200509: Making System Calls (with unix and Python 2.4)

To make a system call, use:

subprocess.Popen([r"gzip","-d", "x.txt.gz"]).wait()


The subprocess module is available only since Python 2.4, and it is
intended to replace several other older ones:

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


The subprocess.Popen([...]) creates a object. The .wait() makes the
code wait until the system call finished. To not wait for it, simply
use:

subprocess.Popen([r"gzip","-d", "x.txt.gz"])


To make a system call and get its output, do, for example:

last_line=subprocess.Popen([r"tail","-n 1", "x.txt"],
stdout=subprocess.PIPE).communicate()[0]


The “communicate()” automatically makes the call wait.

In the following example, we make a system call to decompress a gzip
file (and wait for it), then get the last line of the file using
“tail”, then gzip it again.

# -*- coding: utf-8 -*-
# Python

import subprocess

subprocess.Popen([r"gzip","-d", "x.txt.gz"]).wait()

last_line=subprocess.Popen([r"tail","-n 1", "x.txt"],
stdout=subprocess.PIPE).communicate()[0]

subprocess.Popen([r"gzip","x.txt"]).wait()

print last_line


Reference: http://www.python.org/doc/2.4/lib/module-subprocess.html
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