Python subprocess examples

Description

This is a collection of python subprocess examples for easy usage. Although I found the python documents are very comprehensive, I found there is not much examples of showing how to use. This document act as a note of how to use python subprocess package.

Environment settings

Main functions

• subprocess.run()

  the code is executed in IDLE command line window.

  >>> import subprocess as sb
  >>> bash_exec = "C:\\Program Files\\Git\\bin\\bash.exe"
  >>> sb.run([bash_exe, '-c','pwd'], stdout=sb.PIPE)
  

  CompletedProcess(args=[‘C:\Program Files\Git\bin\bash.exe’, ‘-c’, ‘pwd’], returncode=0, stdout=b’/c/Users/xxx/Anaconda3/Scripts\n’)

  NOTE:

  • need to point to bash exe
  • bash needs a "-c" condition in order to run it, otherwise a returncode=126 will return. Cannot using -i tag for bash command execution.
  • return result will not be shown if stdout is not set.

• subprocess.Popen(*)

  However, I personally feel that the Popen is the most important tool of this subprocess package.

  Basic syntex

  >>> p = sb.Popen([bash_exe, '-c', 'pwd'], stdout=sb.PIPE, stderr=sb.PIPE)
  >>> p.communicate()
  

  (b’/c/Users/xxx/Anaconda3/Scripts\n’, b’’)

  this is working.

  The other one I thought it would work but actually not

  >>> p = sb.Popen(['-c','pwd'],executable=bash_exe, stdout=sb.PIPE, stderr=sb.PIPE)
  >>> p.communicate()
  

  (b’’, b’/usr/bin/pwd: /usr/bin/pwd: cannot execute binary file\n’)

  The error turn out to be a bit wired. For now, I am not sure how is this happened.

  env might be one very useful attributes when some info needs to be preloaded before executing the command.

  >>> p = sb.Popen([bash_exe, '-c', 'echo $NAME'], stdout=sb.PIPE, stderr=sb.PIPE)
  >>> p.communicate()
  

  (b’\n’, b’')

  >>> p = sb.Popen([bash_exe, '-c', 'echo $NAME'], stdout=sb.PIPE, stderr=sb.PIPE, env = {'NAME':'superman'})
  >>> p.communicate()
  

  (b’superman\n’, b’')

  context manager

  with sb.Popen([bash_exe, '-c', 'echo $NAME'], stdout=sb.PIPE, stderr=sb.PIPE, env = {'NAME':'superman'}) as proc:
  	proc.communicate()
  

  (b’superman\n’, b’')

  Not working:

  >>>with sb.Popen([bash_exe, '-c',], stdout=sb.PIPE, stderr=sb.PIPE, stdin = sb.PIPE, env = {'NAME':'superman'}) as proc:
  	proc.communicate(input=input())
  
  
  pwd
  

  (b’’, b’/usr/bin/bash: -c: option requires an argument\n')

  Eh, looks I can write a basic bash CLI?

  import subprocess as sb
  
  command = ''
  kernal = "C:\\Program Files\\Git\\bin\\bash.exe"
  
  while command!="exit":
      command = input('%: ')
      proc = sb.Popen([kernal,'-c',command], stdout=sb.PIPE, stderr=sb.PIPE, encoding = 'utf-8')
      out,err = proc.communicate()
      if err =='':
          print(out)
      else:
          print(err)
  

  However, if you input “cd” command, it actually doesn’t change its directory. SO, this CLI sucks.

  One easy solution is to use some libraries such as Pespect on Windows.

  NOTE:

  (TODO)

  this seems a good solution for building an interactive shell.

  ​