Wednesday, September 22, 2004

Some in-class exercises in writing short shell scripts. 

Exercises 1 and 2 are pretty easy; you should be able to handle 
each of them with relative ease.  Exercise 3 is more challenging.
If you're having trouble with any of them, please ask for help.

1. Write a script that takes two or more numbers specified as
arguments on the command line and uses a 'while' loop to print out
each number that is evenly divisible by the first number in the list.

% divides 3 5 7 9 11 12
3
9
12

2. Write a script that that takes the name of an existing directory
(which we'll refer to as the 'target' directory) as its only argument,
creates a new directory named 'images' in the target directory if a
directory of that same name doesn't already exist, and then uses a
'foreach' loop to copy every file with extension ".gif" or ".jpg" in
the target directory to the 'images' directory.  To test your script,
create a directory called 'test' and then use the 'touch' command to
create a bunch of files with extension ".gif" and ".jpg".

3. Write a script that takes the names of two text files as its only
arguments.  The first file is just an ordinary text file that might
contain, for example, the text of a mail message or a term paper.  The
second file is of a special form: each line consists of a commonly
mispelled word followed by a space followed by the correct spelling of
the word, for example

% cat mspl.txt
tey the
teh the
arguement argument
lawer lawyer
plaintif plaintiff

Your script should correct the spelling in the ordinary text file using
the corrections in the file named by the second argument.  For example, 

% cat msg.txt
teh lawer's arguement was pretty weak
but tey plaintif still got a big settlement
% spellfix msg.txt mspl.txt
the lawyer's argument was pretty weak
but the plaintiff still got a big settlement

Here are some hints that will help in writing your script

% set fix = ( `cat mspl.txt` )
% echo "$fix[1] => $fix[2]"
tey => the

Note that the following straightforward approach to making the 
substitutions doesn't work:

% echo "tey door closed" | sed 's/$foo[1]/$foo[2]/g'
tey door closed

The problem is that 'sed' has it's own interpretation of $ (it is used
in a regular expression to match the end of a line) which is different
from the shell's interpretation.  Somehow we want to force the shell
to 'interpolate' shell variables by looking up their values and
inserting the values into the 'sed' command.  One solution involves
the careful use of quotes.  Note that 

% echo "sed 's/$fix[1]/$fix[2]/g'"
sed 's/tey/the/g'

The outer double quotes render the inner single quotes as literals -
that is to say they don't behave like single quotes usually do in
suppressing the interpolation of shell variables.  This means that the
shell variables do get interpolated.  Now we can expand on this and
use 'eval' to force evaluation of the quoted string as follows:

% eval "echo 'tey door closed' | sed 's/$fix[1]/$fix[2]/g'"
the door closed

The shell command 'eval' is often used in exactly this fashion. There
are many possible solutions to this exercise; I want you to solve this
problem using a 'while' loop that repeatedly uses 'sed' to rewrite the
input file by making a different substitution on each iteration. Note
that you'll probably want to use one or more temporary files to manage
this iterative solution.