Pipe subprocess standard output to a variable [duplicate]

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 > 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 > 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 <module>     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 

Answer 1

To get the output of ls, use stdout=subprocess.PIPE.

>>> proc = subprocess.Popen('ls', stdout=subprocess.PIPE) >>> output = proc.stdout.read() >>> 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.

>>> proc = subprocess.Popen(['cdrecord', '--help'], stderr=subprocess.PIPE) >>> output = proc.stderr.read() >>> 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().

>>> proc = subprocess.Popen(['cdrecord', '--help'], stdout=subprocess.PIPE, stderr=subprocess.PIPE) >>> out, err = proc.communicate() >>> print 'stdout:', out stdout:  >>> 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 #. ... 

Answer 2

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)