 | Level: Intermediate James Knowlton (james@jamesknowlton.com), Software Quality Engineer, Automated Data Processing
04 Sep 2007
Updated 07 Sep 2007 Adopt Python to manage UNIX® systems while incorporating concepts of good program
design. Python is an easy-to-learn, open source scripting language that lets system
administrators do their job more quickly. It can also make tasks more fun.
Introduction
As a system administrator, you run across numerous challenges and problems.
Managing users, disk space, processes, devices, and backups can cause many
system administrators to lose their hair, good humor, or sanity. Shell scripts can help, but
they often have frustrating limitations. This is where a full-featured scripting
language, such as Python, can turn a tedious task into an easy and, dare I say it,
fun one.
The examples in this article demonstrate different Python features that you can
put to practical use. If you work through them, you'll be well on your way to
understanding the power of Python.
About modules
A module is an important Python concept. Basically, a module is a resource
you import in order to use it. This process is comparable to taking a piece of
paper out of a file cabinet and putting it on your desk, ready for use. You import
modules using the import command, which appears at the
top of each of the example programs. Modules are available for database
connectivity, network programming, operating system services, and hundreds of
other useful areas.
Put Python to work
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Why Python?
Python has many features to recommend it:
- It's free, being open source.
- It's easy to learn. Many users find its syntax much more English-like than
other scripting languages.
- It's mature. Python has been around a long time, which means its code is
stable, lots of modules add functionality, and robust documentation is
available on the Web.
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Python is a full-featured, robust programming language and, as such, it has tons
of features. Learning it could be a task of epic proportions. However, remember
that many Python features, such as the GUI toolkits, are of limited value to
system administrators. That's why this article uses specific examples: They
demonstrate the skills you need to effectively write Python scripts to manage
systems.
Notes on the examples
- Each example includes a
try: and an
except: with a surrounding block of code. This is an
implementation of rudimentary error handling. Python has extensive support for
handling all types of exceptions but, for the purposes of these example
programs,
I've kept it simple.
- These examples were run on Python 2.5 running on a Linux® box, but
they should work on any UNIX®/Linux machine.
- You'll undoubtedly think of ways these scripts can be improved. This is good!
The nature of Python scripts is that they can be easily modified and customized
without needing to recompile code.
Example 1: Search for
files and show permissions in a friendly format
The first example program (see Listing 1) searches for files that match a pattern
(based on user input) and displays the results to the screen, along with the
permissions assigned to the particular files. At first, you might think this program
doesn't do much more than execute a find command;
however, it displays results in a customized way, and your options for displaying this
enhanced find are limitless. The example shows you how to take a system command
and make it better (or at least more customized).
The script basically performs three tasks:
- Get the search pattern from the user.
- Perform the search.
- Present the results to the user.
In writing the script, constantly ask yourself this question, "Which task is this
code supporting?" Asking yourself this question can increase the focus of your
work and efficiency.
Listing 1. Search for files and list results with file permissions
import stat, sys, os, string, commands
#Getting search pattern from user and assigning it to a list
try:
#run a 'find' command and assign results to a variable
pattern = raw_input("Enter the file pattern to search for:\n")
commandString = "find " + pattern
commandOutput = commands.getoutput(commandString)
findResults = string.split(commandOutput, "\n")
#output find results, along with permissions
print "Files:"
print commandOutput
print "================================"
for file in findResults:
mode=stat.S_IMODE(os.lstat(file)[stat.ST_MODE])
print "\nPermissions for file ", file, ":"
for level in "USR", "GRP", "OTH":
for perm in "R", "W", "X":
if mode & getattr(stat,"S_I"+perm+level):
print level, " has ", perm, " permission"
else:
print level, " does NOT have ", perm, " permission"
except:
print "There was a problem - check the message above"
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The program follows these steps:
- Ask the user for a search pattern (lines 7-9).
- Print a listing of files found (lines 12-14).
- Using the
stat module, get permissions for each
file found and display them to the screen (lines 15-23).
When the program is run, the output looks like that shown in Listing 2.
