Python Scripting in Unix: Lab 4 - Building a Simple Filter - Prof. Derek Harter, Lab Reports of Computer Systems Networking and Telecommunications

Download Python Scripting in Unix: Lab 4 - Building a Simple Filter - Prof. Derek Harter and more Lab Reports Computer Systems Networking and Telecommunications in PDF only on Docsity! Spring 2008 CSci 553 Lab 4 Lab 04 Basic Scripting With the Python Language Goals In lab 4 we will endeavor to learn some of the basics of using a high level scripting language in the Unix software development environment. You will build several scripts to perform small tasks using the Python scripting language. Thus the goals of this lab are: 1) Learn the basics of the Python high level scripting language 2) Learn about using scripts to perform basic filtering and I/O tasks 3) Become more familiar with standard Unix filtering processes and commands. Instructions For this and all future labs, all of your work will be done and submitted through your own student repository on the nisl class server. If you have not done already, you need to check out a working copy of your personal repository before you can begin this lab. Use the in-class lab time not only to find the answers to the questions and complete the given tasks, but to practice your skills in using the Python programming language, executing scripts and commands from the command line, and building small filters to accomplish tasks in the Unix programming enviornment.. The first portion of the lab is to be completed in class so that you may ask the instructor for help and suggestions for any problems you may be experiencing. Exercise 4.0: Preliminaries Check out a working copy of your students repository if you haven't already done so. Your student repository will have a url designation of the form: http://nisl.tamu-commerce.edu/repo/csci553/username Where you need to substitute your own nisl account username for the last portion above. In your working copy, create and add to your repository a directory called lab04 (make sure you name it exactly as shown, make sure you not only create the directory, but you run the appropriate command to add the directory to be under version control). All work for this lab is to be added and checked into the lab04 directory of your repository. Exercise 4.1: Hello World a la Python Create a basic hello world python script. You should do the following:  Create a file called hello.py, add it to your repository.  Use the #! method we described in class to specify that the file should be interpreted using the python interpreter.  Make the script executable.  Also make sure you do an appropriate action so that your python scripting commands can be found in your path. I would suggest for the duration of this lab that you add the current working directory '.' to the end of your path.  Have the script print out something, like “Hello World!” or whatever you wish.  When your script can be successfully executed from the command line, commit your new file to the repository. Spring 2008 CSci 553 Lab 4 Exercise 4.2: Building a Simple Filter There are many methods for building a simple filter in Python, to read lines from standard input and write results to standard output. We will use the fileinput python library to create and perform simple filtering tasks. Create and add a file called simplefilter.py to your lab04 working directory. For this step and for all future steps, you should make the script interpretable by the python interpreter and executable from the command line by performing the tasks of part 4.1 on your script file. Add the following lines to your simplefilter.py script: import fileinput for line in fileinput.input(): print "Got line: <%s>" % line The fileinput library is a set of routines for doing exactly this very common task of reading lines of text from standard input so that they can be processed and filtered. The import keyword is how you tell a python script that you want to use commands or functions from a given library (look at the library reference manual on the python web site: http://docs.python.org/lib/lib.html for documention on the fileinput and all other python libraries). You can run your simplefilter.py script interactively to see how it works: [harry@nisl lab04]$ simplefilter.py Hello world This is my second line yet another line Got line: <Hello world > Got line: <This is my second line > Got line: <yet another line > [harry@nisl lab04]$ In the above example I typed in the first 3 lines (in italics). I then typed in a Ctrl-d. After entering Ctrl-d, the text will be sent to the python script, and you will get the results shown where the text of the 3 lines that were typed in are displayed to standard output. You may need to enter a Ctrl-d on a line by itself in order to exit from running the script interactively. Here is another way that you can use a pipe to test your simple filter. Create a file and type in some lines of text. Then use the cat command and a pipe to send the input into your filter: [harry@nisl lab04]$ vi junk ... I edited the file called junk here and typed in a few lines of text ... [harry@nisl lab04]$ cat junk First line another line This is my last line [harry@nisl lab04]$ cat junk | simplefilter.py Got line: <First line > Got line: <another line > Got line: <This is my last line > Spring 2008 CSci 553 Lab 4 >>> orig = "Hi!" >>> for char in orig: ... print char ... H i ! Notice that in the for loop, each char is accessed one by one, and printed out to standard output. The same task can be accomplished by using an explicit index variable in a loop: >>> for i in range(0, len(orig)): ... print orig[i] ... H i ! This could especially useful to, for example, access the characters starting from the end and progressing to the beginning of the string: >>> i = len(orig)-1 >>> while (i >= 0): ... print orig[i] ... i -= 1 ... ! i H One other example/hint that might be useful in implementing your reverse() function. Recall that strings are overloaded in Python so that you can use the + operator to concatenate strings together. So for example, you could append or prepend a character to a string doing things like: >>> orig+'M' 'Hi!M' >>> 'M'+orig 'Mhi!' With those example in mind, you are to write a filter that will read lines from standard input, and write the reversed version of the line to standard output. Create an interpretable and executable python script file called reverse.py for this portion of the lab. You file should look something like this: #!/usr/bin/env python import fileinput # # your reverse function defined here # for line in fileinput.input(): print reverse(line) Of course you need to provide your implementation of the reverse() function at the location indicated. As stated above, your reverse() function takes a string as input. It should returned the reversed string as the result of calling the function. Also, to make your output more readable, you should strip the newline from your line before calling the reverse function, as you did in part 4.2.1 Once your reverse function is working properly, you should be able to run it with a file or interactively, and get a result similar to this: Spring 2008 CSci 553 Lab 4 [harry@nisl lab04]$ cat junk First line another line This is my last line [harry@nisl lab04]$ cat junk | reverse.py enil tsriF enil rehtona enil tsal ym si sihT Once you are satisfied with your reverse.py script, commit it to your repository. Exercise 4.4: Palindrome Filter in Python In this part of the lab we will reimplement the palindrome filter from lab 3 as a Python script. I have created the code to do the command line option parsing for you. Your task will be to implement the logic in order to perform palindrome filtering, as well as the logic to support the various command line arguments. Copy the Python script from /home/csci553/classfiles/palindrome.py to your lab04 directory. Also go ahead and add and commit the file to your repository. You should perform and test each of the following tasks in order on the file, and after each one you should commit your changes to the repository. 4.4.1 Implement Basic Palindrome Test The script provided does nothing yet except for parsing command line arguments. It simply echos all lines from standard input back to standard output. Your first task is to implement palindrome filtering. You need to test each line to see if it is equal to the reverse of the line. You should copy and use your reverse function from part 4.3 to accomplish this. Also remember that, as in lab03, your reverse() function will probably not work well unless you strip off the trailing newline from the lines read in from standard input. You will need to do this, as you learned in part 4.1, before calling your reverse() function and comparing the reversed string to the original line. Remember to commit you code once you have your basic palindrome logic working. 4.4.2 Implement the minPalindromeLength / singleton Options This is a simple option to implement. Add code to skip lines that are less than the minimum length that is being filtered for. Hint: loops in Python support break and continue keywords as in the C and Java programming languages. Once again, commit your changes when you have the length test working. 4.4.3 Implement the ignoreCase Option Also a relatively simple option to implement. Using a string method, such as upper or lower, have your palindrome filter ignore differences in case by converting the line and its reversed string to one or the other before testing for equality. Make sure that you don't break the functionality to not ignore case when this option is not provided. Once again, commit your changes when you have the ignoreCase option working. 4.4.4 Implement the invertMatch Option Implement the logic to invert the normal sense of matching, so that when the -v flag is given your filter finds non-palindromes. Commit your changes when you succeed implementing this option. Spring 2008 CSci 553 Lab 4 Lab 04 Extra Credit: Reimplement the wc Command as a Python Script The following task can be undertaken for extra credit. The extra credit will be worth 15 extra points for undergraduate students, and 5 extra points for graduate students. The task is to reimplement the wc command as a python script. Call your script wc.py. You should be able to use the palindrome.py as a base for your work. You need to support the -l, -w and -c options. In fact, asking for help from your wc.py filter should produce the following usage message: [harry@nisl lab04]$ wc.py -h Usage: wc.py [options] options: -h, --help display this help and exit -c, --bytes print the byte counts -l, --lines print the newline counts -w, --words print the word counts You will need to, of course, correctly specify and parse the options using the optparse Python library, as well as implement the logic to display the counts. Your implementation should give exactly the same results when filtering standard input as the wc command: [harry@nisl lab04]$ cat hello.py | wc 3 6 51 [harry@nisl lab04]$ cat hello.py | wc -w 6 [harry@nisl lab04]$ cat hello.py | wc.py 3 6 51 [harry@nisl lab04]$ cat hello.py | wc.py -w 6 To get full credit you need to implement the logic and the indicated options, and add and commit your wc.py script to your lab04 repository directory.