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= 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 [[http://docs.python.org/lib/module-sys.html|sys]] provides access to system-specific parameters  
by the interpreter.

=== Example argv ===
{{{#!python
#system1.py
import sys
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 
[[http://docs.python.org/lib/module-optparse.html|optparse]]

== Module os ==
The module [[http://docs.python.org/lib/module-os.html|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
 
== Module fnmatch ==

The module [[http://docs.python.org/library/fnmatch.html|fnmatch]] provides support for Unix shell-style wildcards

== Module glob ==

The module [[http://docs.python.org/lib/module-glob.html|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.
{{{#!python
#!/usr/bin/env python
import os,glob

filelist=glob.glob('*.pdf')
for f in filelist:
    psfilename=f.split('.')[0]+'.ps'
    cmd='pdftops '+f+' '+psfilename
    print cmd
    os.system(cmd)
}}}
== Module shutil — High-level file operations ==
The [[http://docs.python.org/library/shutil.html|shutil]] module offers a number of high-level operations on files and collections of files. In particular, functions are provided which support file copying and removal.

== 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 [[http://docs.python.org/lib/module-pipes.html|pipes]]

= System programming: walk example =

The following script walks through a directory tree and looks for all files with the matching extension:
{{{#!python
#!/usr/bin/env python
import os,fnmatch,sys

# Usage:
# ./walkdir.py directory extension

dir,ext=sys.argv[1],sys.argv[2]
for root, dirs, files in os.walk(dir):
    f=fnmatch.filter(files,'*.'+ext)
    if type(f)==type([]):
        for fi in f:
            print root+fi
}}}

Save the file as {{{walkdir.py}}} and use {{{chmod +x walkdir.py}}} to set the execution permissions of the file. The first magic line starts the python interpreter. The script can be exectuted on the bash shell using:

{{{
./walkdir.py $HOME/subdir ps
}}}

Without the magic line, the script has to be run like this:
{{{
python walkdir.py $HOME/sync/ ps
}}}
Or within {{{ipython}}} using {{{run}}}