Python System Services and System Calls

July 30, 2009

All operating systems have services. At the highest levels, these give users abilities like running programs, viewing and setting environment variables, and renaming files on the hard drive. High level languages like C++, Java, and Python have libraries which give the programmer access to services. These libraries usually contain classes, which contain methods and properties, which are the abstractions of the operating system services.

In Python, many of these services are contained in the modules sys, os, and os.path. This article will explore some of the most common and useful of these system tools.

Here are some examples from the sys module, which might be useful:

$ python
>>> import sys
>>> sys.stdin
<open file '<stdin>', mode 'r' at 0xb7d44020>

>>> sys.platform
'linux2'

>>> sys.version
'2.5.1 (r251:54863, May  2 2007, 16:56:35) \n[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)]'

>>> sys.getrecursionlimit()
1000

#!/usr/bin/env python
# sample.py
import sys

arglist = sys.argv
print arglist
sys.exit()

$ python sample.py filea fileb
['sample.py', 'filea', 'fileb']

sys.argv is a list of the command line arguments to the program. We can see that the first item in the list is the program itself. sys.exit() exits the python interpreter.

The 'os' module contains many useful tools for file manipulation, such as removing, renamimg, and seeking within a file. Let's write a simple program which finds if all the files in one directory are also in another directory.

#!/usr/bin/env python
import sys, os

arglist = sys.argv
if len(arglist) != 3:
   print "Need two dir names as args."
   print
   sys.exit()

dir1 = arglist[1]
dir2 = arglist[2]

if not os.path.isdir(dir1):
   print dir1,'is not a directory'
   sys.exit()
if not os.path.isdir(dir2):
   print dir2,'is not a directory'
   sys.exit()

list1 = os.listdir(dir1)
list2 = os.listdir(dir2)
list1.sort()

for filen in list1:
    if not os.path.exists(dir2 + '/' + filen):
        print filen

Python System Services and System Calls
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