Reading the last n lines of a file in Ruby?

Question

I need to read the last 25 lines from a file (for displaying the most recent log entries). Is there anyway in Ruby to start at the end of a file and read it backwards?

Answer 1

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]$du -sh 11M.txt
11M     11M.txt
[john@awesome]$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.rpm

real    0m0.012s
user    0m0.000s
sys     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.rpm

real    0m0.350s
user    0m0.000s
sys     0m0.130s

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