CS32, Fall 2012

Lab02:
Shell scripts

Goals for this lab

By the time you have completed this lab, you should be able to

Step by Step Instructions

Step 0: Choose initial pair programming roles, and log on

Let the person who was not the first pilot last week be the pilot first this time. Switch roles after awhile, before the pilot gets tired, and before the navigator gets bored or distracted.

If your regular partner is more than 5 minutes late, ask the TA to pair you with someone else for this week.

Step 1: Create a directory for this lab in the pilot's account

This lab's pilot should log in. You will (both) work in this account for the rest of the lab.

Create a ~/cs32/lab02 directory and make it your current directory:

mkdir ~/cs32/lab02
cd ~/cs32/lab02

Step 2: Practice creating and running a simple script

Recall (from lectures/readings) that a script is a collection of shell commands and control structures. In fact a script is like a program written in the language of a particular shell. The shell interprets the script, and executes the commands in the order prescribed by the program. Any sequence of shell commands that are stored in a file can (basically) qualify as a script.

Use an editor (e.g., emacs) to create a file named rootcontents.sh - then type just the following two lines in it, and save the file: 

cd /
ls
Now tell an appropriate shell to execute it, like sh for instance (user input is bold):

bash-4.2$ sh rootcontents.sh 
bin   cgroup  dev  fs	 lib	lost+found  mnt  proc  sbin	srv  tmp  var
boot  cs      etc  home  local	media	    opt  root  selinux	sys  usr

To repeat: "you must tell an appropriate shell to execute it." The point is that the shell is the executing program, and the script is just the specific instructions that program is asked to execute. The default shell will run it if you don't specify:

bash-4.2$ ./rootcontents.sh
bash: ./rootcontents.sh: Permission denied

Oops, first we must change the permissions of the script file to make it executable. Then bash, our default shell, will execute it:

bash-4.2$ chmod 755 rootcontents.sh 
bash-4.2$ ./rootcontents.sh
bin   cgroup  dev  fs	 lib	lost+found  mnt  proc  sbin	srv  tmp  var
boot  cs      etc  home  local	media	    opt  root  selinux	sys  usr
bash-4.2$ pwd
/cs/class/cs32/labs/lab02

NOTICE TWO IMPORTANT THINGS:

  1. After the last run we typed pwd and found out our current directory was still local to our account, and not the root (/) as specified by `cd /` in the script.
    Do you know why? Think about it, then click here for the answer.
  2. This script was very simple, using commands standard to all shells and with no special options or syntax. So any shell can execute it without errors. Most scripts use syntax that can vary between shells though, so script-writers usually include a shebang as the first line of the script.

Fix the script now (as per important thing #2) by typing the following at the very start of rootcontents.sh:

#!/bin/sh

Then run the script again to verify it still works. The shebang must be the very first line of the script, and it must not even have any spaces before it.

Normally you would follow the shebang with a comment. In keeping with this convention, add the following comment (begins with '#') right after the shebang as shown - substituting your actual name(s), of course:

#!/bin/sh
# Very simple script to display the root's contents
# YOUR NAME(S), 10/11/2012

Step 3: Learn (or hopefully review) more sh scripting syntax

Sufficient sh scripting techniques were presented in lecture to handle the required in-lab work of Step 4. In case you missed it, here is a link to the demo scripts presented in lecture. Additional techniques might be required for the after-lab tasks though, so here is a short sh script primer, and here is a link to a nice bash scripting tutorial (bash scripts are basically identical to sh scripts).

Peruse/review these materials now, and then refer back to them if necessary to complete the rest of the lab work.

Also remember that shell scripts just contain shell commands, and that these commands can be tested interactively at the shell prompt. In other words, don't be afraid to try stuff before including it in your script. Use the same continuation character you use for C programs, '\', to continue long lines (or control structures).

Step 4: Write an sh script to list writable items in a directory

SWITCH ROLES WITH YOUR PARTNER now if you have not already done that. Please remember to do so frequently in every lab, and we promise to stop pestering you about it. Thanks!

Write a script named listwritable (most scripts don't have extensions like .sh) to list the writable items in the directory specified by the user at the command line. For example, assuming that listwritable has its permissions set to execute, then the following command would print the names of just the writable items in the directory named myfiles:

./listwritable myfiles

Here are two sample runs, one for the current directory ('.'), and one for the root:

bash-4.2$ ./listwritable .
   listwritable
   rootcontents.sh
bash-4.2$ ./listwritable /
   tmp

This in-lab part does not have to do any error checking. For example, if the user does not specify a command line argument, or if the argument is not a readable directory name, then the script's behavior may be unpredictable. That's okay for now.

Be sure to include the shebang line at the beginning of the script.

Also include a comment with your name(s) and the current date. Please do this for every file you create in CS 32 labs in the future too. Thanks.

Make the file executable - type 'chmod 755 listwritable' - then test it a few times until you are sure it is working, and leave the terminal window open for the TA to see.

Step 5: Show off your work and get credit for the in-lab requirements

Get your TA's attention to inspect your work, and to record completion of your in-lab work.

Don't leave early though ... begin the after-lab work below.

Step 5a. ONLY IF YOU RAN OUT OF TIME TO HAVE THE TA INSPECT YOUR WORK

If you must complete this assignment at CSIL, then submit it with the turnin program - but do NOT turn it in if the TA already checked you off. You MUST have both your name and your partner's name in the file in order to receive credit. Remember that the original pilot needs to do this step, since that is whose account you have been using in Cooper Lab.

Bring up a terminal window on CSIL, and cd into the original pilot's cs32 directory, and cd again into the lab02 directory. Then type the following to turn in your version of listwritable:

turnin lab02@cs32 listwritable

Respond "yes" when the program asks if you want to turn in (be sure to read the list of files you are turning in), and then wait for the message indicating success.

Evaluation and Grading

Each pair of students must accomplish the following to earn full credit for this lab:

After lab-work is done

Homework - to be done before lab next week: October 18

This week the homework is (mostly) to add features to the listwritable script, as described in these hw2 instructions. You will use the turnin program to submit your enhanced script, and that must be done before your next lab - the TA will check for it then. A short written part is also required, to be turned in at your lab next week. Please do NOT print the homework in Cooper Lab.

Each student must complete the paper part of the homework, even if you do the script part in pairs.

Optional - just for fun and/or to keep busy

Write a script named sumargs to sum the arguments entered on the command line (assume the user enters nothing but numbers). Use the expr command to do simple calculations in a script, as demonstrated in the multi demo script. Example sumargs runs should look like this:

bash-4.2$ ./sumargs 75 6 -9
sum is 72
bash-4.2$ ./sumargs
sum is 0

The user did not enter any arguments in the second run, so the sum is 0.

Want/need more to do? Why not write a script named calcargs that will perform different calculations on the arguments. Perhaps provide a menu to give the user a choice to add, subtract, multiply or divide the arguments. See the menu demo script for an example menu and use of the read command to get user responses. Add error checking if you want to be proud of your script!

Prepared by Michael Costanzo.