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8.20. 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 very hard, but it's very easy with the laston program. Here's an example:

% laston gnat
gnat  UID 314 at Mon May 25 08:32:52 1998 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
# laston - find out when given user last logged on
use User::pwent;
use IO::Seekable qw(SEEK_SET);

open (LASTLOG, "/var/log/lastlog") or die "can't open /usr/adm/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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