redirecting output of process to a file using subprocess.Popen()

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  • rparimi@gmail.com
    #1

    redirecting output of process to a file using subprocess.Popen()

    I am trying to redirect stderr of a process to a temporary file and
    then read back the contents of the file, all in the same python
    script. As a simple exercise, I launched /bin/ls but this doesn't
    work:

    #!/usr/bin/python
    import subprocess as proc
    import tempfile
    name = tempfile.NamedT emporaryFile(mo de='w+b')
    print 'name is '+ name.name

    cmd = []
    cmd.append('/bin/ls')
    cmd.append('-l')
    cmd.append('/tmp')
    p = proc.Popen(cmd, stdout=name, stderr=proc.STD OUT, close_fds=True)
    while True:
    ret = p.poll()
    if (ret is not None):
    output = name.readlines( )
    print 'out = ', output
    break

    $python sub.py
    name is /tmp/tmpjz4NJY
    out = []


    I tried calling flush() on the file object but this didn't help
    either. Tried closing and re-opening the file, but closing the file
    object results in it getting deleted. Can the above be made to work by
    using tempfiles?

    thanks
    Tags: None
    • skeept
      #2
      Re: redirecting output of process to a file using subprocess.Pope n()

      On Jul 9, 7:32 pm, rpar...@gmail.c om wrote:
      I am trying to redirect stderr of a process to a temporary file and
      then read back the contents of the file, all in the same python
      script. As a simple exercise, I launched /bin/ls but this doesn't
      work:
      >
      #!/usr/bin/python
      import subprocess as proc
      import tempfile
      name = tempfile.NamedT emporaryFile(mo de='w+b')
      print 'name is '+ name.name
      >
      cmd = []
      cmd.append('/bin/ls')
      cmd.append('-l')
      cmd.append('/tmp')
      p = proc.Popen(cmd, stdout=name, stderr=proc.STD OUT, close_fds=True)
      while True:
         ret = p.poll()
         if (ret is not None):
            output = name.readlines( )
            print 'out = ', output
            break
      >
      $python sub.py
      name is /tmp/tmpjz4NJY
      out =  []
      >
      I tried calling flush() on the file object but this didn't help
      either. Tried closing and re-opening the file, but closing the file
      object results in it getting deleted. Can the above be made to work by
      using tempfiles?
      >
      thanks

      your script works just fine.
      The problem is that you have to move to the beggining of the file to
      read the actual contents of
      the file.
      name.seek(0)

      The whole program would be:

      #!/usr/bin/python
      import subprocess as proc
      import tempfile
      name = tempfile.NamedT emporaryFile(mo de='w+b')
      print 'name is '+ name.name

      cmd = []
      cmd.append('/bin/ls')
      cmd.append('-l')
      cmd.append('/tmp')
      p = proc.Popen(cmd, stdout=name, stderr=proc.STD OUT, close_fds=True)
      while True:
      ret = p.poll()
      if (ret is not None):
      name.seek(0)
      output = name.readlines( )
      print 'out = ', output
      break

        Comment

        • Matt Nordhoff
          #3
          Re: redirecting output of process to a file using subprocess.Pope n()

          skeept wrote:
          On Jul 9, 7:32 pm, rpar...@gmail.c om wrote:
          >I am trying to redirect stderr of a process to a temporary file and
          >then read back the contents of the file, all in the same python
          >script. As a simple exercise, I launched /bin/ls but this doesn't
          >work:
          >>
          >#!/usr/bin/python
          >import subprocess as proc
          >import tempfile
          >name = tempfile.NamedT emporaryFile(mo de='w+b')
          >print 'name is '+ name.name
          >>
          >cmd = []
          >cmd.append('/bin/ls')
          >cmd.append('-l')
          >cmd.append('/tmp')
          >p = proc.Popen(cmd, stdout=name, stderr=proc.STD OUT, close_fds=True)
          >while True:
          > ret = p.poll()
          > if (ret is not None):
          > output = name.readlines( )
          > print 'out = ', output
          > break
          >>
          >$python sub.py
          >name is /tmp/tmpjz4NJY
          >out = []
          >>
          >I tried calling flush() on the file object but this didn't help
          >either. Tried closing and re-opening the file, but closing the file
          >object results in it getting deleted. Can the above be made to work by
          >using tempfiles?
          >>
          >thanks
          >
          >
          your script works just fine.
          The problem is that you have to move to the beggining of the file to
          read the actual contents of
          the file.
          name.seek(0)
          >
          The whole program would be:
          >
          #!/usr/bin/python
          import subprocess as proc
          import tempfile
          name = tempfile.NamedT emporaryFile(mo de='w+b')
          print 'name is '+ name.name
          >
          cmd = []
          cmd.append('/bin/ls')
          cmd.append('-l')
          cmd.append('/tmp')
          p = proc.Popen(cmd, stdout=name, stderr=proc.STD OUT, close_fds=True)
          while True:
          ret = p.poll()
          if (ret is not None):
          name.seek(0)
          output = name.readlines( )
          print 'out = ', output
          break
          This is an aside, but why the loop?

          ....
          p = ...
          p.wait()
          name.seek(0)
          ....

          (p.wait() returns p.returncode, just like p.poll() does, but you aren't
          using it anyway...)
          --

            Comment

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