Learning Perl, Second Edition

A.13. Chapter 14

1. Here's one way to do it:

   if (`date` =~ /^S/) {
       print "Go play!\n";
   } else {
       print "Get to work!\n";
   }

   It just so happens that the first output character of the date command is an S only on the weekend (Sat or Sun), which makes this program trivial. We invoke date, then use a regular expression to see if the first character is an S. Based on that, we print one message or the other.

2. Here's one way to do it:

   open(PW,"/etc/passwd");
   while (<PW>) {
       chomp;
       ($user,$gcos) = (split /:/)[0,4];
       ($real) = split(/,/, $gcos);
       $real{$user} = $real;
   }
   close(PW);
   
   open(WHO,"who|") || die "cannot open who pipe";
   while (<WHO>) {
       ($login, $rest) = /^(\S+)\s+(.*)/;
       $login = $real{$login} if $real{$login};
       printf "%-30s %s\n",$login,$rest;
   }

   The first loop creates a hash %real that has login names for keys and the corresponding real names as values. This hash is used in the following loop to change the login name into a real name.

   The second loop scans through the output resulting from opening the who command as a filehandle. Each line of who's output is broken apart using a regular expression match in a list context. The first word of the line (the login name) is replaced with the real name from the hash, but only if it exists. When that's all done, a nice printf puts the result onto STDOUT.

   You can replace the filehandle open and the beginning of the loop with just

   foreach $_ (`who`) {

   to accomplish the same result. The only difference is that the version with the filehandle can begin operating ...