1. CSC 352– Unix Programming, Fall 2012
Weeks 13 & 14, 2012
Perl Programming (Highlights only), Chapter 15

2. Project 5 Project 5 is a redo of Project 3 in Perl.
~parson/unix/perl5.zip has the handout code.
perl5/diffcheck is the assignment 3 difference checker as a Bash script.
perl5/diffcheck.pl is the assignment 5 difference checker as a Perl script.
Use diffcheck and diffcheck.pl as a Rosetta Stone. It shows you Bash scripting constructs and their Perl counterparts.
perl5/replacestr is my assignment 3 solution as a Bash script.
perl5/replacestr.pl is your starting point for a Perl script.

3. diffcheck.pl part 1 $USAGE="USAGE: diffcheck.pl DIRNAME1 DIRNAME2";
Perl assignments use a $ for defining and using any Perl variable.
Perl terminates each command with a semicolon.
Do not leave spaces around the = operator.

4. diffcheck.pl part 2 if ($#ARGV != 1) { }
print STDERR "$USAGE\n";
print "NUMARGS $#ARGV\n";
exit 1; }
$#ARGV is the index of the final command-line argument; it is one less than Shell’s $#.
if, while, and foreach programming control constructs require { curly braces } around their code blocks, just like C++.
Use print, not echo as in Shell. Put double quotes around the string, and redirect to STDERR if needed. Note the \n.

5. diffcheck.pl part 3 $DIR1=shift; $DIR2=shift; if (! (-d $DIR1)) { }
print STDERR "ERROR. $DIR1 is not a valid directory.\n";
exit 1; }
shift returns the next command line argument and shifts it away (discards it).
-d tests for a directory. See Table 15.2 in the text. <, <=, ==, >=, > and != are for numeric comparison. lt, le, eq, ge, gt and ne are for string comparison. This is the opposite of the Shell!

6. diffcheck.pl part 4 $STARTDIR=`pwd`; # note the starting directory
chomp($STARTDIR); # Get rid of any trailing (\n).
Using backticks runs a command via the Shell.
The returned result includes the trailing newline char (\n).
Use chomp to strip that \n out of the returned string.
foreach $file (`find . -type f -print`) {
The foreach command loops over every element in an array.
A backticked Shell command that returns multiple lines actually returns a Perl array, instead of a Shell-like string.
Always use chomp on anything returned via backticks.

7. diffcheck.pl part 5 foreach $file (`find . -type f -print`) { }
chomp($file); # strip away leading & trailing white space in file name
print "DIFF $file ../$DIR2/$file\n";
system("diff $file ../$DIR2/$file 2>&1"); }
Note use of chomp on each string returned from a backticked Shell command.
system() function runs its string in the Shell without returning a value.

8. While loop from my replacestr.pl
while ($#ARGV > 0) {
This really says, “while the number of command line arguments is > 1”.
$#ARGV is the index of the last element in the command line array, where the array starts at element 0. When $#ARGV == 0, there is in fact 1 element in the command line array.
Use something like $myvar=shift; to read the next command line argument into your variable called $myvar and to shift it out of the command line array. This shift decrements $#ARGV.
The while loop uses curly brace delimiters, just like the if and foreach programming constructs; if also supports else and elsif, see Figure 15.2 in the textbook.

9. Handout replacestr.pl
foreach $file (`find $DIRNAME -type f -print | xargs grep -l "$PAT"`) { chomp($file);
Because the backticks return an array of strings, we will combine the find and the grep in a single backtick to the Shell.
The xargs command takes the stdout of the previous command in the pipeline and plugs those strings in as command line arguments for the command that xargs runs.
In this case find supplies a list of regular files to xargs, which plugs them in as command line arguments for grep.
Otherwise, we would have to join the array as shown on the next page.

10. You won’t need to do this:
@allarray=`find $DIRNAME -type f -print`;
Get all regular file paths into
Discard the \n on every element in the array.
$all=join("
Join the file paths into a single, space-separated string.
@matchesarray=`grep -l "$PAT" $all`;
Find matching files using grep –l, save
foreach $file {
Within loop chomp($file); and do other replacestr.pl steps.

11. Assignment 5
Assignment 5 is due by the end of Saturday, December 8 via gmake turnitin. There is another handout for the assignment.
The comments in replacestr.pl outline your code, just like replacestr in assignment 3.
You have my Shell file replacestr in this assignment handout.
You need to use Perl syntax, control constructs, and command line argument processing.
Use backticks to run Shell command lines similar to the solution in replacestr.