Pipe subprocess standard output to a variable

Question

I want to run a command in pythong  using the subprocess module  and store the output in a variable  However  I do not want the command s output to be printed to the terminal  For this code   def storels       a   subprocess Popen  ls  shell True  storels     I get the directory listing in the terminal  instead of having it stored in a  I ve also tried    def storels           subprocess Popen  ls  gt  tmp  shell True         a   open    tmp           Rest of Code   storels     This also prints the output of ls to my terminal  I ve even tried this command with the somewhat dated os system method  since running ls  gt  tmp in the terminal doesn t print ls to the terminal at all  but stores it in tmp  However  the same thing happens   Edit   I get the following error after following marcog s advice  but only when running a more complex command  cdrecord --help  Python spits this out   Traceback  most recent call last     File    install py   line 52  in  lt module gt      burntrack2  hi     File    install py   line 46  in burntrack2     a   subprocess Popen  cdrecord --help  stdout   subprocess PIPE    File   usr lib python2 6 subprocess py   line 633  in   init       errread  errwrite    File   usr lib python2 6 subprocess py   line 1139  in  execute child     raise child exception OSError   Errno 2  No such file or directory

User · Accepted Answer

To get the output of ls  use stdout subprocess PIPE    gt  gt  gt  proc   subprocess Popen  ls   stdout subprocess PIPE   gt  gt  gt  output   proc stdout read    gt  gt  gt  print output bar baz foo   The command cdrecord --help outputs to stderr  so you need to pipe that indstead  You should also break up the command into a list of tokens as I ve done below  or the alternative is to pass the shell True argument but this fires up a fully-blown shell which can be dangerous if you don t control the contents of the command string    gt  gt  gt  proc   subprocess Popen   cdrecord    --help    stderr subprocess PIPE   gt  gt  gt  output   proc stderr read    gt  gt  gt  print output Usage  wodim  options  track1   trackn Options      -version    print version information and exit     dev target  SCSI target to use as CD DVD-Recorder     gracetime   set the grace time before starting to write to          If you have a command that outputs to both stdout and stderr and you want to merge them  you can do that by piping stderr to stdout and then catching stdout   subprocess Popen cmd  stdout subprocess PIPE  stderr subprocess STDOUT    As mentioned by Chris Morgan  you should be using proc communicate   instead of proc read      gt  gt  gt  proc   subprocess Popen   cdrecord    --help    stdout subprocess PIPE  stderr subprocess PIPE   gt  gt  gt  out  err   proc communicate    gt  gt  gt  print  stdout    out stdout    gt  gt  gt  print  stderr    err stderr Usage  wodim  options  track1   trackn Options      -version    print version information and exit     dev target  SCSI target to use as CD DVD-Recorder     gracetime   set the grace time before starting to write to

User · Answer

With a   subprocess Popen  cdrecord --help  stdout   subprocess PIPE    you need to either use a list or use shell True   Either of these will work   The former is preferable   a   subprocess Popen   cdrecord    --help    stdout subprocess PIPE   a   subprocess Popen  cdrecord --help   shell True  stdout subprocess PIPE    Also  instead of using Popen stdout read Popen stderr read  you should use  communicate    refer to the subprocess documentation for why    proc   subprocess Popen   cdrecord    --help    stdout subprocess PIPE  stderr subprocess PIPE  stdout  stderr   proc communicate

User · Answer

If you are using python 2 7 or later  the easiest way to do this is to use the subprocess check output   command   Here is an example   output   subprocess check output  ls     To also redirect stderr you can use the following   output   subprocess check output  ls   stderr subprocess STDOUT     In the case that you want to pass parameters to the command  you can either use a list or use invoke a shell and use a single string   output   subprocess check output   ls    -a    output   subprocess check output  ls -a   shell True

[python] Pipe subprocess standard output to a variable

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