Running shell command and capturing the output

Question

I want to write a function that will execute a shell command and return its output as a string, no matter, is it an error or success message. I just want to get the same result that I would have gotten with the command line.

What would be a code example that would do such a thing?

For example:

def run_command(cmd):
    # ??????

print run_command('mysqladmin create test -uroot -pmysqladmin12')
# Should output something like:
# mysqladmin: CREATE DATABASE failed; error: 'Can't create database 'test'; database exists'

Answer 1

Your Mileage May Vary, I attempted @senderle's spin on Vartec's solution in Windows on Python 2.6.5, but I was getting errors, and no other solutions worked. My error was: WindowsError: [Error 6] The handle is invalid.

I found that I had to assign PIPE to every handle to get it to return the output I expected - the following worked for me.

import subprocess

def run_command(cmd):
    """given shell command, returns communication tuple of stdout and stderr"""
    return subprocess.Popen(cmd, 
                            stdout=subprocess.PIPE, 
                            stderr=subprocess.PIPE, 
                            stdin=subprocess.PIPE).communicate()

and call like this, ([0] gets the first element of the tuple, stdout):

run_command('tracert 11.1.0.1')[0]

After learning more, I believe I need these pipe arguments because I'm working on a custom system that uses different handles, so I had to directly control all the std's.

To stop console popups (with Windows), do this:

def run_command(cmd):
    """given shell command, returns communication tuple of stdout and stderr"""
    # instantiate a startupinfo obj:
    startupinfo = subprocess.STARTUPINFO()
    # set the use show window flag, might make conditional on being in Windows:
    startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
    # pass as the startupinfo keyword argument:
    return subprocess.Popen(cmd,
                            stdout=subprocess.PIPE, 
                            stderr=subprocess.PIPE, 
                            stdin=subprocess.PIPE, 
                            startupinfo=startupinfo).communicate()

run_command('tracert 11.1.0.1')

Answer 2

I had a slightly different flavor of the same problem with the following requirements:

1. Capture and return STDOUT messages as they accumulate in the STDOUT buffer (i.e. in realtime).
   • @vartec solved this Pythonically with his use of generators and the 'yield'
     keyword above
2. Print all STDOUT lines (even if process exits before STDOUT buffer can be fully read)
3. Don't waste CPU cycles polling the process at high-frequency
4. Check the return code of the subprocess
5. Print STDERR (separate from STDOUT) if we get a non-zero error return code.

I've combined and tweaked previous answers to come up with the following:

import subprocess
from time import sleep

def run_command(command):
    p = subprocess.Popen(command,
                         stdout=subprocess.PIPE,
                         stderr=subprocess.PIPE,
                         shell=True)
    # Read stdout from subprocess until the buffer is empty !
    for line in iter(p.stdout.readline, b''):
        if line: # Don't print blank lines
            yield line
    # This ensures the process has completed, AND sets the 'returncode' attr
    while p.poll() is None:                                                                                                                                        
        sleep(.1) #Don't waste CPU-cycles
    # Empty STDERR buffer
    err = p.stderr.read()
    if p.returncode != 0:
       # The run_command() function is responsible for logging STDERR 
       print("Error: " + str(err))

This code would be executed the same as previous answers:

for line in run_command(cmd):
    print(line)

Answer 3

This is a tricky but super simple solution which works in many situations:

import os
os.system('sample_cmd > tmp')
print open('tmp', 'r').read()

A temporary file(here is tmp) is created with the output of the command and you can read from it your desired output.

Extra note from the comments: You can remove the tmp file in the case of one-time job. If you need to do this several times, there is no need to delete the tmp.

os.remove('tmp')

Answer 4

If you need to run a shell command on multiple files, this did the trick for me.

import os
import subprocess

# Define a function for running commands and capturing stdout line by line
# (Modified from Vartec's solution because it wasn't printing all lines)
def runProcess(exe):    
    p = subprocess.Popen(exe, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    return iter(p.stdout.readline, b'')

# Get all filenames in working directory
for filename in os.listdir('./'):
    # This command will be run on each file
    cmd = 'nm ' + filename

    # Run the command and capture the output line by line.
    for line in runProcess(cmd.split()):
        # Eliminate leading and trailing whitespace
        line.strip()
        # Split the output 
        output = line.split()

        # Filter the output and print relevant lines
        if len(output) > 2:
            if ((output[2] == 'set_program_name')):
                print filename
                print line

Edit: Just saw Max Persson's solution with J.F. Sebastian's suggestion. Went ahead and incorporated that.

Answer 5

eg, execute('ls -ahl') differentiated three/four possible returns and OS platforms:

1. no output, but run successfully
2. output empty line, run successfully
3. run failed
4. output something, run successfully

function below

def execute(cmd, output=True, DEBUG_MODE=False):
"""Executes a bash command.
(cmd, output=True)
output: whether print shell output to screen, only affects screen display, does not affect returned values
return: ...regardless of output=True/False...
        returns shell output as a list with each elment is a line of string (whitespace stripped both sides) from output
        could be 
        [], ie, len()=0 --> no output;    
        [''] --> output empty line;     
        None --> error occured, see below

        if error ocurs, returns None (ie, is None), print out the error message to screen
"""
if not DEBUG_MODE:
    print "Command: " + cmd

    # https://stackoverflow.com/a/40139101/2292993
    def _execute_cmd(cmd):
        if os.name == 'nt' or platform.system() == 'Windows':
            # set stdin, out, err all to PIPE to get results (other than None) after run the Popen() instance
            p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
        else:
            # Use bash; the default is sh
            p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True, executable="/bin/bash")

        # the Popen() instance starts running once instantiated (??)
        # additionally, communicate(), or poll() and wait process to terminate
        # communicate() accepts optional input as stdin to the pipe (requires setting stdin=subprocess.PIPE above), return out, err as tuple
        # if communicate(), the results are buffered in memory

        # Read stdout from subprocess until the buffer is empty !
        # if error occurs, the stdout is '', which means the below loop is essentially skipped
        # A prefix of 'b' or 'B' is ignored in Python 2; 
        # it indicates that the literal should become a bytes literal in Python 3 
        # (e.g. when code is automatically converted with 2to3).
        # return iter(p.stdout.readline, b'')
        for line in iter(p.stdout.readline, b''):
            # # Windows has \r\n, Unix has \n, Old mac has \r
            # if line not in ['','\n','\r','\r\n']: # Don't print blank lines
                yield line
        while p.poll() is None:                                                                                                                                        
            sleep(.1) #Don't waste CPU-cycles
        # Empty STDERR buffer
        err = p.stderr.read()
        if p.returncode != 0:
            # responsible for logging STDERR 
            print("Error: " + str(err))
            yield None

    out = []
    for line in _execute_cmd(cmd):
        # error did not occur earlier
        if line is not None:
            # trailing comma to avoid a newline (by print itself) being printed
            if output: print line,
            out.append(line.strip())
        else:
            # error occured earlier
            out = None
    return out
else:
    print "Simulation! The command is " + cmd
    print ""

Answer 6

Vartec's answer doesn't read all lines, so I made a version that did:

def run_command(command):
    p = subprocess.Popen(command,
                         stdout=subprocess.PIPE,
                         stderr=subprocess.STDOUT)
    return iter(p.stdout.readline, b'')

Usage is the same as the accepted answer:

command = 'mysqladmin create test -uroot -pmysqladmin12'.split()
for line in run_command(command):
    print(line)