Tail command switches with example in Unix/Linux

10+ useful tail command’s switches with example – Unix/Linux

The tail command

By default, the tail command prints the last 10 lines from the file which is given followed by the tail command.

In the case of more than one file with tail command displays separate output with a header identifying the file name. Tail command has a lot of useful and also very helpful switches.

In this article, we discuss about the usages of Linux tail command.

Useful switches with example:

“I’ve copied the ‘/etc/passwd’ and ‘/etc/shadow’ files to pass.txt and sha.txt respectively to demonstrate examples.”

1. tail : The tail command alone

Which displays the last 10 line by-default.

Example:

# tail pass.txt
cpanelphppgadmin:x:32004:32004::/var/cpanel/userhomes/cpanelphppgadmin:/usr/local/cpanel/bin/noshell
......................
......................
tomcat:x:495:99:Apache Tomcat:/home/tomcat:/bin/nologin
crybit:x:503:504::/home/crybit:/usr/local/cpanel/bin/noshell

2. tail : More than one file

Displays separate output with a header identifying the file name.

Example:

# tail pass.txt sha.txt
==> pass.txt <== 
cpanelphppgadmin:x:32004:32004::/var/cpanel/userhomes/cpanelphppgadmin:/usr/local/cpanel/bin/noshell 
...................... 
...................... 
tomcat:x:495:99:Apache Tomcat:/home/tomcat:/bin/nologin 
crybit:x:503:504::/home/crybit:/usr/local/cpanel/bin/noshell 
==> sha.txt <==
cpanelphppgadmin:*:16073::::::
......................
......................
tomcat:!!:16074::::::
crybit:$6$xbycY1hCeTldon5.$.J6ZtPA77351rLQdo23tKi4o5cTrjlJOOzkZJs11AeMqeGlsnTdLzuo2x7HtOzf0JigQ42AlrfTRzj5Bdcl/D.:16085:0:99999:7:::

3. tail -n : Displays last “n” lines from the text file

Example:

# tail -n3 pass.txt
cpses:x:496:497::/var/cpanel/cpses:/sbin/nologin
tomcat:x:495:99:Apache Tomcat:/home/tomcat:/bin/nologin
crybit:x:503:504::/home/crybit:/usr/local/cpanel/bin/noshell

4. -c, –bytes=K

Output the last K bytes; alternatively, use -c +K to output bytes starting with the Kth of each file.

Example:

# tail -c 100  pass.txt
pache Tomcat:/home/tomcat:/bin/nologin
crybit:x:503:504::/home/crybit:/usr/local/cpanel/bin/noshell

5. -f, –follow[={name|descriptor}]

Output appended data as the file grows; -f, –follow, and –follow=descriptor are equivalent

6. -F same as –follow=name –retry

7. –max-unchanged-stats=N

with –follow=name, reopen a FILE which has not changed size after N (default 5) iterations to see if it has been unlinked or renamed (this is the usual case of rotated log files). With inotify, this option is rarely useful.

8. –pid=PID

With -f, terminate after process ID, PID dies

9. -q, –quiet, –silent

Never output headers giving file names

10. –retry

Keep trying to open a file even when it is or becomes inaccessible; useful when following by name, i.e., with –follow=name

11. -s, –sleep-interval=N

With -f, sleep for approximately N seconds (default 1.0) between iterations.

12. –version

Displays the version.

13. –help

Displays the help messages.

The “-f” switch is very useful for analyzing the log files dynamically.

Examples: Watching apache error log dynamically.

# tail -f /usr/local/apache/logs/error_log
Or
# tailf /usr/local/apache/logs/error_log

That’s it. 🙂

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