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8.26. Program: laston

When you log in to a Unix system, it tells you when you last logged in. That information is stored in a binary file called lastlog. Each user has their own record; UID 8 is at record 8, UID 239 at record 239, and so on. To find out when a given user last logged in, convert their login name to a number, seek to their record in that file, read, and unpack. Doing so with shell tools is hard, but with the laston program, it's easy. Here's an example:

% laston gnat
gnat  UID 314 at Mon May 25 08:32:52 2003 on ttyp0 from below.perl.com

The program in Example 8-9 is much newer than the tctee program in Example 8-8, but it's less portable. It uses the Linux binary layout of the lastlog file. You'll have to change this for other systems.

Example 8-9. laston

  #!/usr/bin/perl -w
  # laston - find out when given user last logged on
  use User::pwent;
  use IO::Seekable qw(SEEK_SET);
  open (LASTLOG, "< :raw", "/var/log/lastlog")
      or die "can't open /var/log/lastlog: $!";
  $typedef = "L A12 A16";  # linux fmt; sunos is "L A8 A16"
  $sizeof  = length(pack($typedef, ( )));
  for $user (@ARGV) {
      $U = ($user =~ /^\d+$/) ? getpwuid($user) : getpwnam($user);
      unless ($U) { warn "no such uid $user\n"; next; }
      seek(LASTLOG, $U->uid * $sizeof, SEEK_SET) or die "seek failed: $!";
      read(LASTLOG, $buffer, $sizeof) =  = $sizeof        or next;
      ($time, $line, $host) = unpack($typedef, $buffer);
      printf "%-8s UID %5d %s%s%s\n", $U->name, $U->uid,
            $time ? ("at " . localtime($time)) : "never logged in",
            $line && " on $line", 
            $host && " from $host";

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