Listing
2. Output of the first example
$ python example1.py
Enter the file pattern to search for:
j*.py
FILES FOUND FOR PATTERN j*.py :
jim.py
jim2.py
================================
Permissions for file jim.py :
USR R
USR W
USR X
GRP -
GRP -
GRP -
OTH -
OTH -
OTH -
Permissions for file jim2.py :
USR R
USR W
USR X
GRP R
GRP -
GRP X
OTH R
OTH -
OTH X |
Example 2: Perform
operations on a tar archive that is based on menu selection
The previous example prompted the user for a search pattern to use. Another way
to get information from the user is through a command-line argument. The program
in Listing 3 shows how to do that in Python: The code takes a tar filename as a
command-line argument and then prompts the user with several options.
This example also shows a new way to attack the problem. The first example used the
command module to run the find command and capture the
output. This approach can be clumsy and isn't very "Pythonic." This example uses
the tarfile module to open a tar file, which has the advantage of allowing you to
use Python attributes and methods as you manipulate the file. With many modules
provided by Python, you can do things that can't be done through the command line.
This is a good example of implementing a menu system in Python. The program
performs different actions based on your selection:
- If you press 1, the program prompts you for the file name in the
archive to extract the current directory to and then extracts the file.
- If you press 2, the program prompts you for the file name and then displays
the file information.
- If you press 3, the program lists all the files in the archive.
Listing 3. Perform actions on a tar archive based on your menu selection
import tarfile, sys
try:
#open tarfile
tar = tarfile.open(sys.argv[1], "r:tar")
#present menu and get selection
selection = raw_input("Enter\n\
1 to extract a file\n\
2 to display information on a file in the archive\n\
3 to list all the files in the archive\n\n")
#perform actions based on selection above
if selection == "1":
filename = raw_input("enter the filename to extract: ")
tar.extract(filename)
elif selection == "2":
filename = raw_input("enter the filename to inspect: ")
for tarinfo in tar:
if tarinfo.name == filename:
print "\n\
Filename:\t\t", tarinfo.name, "\n\
Size:\t\t", tarinfo.size, "bytes\n\
elif selection == "3":
print tar.list(verbose=True)
except:
print "There was a problem running the program" |
The program follows these steps:
- Open the tar file (line 5).
- Present the menu and get the user selection (lines 8-11).
- If you press 1 (lines 14-16), extract a file from the archive.
- If you press 2 (lines
17-23), present information about a selected file.
- If you press 3
(lines 24-25), present information about all the files in the archive.
The output is shown in Listing 4.
Listing
4. User menu for second example
$ python example2.py jimstar.tar
Enter
1 to extract a file
2 to display information on a file in the archive
3 to list all the files in the archive |
Example 3: Check for a
running process and show information in a friendly format
One of the most important duties of a system administrator is checking on running
processes. The script in Listing 5 gives you some ideas. The
program takes advantage of UNIX's ability to run a grep
command on output generated by a command, which lets you automatically narrow the
data Python has to parse.
This program also uses the string module. Get to know this module—you'll use
it often.
Listing 5. Display information on a running process in a friendly format
import commands, os, string
program = raw_input("Enter the name of the program to check: ")
try:
#perform a ps command and assign results to a list
output = commands.getoutput("ps -f|grep " + program)
proginfo = string.split(output)
#display results
print "\n\
Full path:\t\t", proginfo[5], "\n\
Owner:\t\t\t", proginfo[0], "\n\
Process ID:\t\t", proginfo[1], "\n\
Parent process ID:\t", proginfo[2], "\n\
Time started:\t\t", proginfo[4]
except:
print "There was a problem with the program." |
The program follows these steps:
- Get the name of a process to check and assign it to a variable (line 3).
- Run the
ps command and assign the results to a
list (lines 7-8).
- Display detailed information about the process with English terms (lines
11-16).
The output is shown in Listing 6.
Listing 6.
Output of the third example
$ python example3.py
Enter the name of the program to check: xterm
Full path: /usr/bin/xterm
Owner: knowltoj
Process ID: 3220
Parent process ID: 4308
Time started: 16:51:46 |
Example 4: Check
userids and passwords for policy compliance
Managing security is a critical part of the job for any system administrator. Python makes
this job easier, as the last example illustrates.
