 and would probably be able to apply this for my usecase. On the other hand, I imagine it should be simpler to achieve what I want with the standard library and wonder if I'm missing something. What do you recommend?

---

• Araq (orginal) [2019-07-09T13:18:47+02:00] view original

  I would adapt execCmdEx for your purpose:

  proc myExec(command, wd: string): tuple[output: string, exitCode: int] =
    var p = startProcess(command, workingDir = wd, options = {poStdErrToStdOut})
    var outp = outputStream(p)
    close inputStream(p)
    
    result = ("", -1)
    var line = newStringOfCap(120).TaintedString
    while true:
      if outp.readLine(line):
        result[0].string.add(line.string)
        result[0].string.add("\n")
      else:
        result[1] = peekExitCode(p)
        if result[1] != -1: break
    close(p)
  Run

---

• juancarlospaco (orginal) [2019-07-09T15:51:56+02:00] view original

  Check Firejail on Nimble.

---

• sschwarzer (orginal) [2019-07-09T23:55:53+02:00] view original

  @Araq Thanks, I'll try it. :-)

---

• sschwarzer (orginal) [2019-07-10T00:11:24+02:00] view original

  @juancarlospaco Thanks!

  firejail seems to do much more than I want, and from reading a bit about the Nim package and the underlying library, I must admit that I don't even understand how I would apply it to my usecase. I only want to call the external process from a small command line program. (That said, my program won't be executed by the user directly, but implicitly from another program the user runs. I can elaborate on this if you're interested.)

---