A.13 Chapter 14, Process Management

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 as soon as who starts spitting out characters, while the version with who in backquotes must wait for who to finish.

3. 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(LPR,"|lpr") || die "cannot open LPR pipe";
   open(WHO,"who|") || die "cannot open who pipe";
   while (<WHO>) {
   # or replace previous two lines with: foreach $_ (`who`) {
       ($login, $rest) = /^(\S+)\s+(.*)/;
       $login = $real{$login} if $real{$login};
       printf LPR "%-30s %s\n",$login,$rest;
   }

   The difference between this program and the program from the previous exercise is that we've added an LPR filehandle opened onto an lpr process, and modified the printf statement to send the data there instead of STDOUT.

4. Here's one way to do it:

   sub mkdir {
       !system "/bin/mkdir", @_;
   }

   Here, the mkdir command is given the arguments directly from the arguments to the subroutine. The return value must be logically negated, however, because a nonzero exit status from system must translate into a false value for the Perl caller.

5. Here's one way to do it:

   sub mkdir {
       my($dir, $mode) = @_;
       (!system "/bin/mkdir", $dir) && chmod($mode, $dir);
   }

   First, the arguments to this routine are named as $dir and $mode. Next, we invoke mkdir on the directory named by $dir. If that succeeds, the chmod operator gives the proper mode to the directory.