The program in Listing 7 uses the pwd module to access the password database. It
checks userids and passwords for security policy compliance (in this case, that
userids are at least six characters long and passwords are at least eight
characters long).
There are two caveats:
- This program works only if you have full rights to /etc/passwd.
- If you use shadow passwords, this script won't work (however, Python 2.5
does have a spwd module that does the job).
Listing 7. Check userids and passwords for compliance with security policy
import pwd
#initialize counters
erroruser = []
errorpass = []
#get password database
passwd_db = pwd.getpwall()
try:
#check each user and password for validity
for entry in passwd_db:
username = entry[0]
password = entry [1]
if len(username) < 6:
erroruser.append(username)
if len(password) < 8:
errorpass.append(username)
#print results to screen
print "The following users have an invalid userid (less than six characters):"
for item in erroruser:
print item
print "\nThe following users have invalid password(less than eight characters):"
for item in errorpass:
print item
except:
print "There was a problem running the script." |
The program follows these steps:
- Initialize the counter lists (lines 4-5).
- Open the password database and assign data to a list (line 8).
- Check users and passwords for validity (lines 12-18).
- Output invalid users and passwords (lines 21-26).
The output is shown in Listing 8.
Listing 8.
Output of the fourth example
$ python example4.py
The following users have an invalid userid (less than six characters):
Guest
The following users have invalid password(less than eight characters):
Guest
johnsmith
joewilson
suejones
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Other uses for scripts
You can use Python in a number of ways to manage systems. One of
the best things you can do is analyze your work, determine which tasks you perform
repeatedly, and explore whether Python modules are available to help you with
those tasks—almost certainly they are.
Some specific areas where Python can be of great help are as follows:
-
Managing servers: Checks patch levels for a particular application
across a set of servers and updates them automatically.
-
Logging: Sends an e-mail automatically if a particular type of
error shows up in the syslog.
-
Networking: Makes a Telnet connection to a server and monitors
the status of the connection.
-
Testing Web applications: Uses freely available tools to emulate a
Web browser and verifies Web application functionality and performance.
These are just a few examples—I'm sure you can add useful ideas of your
own.
Summary
With its easy-to-learn language; its ability to handle files, processes, strings,
and numbers; and its almost endless array of helper modules, Python is a scripting
language that looks like it was made for system administrators. It's a valuable
tool for any system administrator's toolbox.
Resources Learn
-
The Python tutorial: This tutorial is a great
source of basic language information.
- "Discover
Python" (Robert J. Brunner, developerWorks): Read all the articles in the developerWorks "Discover Python" series.
-
Official Python
Web site: This site provides a wealth of information and downloads.
-
The Python Cookbook: This
site is maintained by ActiveState and is a user community with user-contributed
scripts on virtually every programming topic.
-
Planet Python: This site covers all things Python.
-
Popular content:
See what AIX® and UNIX content your peers find interesting.
-
AIX and
UNIX:
The AIX and UNIX developerWorks zone provides a wealth of information relating to
all aspects of AIX systems administration and expanding your UNIX skills.
-
New to AIX and UNIX?:
Visit the "New to AIX and UNIX" page to learn more about AIX and UNIX.
-
AIX Wiki:
A collaborative environment for technical information related to AIX.
- Search the AIX and UNIX library by topic:
- Browse the
Python section
of the Safari Technology Bookstore for books on these and other technical topics.
-
Safari bookstore:
Visit this e-reference library to find specific technical resources.
-
developerWorks technical events and webcasts:
Stay current with developerWorks technical events and webcasts.
-
Podcasts: Tune in and
catch up with IBM technical experts.
Get products and technologies
Discuss
About the author | 
| | James Knowlton is a software quality engineer with Automated Data Processing (ADP), Inc. He leads quality assurance efforts on ADP's computer telephony integration and network video projects. He has extensive experience in open source technologies, including Python, Ruby, PHP, Apache, and MySQL. You can contact him at james@jamesknowlton.com. |
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