The copy command “cp -al” found on all versions of Unix/Linux creates what’s called a “hard link” to a file. The nice thing about this command is that it doesn’t create an actual copy of the file on disk – instead, it creates a “link” or pointer to the file data on the disk. Basically a “snapshot” of that directory in-time. The net result is that you can have 10 “copies’ of a 10G file that only take up a total of 10G.
This nifty behavior makes cp -al, when combined with rsync, ideal for backup systems. One can use the cp -al command to take a “snapshot” of a given directory tree at a given time, at the expense of very little additional disk space. I use this script in concert with my rsync_backup.pl script to keep 21 days of “snapshot” backups of each of my machines.
#!/usr/bin/perl
use POSIX;
# Rotates backup directories w/ cp -al (hardlinks)
# Deletes directories older than $KEEP_DAYS
# Runs each night ahead of backup process
# (c) 2009 eddie@eddieoneverything.com
$KEEP_DAYS=21;
$LOGFILE = "/var/log/rotate_backups";
@BACKUP_DIRS=(
'/mnt/backup/hansel',
'/mnt/backup/tiger',
'/mnt/backup/june'
);
$ts = get_timestamp();
open hLOG, ">>$LOGFILE";
print hLOG "=" x 80, "\n";
print hLOG "Run START at " . `date` . "\n";
print hLOG "=" x 80, "\n";
## Do the rotation
print hLOG "Do today's rotation\n";
foreach $dir ( @BACKUP_DIRS ){
print hLOG "\t" , `date`;
$newfn = $dir ."_" . $ts;
$cmd = "cp -al $dir $newfn";
print hLOG "Execute Command: $cmd\n";
`$cmd`;
}
## Delete old directories
print hLOG "Delete Old Directories\n";
foreach $dir ( @BACKUP_DIRS ){
$dir=~m/^(.+)\/(.+?)$/;
$base= $1;
$stub=$2;
#print "dir is $dir\nBASE: $base\nSTUB:$stub\n";
opendir hDIR, "$base" or die "can't open directory $base";
@dirlist=grep { /^$stub\_/ && !/^$stub$/ } readdir(hDIR);
closedir hDIR;
foreach $d (@dirlist){
print hLOG "\t" , `date`;
$d=~/$stub\_([0-9]+)_([0-9]+)_([0-9]+)_.+$/;
$year= $1;
$month = $2;
$day = $3;
if (dirIsOlder($year, $month, $day)){
#print "$d\n";
$remove_dir= $base . '/' . $d;
$cmd = "rm -Rf \"$remove_dir\"";
print hLOG "Execute command: $cmd\n";
`$cmd`;
}else{
print hLOG "Keep $d\n";
}
}
}
print hLOG "=" x 80, "\n";
print hLOG "Run END at " . `date` . "\n";
print hLOG "=" x 80, "\n";
close hLOG;
# ------------- Subroutines & functions -----------------
sub get_timestamp {
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time);
$mon+=1;
if ($mon < 10) { $mon = "0$mon"; }
if ($mday < 10) { $mday = "0$mday"; }
if ($hour < 10) { $hour = "0$hour"; }
if ($min < 10) { $min = "0$min"; }
if ($sec < 10) { $sec = "0$sec"; }
$year=$year+1900;
return $year . '_' . $mon . '_' . $mday . '__' . $hour . '_' . $min . '_' . $sec;
}
sub dirIsOlder{
($fyear, $fmonth, $fday) = @_;
#print "Check $year - $month - $day \n";
$now = mktime(localtime());
$then = mktime (0, 0 , 0, $fday-1, $fmonth-1, $fyear-1900 , 0, 0);
$diff_sec = $now - $then;
$days_since = $diff_sec / 24 / 60 / 60;
#print "n: $now. t: $then. Diff ($diff_sec) = $days_since\n";
#print "$days_since $fyear-$fmonth-$fday\n";
## Subtract days from today
if ($days_since > $KEEP_DAYS){
return 1;
}else{
return 0;
}
}