Subprocess with and without shell

[Nick Craig-Wood]

George Sakkis <george.sakkis at gmail.com> wrote:
>  I'm trying to figure out why Popen captures the stderr of a specific
>  command when it runs through the shell but not without it. IOW:
> 
>  cmd = [my_exe, arg1, arg2, ..., argN]
>  if 1: # this captures both stdout and stderr as expected
>      pipe = Popen(' '.join(cmd), shell=True, stderr=PIPE, stdout=PIPE)
>  else: # this captures only stdout
>      pipe = Popen(cmd, shell=False, stderr=PIPE, stdout=PIPE)
> 
>  # this prints the empty string if not run through the shell
>  print "stderr:", pipe.stderr.read()
>  # this prints correctly in both cases
>  print "stdout:", pipe.stdout.read()
> 
>  Any hints ?

Post an example which replicates the problem!

My effort works as expected

-- z.py ----------------------------------------------------
#!/usr/bin/python
from subprocess import Popen, PIPE
cmd = ["./zz.py"]
for i in range(2):
    if i: # this captures both stdout and stderr as expected
        print "With shell"
        pipe = Popen(' '.join(cmd), shell=True, stderr=PIPE, stdout=PIPE)
    else: # this captures only stdout
        print "Without shell"
        pipe = Popen(cmd, shell=False, stderr=PIPE, stdout=PIPE)

    # this prints the empty string if not run through the shell
    print "stderr:", pipe.stderr.read()
    # this prints correctly in both cases
    print "stdout:", pipe.stdout.read()
---zz.py----------------------------------------------------
#!/usr/bin/python
import sys
print >>sys.stdout, "Stdout"
print >>sys.stderr, "Stderr"
------------------------------------------------------------

Produces

$ ./z.py 
Without shell
stderr: Stderr

stdout: Stdout

With shell
stderr: Stderr

stdout: Stdout



-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick