Lab 1, Linux programs and Shell scripting

In this lab you will write a bourne shell script that computes the average number of lines contained by files in a given directory.

Lab Preparation

Before you begin, let’s prepare your cs32 directory in your home directory if you don’t already have one. Note that these instructions assume you are using either a CSIL machine or one of the lab machines:

Create the directory:

mkdir cs32

Change the permissions such that only your account can access the files inside this directory. This step is incredibly important. If omitted, other students can peek at your work and you may be held partially responsible.

chmod 700 cs32

Change into the cs32 directory, and create a lab1 directory:

cd cs32
mkdir lab1

Note that you do not need to change the permissions of the lab1 directory or any files inside of cs32 because permissions cascade. If you wanted to lock down your entire account you could run chmod 700 ~; however, no one would be able to access your website if that were the case.

Change into the lab1 directory and create the empty files you will need for this lab:

cd lab1
touch README

Exploring a few linux programs

Below is a list of linux programs you will need to use to complete this lab:

  • cut
  • echo
  • expr
  • ls
  • wc

For each program, do the following:

  1. Use the man program to look up what each does: man PROG
  2. In the README, indicate the program name by providing an example invocation of the program such as man ls. Additionally, describe what that invocation of the program should produce assuming it runs successfully.

Example README for other programs:

mkdir blah
 - Will create the directory blah in the current working directory if blah
   does not already exist.

touch foo bar
- Will update both the access and modification times of the files foo and
  bar. Will create a zero byte version of the files if they do not already

Bourne shell scripting:

To review the bourne shell scripting concepts that you will need to complete this lab, please see the introduction to bourne shell scripting.

In this part of the lab, you will write a bourne shell script that computes the average number of lines for all the regular files in a given directory.

For testing and demonstration purposes, the following commands are how I created a simple test directory:

mkdir tests
echo -e "foo\nbar\nbaz" > tests/file1
touch tests/zero

Running ls -l on the tests directory should produce a similar output:

total 4
-rw-r--r-- 1 bboe grad 12 Aug  8 22:55 file1
-rw-r--r-- 1 bboe grad  0 Aug  8 22:55 zero

Conceptually, file1 has 3 lines, and zero has zero lines. You can confirm that using wc. Thus the shell script you will write should confirm that the average is 1 (using integer division is required).

Execution via ./ tests should produce the following output:

# files: 2
avg lines: 1

Error checking

Your script should output the following error message verbatim if the number of command line arguments is not 1:


If the provided argument DIRECTORY is not actually a directory, your script should output the following error message verbatim:

{DIRECTORY} is not a directory

The {DIRECTORY} part of the error message should be replaced with whatever the user input as the first argument. Here is an example:

$ ./ nope
nope is not a directory

Successful output

Your program should iterate through the list of regular files contained in DIRECTORY and produce an output containing the number of regular files and the average number of lines contained within those regular files, if any.

For instance, if there are no regular files in DIRECTORY, then the output should be exactly:

# files: 0

If there is a single regular file with 5 lines in DIRECTORY, then the output should be exactly:

# files: 1
avg lines: 5


  1. At some point you should need to extract a portion of a process’s output. Look carefully through the man pages for the programs referenced earlier in the lab on how to do this. Make sure your solution doesn’t depend on numbers being in the same column location.
  2. You also will need to perform some basic math. This isn’t trivial in shell scripts as it relies on external programs. Again, I suggest looking through the previously referenced programs.

Submission Instructions

Please make only one submission per pair; this isn’t essential, it just makes life easier for the TA. In either case, ensure the TA has recorded with whom you are working. For your comfort, you may include both of your names in the README file.

~cs32/submit lab1@cs32 README

You may submit up to 20 times. Please check the feedback email to ensure you submitted correctly, and are satisfied with your final score. If you are not, feel free to revise and submit again.


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