[Python2.7] Summary of how to use subprocess

Overview

There are quite a lot of opportunities to use subprocess, so I will summarize the points I investigated without understanding. This article is for Python 2.7, as I assumed the Python 2.7 environment as my use case at the time of my research.

Basically, please refer to the official document below. 17.1. Subprocess — Subprocess Management — Python 2.7.x Documentation

Especially in the case of Python 3.5 or higher, the usage is different because the general-purpose run () command is added. Please refer to the formula without diversion as it is. 17.5. Subprocess — Subprocess Management — Python 3.5.2 Documentation

Import specification

import

import subprocess

Basic API

subprocess.call -Runtime standard output / standard error is output at the time of execution ・ Returns a return code in both normal and abnormal conditions

Execution sample

import subprocess

print '!!! start'

cmd = 'echo aaa'
retcode = subprocess.call(cmd.split())
print retcode

cmd = 'false'
retcode = subprocess.call(cmd.split())
print retcode

print '!!! end'

output

aaa
!!! start
0
1
!!! end

subprocess.check_call -Runtime standard output / standard error is output at the time of execution ・ Returns 0 return code when normal -In case of abnormality, subprocess.CalledProcessError exception is sent.

Execution sample

import subprocess

print '!!! start'

cmd = 'echo aaa'
retcode = subprocess.check_call(cmd.split())
print retcode

cmd = 'false'
try:
    retcode = subprocess.check_call(cmd.split())
except subprocess.CalledProcessError as e:
    print e.returncode
    print e.cmd
    print e.output

print '!!! end'

output

aaa
!!! start
0
1
['false']
None
!!! end

subprocess.check_output -Runtime standard output / standard error is output at the time of execution ・ Returns standard output when normal -In case of abnormality, subprocess.CalledProcessError exception is sent.

Execution sample

import subprocess

print '!!! start'

cmd = 'echo aaa'
d = subprocess.check_output(cmd.split())
print d,

cmd = 'unexist_command'
try:
    d = subprocess.check_output(
            cmd.split(),
            stderr=subprocess.STDOUT,
            shell=True)
    print d,
except subprocess.CalledProcessError as e:
    print e.returncode
    print e.cmd
    print e.output,

print '!!! end'

output

!!! start
aaa
127
['unexist_command']
/bin/sh: unexist_command: command not found
!!! end

Get standard error when using subprocess.check_output

Specify stderr = subprocess.STDOUT as shown below.

Example of use

    d = subprocess.check_output(
            cmd.split(),
            stderr=subprocess.STDOUT,
            shell=True)

Change the execution directory of subprocess

Using the cwd option

Specifying the execution directory

subprocess.call(cmd, cwd=workdir)

An execution example is shown below.

Execution sample

import subprocess

d1 = subprocess.check_output(['pwd'])                   #Initial position/tmp
d2 = subprocess.check_output(['pwd'], cwd='/tmp/hoge')  #Confirmation with absolute path
d3 = subprocess.check_output(['pwd'])                   #Not affected by other movements
d4 = subprocess.check_output(['pwd'], cwd='./hoge')     #Confirmation with relative path

print 'case 1:'
print d1,
print d2,

print 'case 2:'
print d3,
print d4,

Execution example

case 1:
/tmp
/tmp/hoge
case 2:
/tmp
/tmp/hoge

python subprocess changing directory - Stack Overflow

important point

When fetching an execution command from the outside, especially when shell = True is specified, a command injection attack may occur, so be careful about security.

Supplement: Command Injection Attack / Shell Injection

IPA ISEC Secure Programming Course: Web Application Chapter 6 Input Countermeasures: Command Injection Attack Countermeasures Code injection - Wikipedia

Unknown behavior

-When shell = True is specified in check_output, standard output / standard error may not be obtained. (Empty string is returned)

reference

Use communicate () when receiving output in Python subprocess-Qiita To execute a command in python-with a mug!