how to get the stdout of the script that os.system executes

[wxy212]

i know that in python, os.system(cmd) can be used to execute a script or command, and the return value is 0 if successful, rather than the standard output of the script that it executes.

e.g.

>>> a=os.system("echo hello")
>>> a
0 (rather than "hello")

my question is how to direct the standard output into a variable in my python script.

[KaezarRex]

The only way I've been able to get output from os.system is to use ">" in the command to make the subshell write its output to a file. Then all I have to do is read the file.

>>> os.system("echo hello > output.txt")
 
>>> reader = open("output.txt")
>>> print reader.read()
hello 

If you want to just append to the file instead of truncating then appending, use ">>" instead of ">".

It would be nice if there was a more direct method, so hopefully someone else knows one. I'd be very interested to know what it is.

[Smygis]

It would be nice if there was a more direct method, so hopefully someone else knows one. I'd be very interested to know what it is.

Tere is the commands module.

[code=python]
>>> import commands
>>> print commands.getoutput("echo helllo")
helllo
>>> import commands
>>> print commands.getoutput("echo hello")
hello
>>> txt = commands.getoutput("uname -a")
>>> print txt
Linux Bob 2.6.20-15-server #2 SMP Sun Apr 15 07:41:34 UTC 2007 i686 GNU/Linux
[code]

I also think os.popen works. but i have no experience with that. commands is quick and easy. i think os.popen is more advanced

[bartonc]

i know that in python, os.system(cmd) can be used to execute a script or command, and the return value is 0 if successful, rather than the standard output of the script that it executes.

e.g.

>>> a=os.system("echo hello")
>>> a
0 (rather than "hello")

my question is how to direct the standard output into a variable in my python script.

Since you have not stated your OS, our freind Smygis has given a great solution (if you are running under *nix). I'd recommend, however, that you use the newer subprocess module.

6.8 subprocess -- Subprocess management

New in version 2.4.

The subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several other, older modules and functions, such as:


os.system
os.spawn*
os.popen*
popen2.*
commands.*

[bartonc]

Since you have not stated your OS, our freind Smygis has given a great solution (if you are running under *nix). I'd recommend, however, that you use the newer subprocess module.

Try:

import subprocess as sub
retcode = sub.call(["ls", "-l"])

And let us know how that works out for you.

[bartonc]

Try:

import subprocess as sub
retcode = sub.call(["ls", "-l"])

And let us know how that works out for you.

On Windows, this:

>>> import subprocess as sub
>>> sub.call(['dir'], shell=True)

worked in a Python command-line shell, but not in my IDE (Boa Constructor).

[Smygis]

Try:

import subprocess as sub
retcode = sub.call(["ls", "-l"])

And let us know how that works out for you.

Still, that prints the result. And does not return it.

So no, Ill stick with commands.

Whenever i need something like this. And thats only happend like two times.

[wxy212]

Thanks for all of your replies.

"var = commands.getoutput(cmd)" seems to be good. However, it doesn't check if the command has been executed successfully.

What i want is if it's executed successfully, then write the stdout to a variable, otherwise die with an error message..

If i use "if os.system(cmd) != 0:" to check it before running "var=commands.getoutput(cmd)", then the stdout will not only be written to variable var, but also pop on the prompt screen, which is not what I like..

Does anyone have some idea?

[Smygis]

Thanks for all of your replies.

"var = commands.getoutput(cmd)" seems to be good. However, it doesn't check if the command has been executed successfully.

What i want is if it's executed successfully, then write the stdout to a variable, otherwise die with an error message..

If i use "if os.system(cmd) != 0:" to check it before running "var=commands.getoutput(cmd)", then the stdout will not only be written to variable var, but also pop on the prompt screen, which is not what I like..

Does anyone have some idea?

commands.getstatusoutput

>>> fail = commands.getstatusoutput("ps -q")
>>> notfail = commands.getstatusoutput("ps -A")
>>> fail[0]
256
>>> len(fail[1]) # the output
1517
>>> notfail[0]
 
>>> len(notfail[1]) # the output
3408
 
 

[wxy212]

commands.getstatusoutput

>>> fail = commands.getstatusoutput("ps -q")
>>> notfail = commands.getstatusoutput("ps -A")
>>> fail[0]
256
>>> len(fail[1]) # the output
1517
>>> notfail[0]
 
>>> len(notfail[1]) # the output
3408
 
 

brilliant!! thanks a lot..

[ghostdog74]

note also that commands modules is Unix specific.( AFAIK , correct me if i am wrong)
i use this sometimes

import os
fin, fout = os.popen4(cmd)
print fout.read() #standard out
 

[Smygis]

note also that commands modules is Unix specific.( AFAIK , correct me if i am wrong)
i use this sometimes

import os
fin, fout = os.popen4(cmd)
print fout.read() #standard out
 

commands is a simple unix specific wrapper for os.popen:
The whole content of commands.py:

smygis@Bob:~$ cat /usr/lib/python2.5/commands.py
"""Execute shell commands via os.popen() and return status, output.
 
Interface summary:
 
       import commands
 
       outtext = commands.getoutput(cmd)
       (exitstatus, outtext) = commands.getstatusoutput(cmd)
       outtext = commands.getstatus(file)  # returns output of "ls -ld file"
 
A trailing newline is removed from the output string.
 
Encapsulates the basic operation:
 
      pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')
      text = pipe.read()
      sts = pipe.close()
 
 [Note:  it would be nice to add functions to interpret the exit status.]
"""
 
__all__ = ["getstatusoutput","getoutput","getstatus"]
 
# Module 'commands'
#
# Various tools for executing commands and looking at their output and status.
#
# NB This only works (and is only relevant) for UNIX.
 
 
# Get 'ls -l' status for an object into a string
#
def getstatus(file):
    """Return output of "ls -ld <file>" in a string."""
    return getoutput('ls -ld' + mkarg(file))
 
 
# Get the output from a shell command into a string.
# The exit status is ignored; a trailing newline is stripped.
# Assume the command will work with '{ ... ; } 2>&1' around it..
#
def getoutput(cmd):
    """Return output (stdout or stderr) of executing cmd in a shell."""
    return getstatusoutput(cmd)[1]
 
 
# Ditto but preserving the exit status.
# Returns a pair (sts, output)
#
def getstatusoutput(cmd):
    """Return (status, output) of executing cmd in a shell."""
    import os
    pipe = os.popen('{ ' + cmd + '; } 2>&1', 'r')
    text = pipe.read()
    sts = pipe.close()
    if sts is None: sts = 0
    if text[-1:] == '\n': text = text[:-1]
    return sts, text
 
 
# Make command argument from directory and pathname (prefix space, add quotes).
#
def mk2arg(head, x):
    import os
    return mkarg(os.path.join(head, x))
 
 
# Make a shell command argument from a string.
# Return a string beginning with a space followed by a shell-quoted
# version of the argument.
# Two strategies: enclose in single quotes if it contains none;
# otherwise, enclose in double quotes and prefix quotable characters
# with backslash.
#
def mkarg(x):
    if '\'' not in x:
        return ' \'' + x + '\''
    s = ' "'
    for c in x:
        if c in '\\$"`':
            s = s + '\\'
        s = s + c
    s = s + '"'
    return s