CMPS 12L Introduction to Programming Lab Assignment 3


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In this lab assignment you will create a shell script to compile and run your program, redirecting
its input and output from and to files. Recall from lab2 that a shell script is a text file containing instructions
to be executed by a Unix command interpreter. If necessary re-read the sections of the lab2 project
description pertaining to shell scripts and input-output redirection.
Why is a Unix command interpreter called a shell? A computer is sometimes thought of as consisting of a
series of concentric circles with the hardware in the center and the outer layers representing software
The computer hardware consists of the CPU, memory, and I/O devices. The kernel serves as a bridge
between software applications and the actual data processing that takes place at the hardware level. In the
Unix operating system the main application is the command interpreter which acts as an interface between
the kernel and user. This outermost layer or “shell” carries out user commands typed at the prompt %. There
are many versions of the Unix shell. Some of the most common are
sh Bourne Shell
csh C Shell
tcsh Tenex C Shell
ksh Korn Shell
bash Bourne Again Shell
zsh Z Shell
Each shell has slightly different commands and features. The Unix commands you are already familiar
with work in all shells, so the differences are only in the advanced commands. Go to
to see the pros/cons and histories of various shells.
Unix Kernel
Command Interpreter
To see which shell you are running, do the Unix command ps –p $$. Most likely it will be either bash or
csh. To run a different shell just type its name. For example do ksh then tcsh. Notice that each shell
gives you a slightly different prompt. Type exit twice to get back to your original shell.
Each shell is also a complete programming language with conditional and iterative control structures,
variables, I/O commands, etc. A shell script is just a program in one of these languages. Typically one
specifies the particular shell to be used on the first line of the script as follows.
#! /bin/bash

We will be using the bash shell in this assignment so all examples will begin with the above line. The
character sequence #! is known among Unix initiates as “shebang”. Subsequent lines that begin with the #
character are comments and are ignored by the shell. Traditionally the first program you write in any
language just prints the message “Hello, World!” to standard output. Here is the hello world program as a
bash script.
#! /bin/bash
# example1
echo “Hello, World!”
Create a subdirectory of cs12a called lab3. Create a text file in that directory called example1 containing
the above lines. Make it executable and run it by doing
% chmod 700 example1
% example1
As always the % character represents your command prompt and you do not type it. Notice that just like
any Java program, we start our script with a comment block. A shell script can contain any commands we
might run at the command line. Here’s another example.
#! /bin/bash
# example2
echo -n “Hello $NAME. It is now”
date +”%l:%M %p %Z, %A %B %d, %Y.”
Replace your_name in this script with your own name then run it. There’s a lot going on here so let’s take
it one line at a time.
assigns the variable NAME to be the string “your_name”. Unlike in Java, spaces matter in shell scripts. In
NAME = “your_name”
will not work (try it and see the error message you get). Thus to define a variable we just do VAR=VALUE.
To refer to the value stored in a variable however, we must place a $ before its name. This helps to explain
the next line.
echo -n “Hello $NAME. It is now”
If you do man echo you will see that the –n option omits the newline character at the end of the printed
line. The date command prints out the date and time in a standard format. That format can be altered as
desired. Try date by itself, then do man date to see all the formatting options. The line
date +”%l:%M %p %Z, %A %B %d, %Y.”
prints the date in a slightly more friendly format, as you can see when you run the script. Now consider
another example.
#! /bin/bash
# example3
ls -l
example1 > junk-out
example2 >> junk-out
javac example1 >& comp-errs
Try to predict what the effect of this script will be. If necessary type the commands individually and observe
their effects. The first line ls –l is the only line whose output goes to stdout. Note the option is the letter
“l” not the number “1” Recall from lab2 the meanings of the redirect operators >, >> and >&. The next line
example1 > junk-out runs example1 and sends its output to a new file called junk-out. The line
example2 >> junk-out appends the output of example2 to the same file. The next line runs the javac
compiler on the file example1. Since example1 is not a Java source file, one expects to get only error
messages from the compiler. In fact the errors you get from javac do not go to stdout, but to stderr. The
line javac example1 >& comp-errs places those errors in the file comp-errs. Run the above script and
observe these effects.
A short introduction to writing bash scripts can be found at
You can see more advanced examples at
Chapter 13 of the recommended text Your Unix by Sumitabha Das (mentioned in our course syllabus)
contains a very thorough introduction to programming with sh, the Bourne Shell.
What to turn in
Copy your program from pa1 into the lab3 directory. Write a bash script called RunLawn that
performs the following operations.
1. Print the line compiling to stdout.
2. Compile the program, sending any errors or warnings to the file Lawn-errs.
3. Print the line running Lawn.class to stdout.
4. Run the Java executable Lawn.class, reading input from a file called Lawn-in and sending output to
a file called Lawn-out.
5. Print the line deleting Lawn.class to stdout.
6. Remove the file Lawn.class from the current directory.
Begin by asking yourself how you would perform each of the above steps at the Unix command line.
Obviously item (2) requires the javac compiler, and item (4) requires the java command, which invokes
the Java Virtual Machine (JVM). To test your script prepare an input file called Lawn-in containing the
five numbers you would type in response to the user prompts in, which ask for lengths, widths,
and mowing rate. Run your script and see if the expected output is stored in Lawn-out. Notice that the
user prompts are intermixed with program output making the file Lawn-out a little hard to read. Edit your
source file so as to eliminate those user prompts. A transcript of one possible test run of your
script would appear as follows.
% more Lawn-in
100 200
50 75
% RunLawn
running Lawn.class
deleting Lawn.class
% more Lawn-errs
% more Lawn-out
The lawn area is 16250.0 square feet.
The mowing time is 0 hours 54 minutes 10 seconds.
Note that Lawn-errs is empty since there were no syntax errors. Delete a semi-colon in to
introduce a syntax error and run your script again. Check that the error messages are directed to the file
Lawn-errs. Correct the syntax error before you turn the project in.
Submit the files RunLawn and your edited version of to the assignment name lab3. This project
is somewhat shorter than lab2, but still requires time to get it right, so start early and ask questions in lab
sessions, office hours and on Piazza.