Python Programming/External commands

The traditional way of executing external commands is using os.system():

import os
os.system("dir")
os.system("echo Hello")
exitCode = os.system("echotypo")

The modern way, since Python 2.4, is using subprocess module:

subprocess.call(["echo", "Hello"])
exitCode = subprocess.call(["dir", "nonexistent"])

The traditional way of executing external commands and reading their output is via popen2 module:

import popen2
readStream, writeStream, errorStream = popen2.popen3("dir")
# allLines = readStream.readlines()
for line in readStream:
  print(line.rstrip())
readStream.close()
writeStream.close()
errorStream.close()

The modern way, since Python 2.4, is using subprocess module:

import subprocess
process = subprocess.Popen(["echo","Hello"], stdout=subprocess.PIPE)
for line in process.stdout:
   print(line.rstrip())

Keywords: system commands, shell commands, processes, backtick, pipe.

External links

• 17.1. subprocess — Subprocess management, python.org, since Python 2.4
• 15.1. os — Miscellaneous operating system interfaces, python.org
• 17.5. popen2 — Subprocesses with accessible I/O streams, python.org, deprecated since Python 2.6