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head / tail – Print First / Last Part Of Files
Sometimes you don't want all the output from a command. You may only want the first few lines or the last few lines. The head command prints the first ten lines of a file and the tail command prints the last ten lines. By default, both commands print ten lines of text, but this can be adjusted with the “-n” option:
[me@linuxbox ~]$ head -n 5 ls-output.txt
total 343496
-rwxr-xr-x 1 root root 31316 2007-12-05 08:58 [
-rwxr-xr-x 1 root root 8240 2007-12-09 13:39 411toppm
-rwxr-xr-x 1 root root 111276 2007-11-26 14:27 a2p
-rwxr-xr-x 1 root root 25368 2006-10-06 20:16 a52dec [me@linuxbox ~]$ tail -n 5 ls-output.txt
-rwxr-xr-x 1 root root 5234 2007-06-27 10:56 znew
-rwxr-xr-x 1 root root 691 2005-09-10 04:21 zonetab2pot.py
-rw-r--r-- 1 root root 930 2007-11-01 12:23 zonetab2pot.pyc
-rw-r--r-- 1 root root 930 2007-11-01 12:23 zonetab2pot.pyo
lrwxrwxrwx 1 root root 6 2016-01-31 05:22 zsoelim -> soelim
[me@linuxbox ~]$ head -n 5 ls-output.txt
total 343496
-rwxr-xr-x 1 root root 31316 2007-12-05 08:58 [
-rwxr-xr-x 1 root root 8240 2007-12-09 13:39 411toppm
-rwxr-xr-x 1 root root 111276 2007-11-26 14:27 a2p
-rwxr-xr-x 1 root root 25368 2006-10-06 20:16 a52dec [me@linuxbox ~]$ tail -n 5 ls-output.txt
-rwxr-xr-x 1 root root 5234 2007-06-27 10:56 znew
-rwxr-xr-x 1 root root 691 2005-09-10 04:21 zonetab2pot.py
-rw-r--r-- 1 root root 930 2007-11-01 12:23 zonetab2pot.pyc
-rw-r--r-- 1 root root 930 2007-11-01 12:23 zonetab2pot.pyo
lrwxrwxrwx 1 root root 6 2016-01-31 05:22 zsoelim -> soelim
These can be used in pipelines as well:
[me@linuxbox ~]$ ls /usr/bin | tail -n 5
[me@linuxbox ~]$ ls /usr/bin | tail -n 5
znew zonetab2pot.py zonetab2pot.pyc zonetab2pot.pyo zsoelim
znew zonetab2pot.py zonetab2pot.pyc zonetab2pot.pyo zsoelim
tail has an option which allows you to view files in real-time. This is useful for watch- ing the progress of log files as they are being written. In the following example, we will look at the messages file in /var/log (or the /var/log/syslog file if mes- sages is missing). Superuser privileges are required to do this on some Linux distribu- tions, since the /var/log/messages file may contain security information:
[me@linuxbox ~]$ tail -f /var/log/messages
Feb 8 13:40:05 twin4 dhclient: DHCPACK from 192.168.1.1
Feb 8 13:40:05 twin4 dhclient: bound to 192.168.1.4 -- renewal in 1652 seconds.
Feb 8 13:55:32 twin4 mountd[3953]: /var/NFSv4/musicbox exported to both 192.168.1.0/24 and twin7.localdomain in 192.168.1.0/24,twin7.localdomain
Feb 8 14:07:37 twin4 dhclient: DHCPREQUEST on eth0 to 192.168.1.1 port 67
Feb 8 14:07:37 twin4 dhclient: DHCPACK from 192.168.1.1
Feb 8 14:07:37 twin4 dhclient: bound to 192.168.1.4 -- renewal in 1771 seconds.
Feb 8 14:09:56 twin4 smartd[3468]: Device: /dev/hda, SMART Prefailure Attribute: 8 Seek_Time_Performance changed from 237 to 236 Feb 8 14:10:37 twin4 mountd[3953]: /var/NFSv4/musicbox exported to both 192.168.1.0/24 and twin7.localdomain in 192.168.1.0/24,twin7.localdomain
Feb 8 14:25:07 twin4 sshd(pam_unix)[29234]: session opened for user me by (uid=0)
Feb 8 14:25:36 twin4 su(pam_unix)[29279]: session opened for user root by me(uid=500)
[me@linuxbox ~]$ tail -f /var/log/messages
Feb 8 13:40:05 twin4 dhclient: DHCPACK from 192.168.1.1
Feb 8 13:40:05 twin4 dhclient: bound to 192.168.1.4 -- renewal in 1652 seconds.
Feb 8 13:55:32 twin4 mountd[3953]: /var/NFSv4/musicbox exported to both 192.168.1.0/24 and twin7.localdomain in 192.168.1.0/24,twin7.localdomain
Feb 8 14:07:37 twin4 dhclient: DHCPREQUEST on eth0 to 192.168.1.1 port 67
Feb 8 14:07:37 twin4 dhclient: DHCPACK from 192.168.1.1
Feb 8 14:07:37 twin4 dhclient: bound to 192.168.1.4 -- renewal in 1771 seconds.
Feb 8 14:09:56 twin4 smartd[3468]: Device: /dev/hda, SMART Prefailure Attribute: 8 Seek_Time_Performance changed from 237 to 236 Feb 8 14:10:37 twin4 mountd[3953]: /var/NFSv4/musicbox exported to both 192.168.1.0/24 and twin7.localdomain in 192.168.1.0/24,twin7.localdomain
Feb 8 14:25:07 twin4 sshd(pam_unix)[29234]: session opened for user me by (uid=0)
Feb 8 14:25:36 twin4 su(pam_unix)[29279]: session opened for user root by me(uid=500)
Using the “-f” option, tail continues to monitor the file and when new lines are ap- pended, they immediately appear on the display. This continues until you type Ctrl-c.