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I'm trying to find a way to check the existence of a shell command before its execution.

For example, I'll execute the command ack-grep. So, I'm trying to do:

import subprocess
from subprocess import PIPE

cmd_grep = subprocess.Popen(["ack-grep", "--no-color", "--max-count=1", "--no-group", "def run_main", "../cgedit/"], stdout=PIPE, stderr=PIPE)

Than, if I execute

I receive '' like the output. But I don't have the command ack-grep on my path. So, why Popen is not putting the error message on my .stderr variable?

Also, is there a easyer way to do what I'm trying to do?

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You should probably ask this on stackoverflow – trolle3000 Jun 4 '10 at 16:01
up vote 1 down vote accepted

You can use the subprocess module under Python 3 or the commands module for Python 2 as follow :

status, result = subprocess.getstatusoutput("ls -al")

status, result = commands.getstatusoutput("ls -al")

Then test the value of status.

Examples from the website :

>>> import subprocess
>>> subprocess.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
>>> subprocess.getstatusoutput('cat /bin/junk')
(256, 'cat: /bin/junk: No such file or directory')
>>> subprocess.getstatusoutput('/bin/junk')
(256, 'sh: /bin/junk: not found')
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couldn't you use the "which" command somehow? the which command automatically performs a lookup for an application in the paths. I think you would merely need to call this command and pass the name of the command you want to look up, then parse the results.

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I finally left it working this way:


    cmd_grep = ["ack-grep", "--no-color", "--max-count=1", "--no-group", function_definition, file_path]

    first_exec = Popen(cmd_grep,stdout=PIPE)

    execution = Popen(cmd_sed, shell=True, stdin=first_exec.stdout, stdout=PIPE)


    #use grep instead

    cmd_grep = cmd_grep = r'grep -R -n "' + function_definition + '" ' + file_path

    execution = Popen(cmd_grep + '|' + cmd_sed,shell=True,stdout=PIPE)

output =
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For situations like this, I use:

def find_program(prog_filename, error_on_missing=False):
    bdirs = ['$HOME/Environment/local/bin/',
    paths_tried = []
    for d in bdirs:
        p = os.path.expandvars(os.path.join(d, prog_filename))
        if os.path.exists(p):
            return p
    if error_on_missing:
        raise Exception("*** ERROR: '%s' not found on:\n  %s\n" % (prog_filename, "\n  ".join(paths_tried)))
        return None

Then you can do something like:

grep_path = find_program('ack_grep', False)
if grep_path is None:
    # default to standard system grep
    grep_path = 'grep'
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