Executing commandline commands from Python using subprocess module

Question

I am trying to use the subprocess module in python and trying to fetch the process id of firefox

cmd = "firefox &" 
fire = subprocess.Popen(cmd,shell=True, stdout=subprocess.PIPE, preexec_fn=os.setsid)
fire_task_procs = find_task(fire.pid)
print "fire_task_procs",fire_task_procs

I think I am getting the pid of the commandline argument that I am executing.. am I doing something wrong? I confirmed that it is not the same using the ps aux | grep firefox

Answer 1

If you use shell=True the pid you'll get ist that of the started shell, not that of the process you want, specially as you use & to send the process into background.

You should use the long (list) form of supplying the parameters, without &, as that makes little sense anyway if you combine it with output redirection.

• rnish: If I don't use shell=True, I get the following error: Traceback (most recent call last): File "test.py", line 33, in <module> fire = subprocess.Popen(cmd, stdout=subprocess.PIPE, preexec_fn=os.setsid) File "/usr/lib/python2.7/subprocess.py", line 679, in __init__ errread, errwrite) File "/usr/lib/python2.7/subprocess.py", line 1259, in _execute_child raise child_exception OSError: [Errno 2] No such file or directory
• abarnert: If you don't use shell=True and you pass the string "firefox &", you will get an error, because that's not a reasonable arg string. (Depending on your platform, this may ask firefox to open a file called &, or it may try to find a program named firefox &, neither of which is right.) But if you just drop the &, it is. And ideally, switch to an arg list. (As the docs say a few times, "Providing a sequence of arguments is generally preferred" when you're not using shell=True, especially on POSIX.) So, change cmd to ["firefox"].

Answer 2

Don't use the shell, instead just use

subprocess.Popen(['firefox'], stdout=subprocess.PIPE, preexec_fn=os.setsid)

However, if firefox is already running then this will not work either since in this case firefox will use some IPC to tell the existing process to open a new window and then terminates.