Reading the last n lines of a file in Ruby?

If on a *nix system with tail, you can cheat like this:

last_25_lines = `tail -n 25 whatever.txt`

Is the file large enough that you need to avoid reading the whole thing? If not, you could just do

IO.readlines("file.log")[-25..-1]

If it is to big, you may need to use IO#seek to read from near the end of the file, and continue seeking toward the beginning until you've seen 25 lines.

There is a library for Ruby called File::Tail. This can get you the last N lines of a file just like the UNIX tail utility.

I assume there is some seek optimization in place in the UNIX version of tail with benchmarks like these (tested on a text file just over 11M):

[john@awesome]$du -sh 11M.txt11M     11M.txt[john@awesome]$time tail -n 25 11M.txt/sbin/ypbind/sbin/arptables/sbin/arptables-save/sbin/change_console/sbin/mount.vmhgfs/misc/csait/csait/course/.autofsck/~/usb/cdrom/homebk/staff/staff/faculty/staff/faculty/darlinr/staff/csadm/staff/csadm/service_monitor.sh/staff/csadm/.bash_history/staff/csadm/mysql5/staff/csadm/mysql5/MySQL-server-community-5.0.45-0.rhel5.i386.rpm/staff/csadm/glibc-common-2.3.4-2.39.i386.rpm/staff/csadm/glibc-2.3.4-2.39.i386.rpm/staff/csadm/csunixdb.tgz/staff/csadm/glibc-headers-2.3.4-2.39.i386.rpmreal    0m0.012suser    0m0.000ssys     0m0.010s

I can only imagine the Ruby library uses a similar method.

Edit:

for Pax's curiosity:

[john@awesome]$time cat 11M.txt | tail -n 25/sbin/ypbind/sbin/arptables/sbin/arptables-save/sbin/change_console/sbin/mount.vmhgfs/misc/csait/csait/course/.autofsck/~/usb/cdrom/homebk/staff/staff/faculty/staff/faculty/darlinr/staff/csadm/staff/csadm/service_monitor.sh/staff/csadm/.bash_history/staff/csadm/mysql5/staff/csadm/mysql5/MySQL-server-community-5.0.45-0.rhel5.i386.rpm/staff/csadm/glibc-common-2.3.4-2.39.i386.rpm/staff/csadm/glibc-2.3.4-2.39.i386.rpm/staff/csadm/csunixdb.tgz/staff/csadm/glibc-headers-2.3.4-2.39.i386.rpmreal    0m0.350suser    0m0.000ssys     0m0.130s

still under a second, but if there is a lot of file operations this makes a big difference.