Interaction with the operating system

The modules sys and os provide the basic interface to the operating system. The module os creates a portable abstraction layer which is used by high-level modules like glob, socket, thred, time, fcntl.

Module sys

The module sys provides access to system-specific parameters by the interpreter.

Example argv

   1 #system1.py
   2 import sys
   3 print sys.argv

run system1.py parameter1 parameter2
['system1.py', 'parameter1', 'parameter2']

The script prints the command line arguments that are passed to the script. argv[0] is the script name

A more sophisticated way of evaluating command line arguments is provided by the module optparse

Module os

The module os is a portable operating system interface.

Some examples:

• os.system() Executes the command (a string) in a subshell

• os.mkdir() Creates a directory

• os.remove() Deletes a file

• os.path.isdir() Test if directory

• os.path.isfile() Test if file

• os.path.exists() Test if file or directory exists

• os.path.getsize() Size of a file

• os.path.basename() Base name of pathname

• os.walk() Directory tree generator

System programming: walk example

The following script walks through a directory tree and looks for all files with the matching extension:

   1 #!/usr/bin/env python
   2 import os,fnmatch,sys
   3 
   4 # Usage:
   5 # ./walkdir.py directory extension
   6 
   7 dir,ext=sys.argv[1],sys.argv[2]
   8 for root, dirs, files in os.walk(dir):
   9     f=fnmatch.filter(files,'*.'+ext)
  10     if type(f)==type([]):
  11         for fi in f:
  12             print root+fi

Save the file as walkdir.py and use chmod +x walkdir.py to set the execution permissions of the file. The script can be exectuted on the bash shell using: ./walkdir.py $HOME/subdir ps

Module glob

The module glob finds all the pathnames matching a specified pattern according to the rules used by the Unix shell.

The following example looks for all pdf files in the current working directory and converts them into postscript files.

   1 #!/usr/bin/env python
   2 import os,glob
   3 
   4 filelist=glob.glob('*.pdf')
   5 for f in filelist:
   6     psfilename=f.split('.')[0]+'.ps'
   7     cmd='pdftops '+f+' '+psfilename
   8     print cmd
   9     os.system(cmd)

Unix Specific Services

Features that are unique to the Unix operating system are for example shell pipelines (data streams) that pipe the output of one program to another. The pipeline symbol is |. For example, the command ls -s | sort -rg pipes the output of ls -s to the sort program. The result is a list of filenames sorted by its size

A python pipeline to a Unix programm can be established using the module pipes