The os module

This module represents generic operating system functionality. This module is especially important if
you want to make your programs platform-independent i.e. it allows the program to be written such that
it will run on Linux as well as Windows without any problems and without requiring changes. An example
of this is using the os.sep variable instead of the operation system-specific path separator.
Some of the more useful parts of the os module are listed below Most of them are self-explanatory.

• The os.name string specifies which platform you are using, such as 'nt' for Windows and
'posix' for Linux/Unix users.
• The os.getcwd() function gets the current working directory i.e. the path of the directory from
which the curent Python script is working.
• The os.getenv() and os.putenv() functions are used to get and set environment variables
respectively.
• The os.listdir() function returns the name of all files and directories in the specified directory.
• The os.remove() function is used to delete a file.
• The os.system() function is used to run a shell command.
• The os.linesep string gives the line terminator used in the current platform. For example, Windows
uses '\r\n', Linux uses '\n' and Mac uses '\r'.
• The os.path.split() function returns the directory name and file name of the path.
>>> os.path.split('/home/swaroop/byte/code/poem.txt')
('/home/swaroop/byte/code', 'poem.txt')
• The os.path.isfile() and the os.path.isdir() functions check if the given path refers
to a file or directory respectively. Similarly, the os.path.exists() function is used to check if
a given path actually exists.

You can explore the Python Standard Documentation for more details on these functions and variables.
You can use help(sys), etc. as well.

The sys module

The sys module contains system-specific functionality. we have already seen that the sys.argv list
contains the command-line arguments.
Command Line Arguments

Example 14.1. Using sys.argv
#!/usr/bin/python
# Filename: cat.py
import sys
def readfile(filename):
'''Print a file to the standard output.'''
f = file(filename)
while True:
line = f.readline()
if len(line) == 0:
break
print line, # notice comma
f.close()
# Script starts from here
if len(sys.argv) < 2:
print 'No action specified.'
sys.exit()
if sys.argv[1].startswith('--'):
option = sys.argv[1][2:]
# fetch sys.argv[1] but without the first two characters
if option == 'version':
print 'Version 1.2'
elif option == 'help':
print '''\
This program prints files to the standard output.


Any number of files can be specified.
Options include:
--version : Prints the version number
--help : Display this help'''
else:
print 'Unknown option.'
sys.exit()
else:
for filename in sys.argv[1:]:
readfile(filename)


Output
$ python cat.py
No action specified.
$ python cat.py --help
This program prints files to the standard output.
Any number of files can be specified.
Options include:
--version : Prints the version number
--help : Display this help
$ python cat.py --version
Version 1.2
$ python cat.py --nonsense
Unknown option.
$ python cat.py poem.txt
Programming is fun
When the work is done
if you wanna make your work also fun:
use Python!


How It Works
This program tries to mimic the cat command familiar to Linux/Unix users. You just speicfy the names
of some text files and it will print them to the output.
When a Python program is run i.e. not an interactive mode, there is always at least one item in the
sys.argv list which is the name of the current program being run and is available as sys.argv[0]
since Python starts counting from 0. Other command line arguments follow this item.
To make the program user-friendly we have supplied certain options that the user can specify to learn
more about the program. We use the first argument to check if any options have been specified to our
program. If the --version option is used, the version number of the program is printed. Similarly,
when the --help option is specified, we give a bit of explanation about the program. We make use of
the sys.exit function to exit the running program. As always, see help(sys.exit) for more details.
When no options are specified and filenames are passed to the program, it simply prints out each line of
each file, one after the other in the order specified on the command line.
As an aside, the name cat is short for concatenate which is basically what this program does - it can
print out a file or attach/concatenate two or more files together in the output.