Manage Files Effectively using head, tail and cat Commands in Linux

There are several commands and programs provided by Linux for viewing the contents of file. Working with files is one of the daunting task, most of the computer users be it newbie, regular user, advanced user, developer, admin, etc performs. Working with files effectively and efficiently is an art.

1. head Command

The head command reads the first ten lines of a any given file name. The basic syntax of head command is:

head [options] [file(s)]

For example, the following command will display the first ten lines of the file named ‘/etc/passwd‘.

# head /etc/passwd 

root:x:0:0:root:/root:/bin/bash 

daemon:x:1:1:daemon:/usr/sbin:/bin/sh 

bin:x:2:2:bin:/bin:/bin/sh 

sys:x:3:3:sys:/dev:/bin/sh 

sync:x:4:65534:sync:/bin:/bin/sync 

games:x:5:60:games:/usr/games:/bin/sh 

man:x:6:12:man:/var/cache/man:/bin/sh 

lp:x:7:7:lp:/var/spool/lpd:/bin/sh 

mail:x:8:8:mail:/var/mail:/bin/sh 

news:x:9:9:news:/var/spool/news:/bin/sh

If more than one file is given, head will show the first ten lines of each file separately. For example, the following command will show ten lines of each file.

# head /etc/passwd /etc/shadow

==> /etc/passwd <== root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm:x:3:4:adm:/var/adm:/sbin/nologin lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin sync:x:5:0:sync:/sbin:/bin/sync shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown halt:x:7:0:halt:/sbin:/sbin/halt mail:x:8:12:mail:/var/spool/mail:/sbin/nologin uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin ==> /etc/shadow <==

root:$6$85e1:15740:0:99999:7:::

bin:*:15513:0:99999:7:::

daemon:*:15513:0:99999:7:::

adm:*:15513:0:99999:7:::

lp:*:15513:0:99999:7:::

sync:*:15513:0:99999:7:::

shutdown:*:15513:0:99999:7:::

halt:*:15513:0:99999:7:::

mail:*:15513:0:99999:7:::

uucp:*:15513:0:99999:7:::

If it is desired to retrieve more number of lines than the default ten, then ‘-n‘ option is used along with an integer telling the number of lines to be retrieved. For example, the following command will display first 5 lines from the file ‘/var/log/yum.log‘ file.

# head -n5 /var/log/yum.log

Jan 10 00:06:49 Updated: openssl-1.0.1e-16.el6_5.4.i686

Jan 10 00:06:56 Updated: openssl-devel-1.0.1e-16.el6_5.4.i686

Jan 10 00:11:42 Installed: perl-Net-SSLeay-1.35-9.el6.i686

Jan 13 22:13:31 Installed: python-configobj-4.6.0-3.el6.noarch

Jan 13 22:13:36 Installed: terminator-0.95-3.el6.rf.noarch

In fact, there is no need to use ‘-n‘ option. Just the hyphen and specify the integer without spaces to get the same result as the above command.

# head  -5 /var/log/yum.log

Jan 10 00:06:49 Updated: openssl-1.0.1e-16.el6_5.4.i686

Jan 10 00:06:56 Updated: openssl-devel-1.0.1e-16.el6_5.4.i686

Jan 10 00:11:42 Installed: perl-Net-SSLeay-1.35-9.el6.i686

Jan 13 22:13:31 Installed: python-configobj-4.6.0-3.el6.noarch

Jan 13 22:13:36 Installed: terminator-0.95-3.el6.rf.noarch

The head command can also display any desired number of bytes using ‘-c‘ option followed by the number of bytes to be displayed. For example, the following command will display the first 45 bytes of given file.

# head -c45 /var/log/yum.log

Jan 10 00:06:49 Updated: openssl-1.0.1e-16.el

2. tail Command

The tail command allows you to display last ten lines of any text file. Similar to the head command above, tail command also support options  ‘n‘ number of lines and ‘n‘ number of characters.

The basic syntax of tail command is:

# tail [options] [filenames]

For example, the following command will print the last ten lines of a file called ‘access.log‘.

# tail access.log 

1390288226.042      0 172.16.18.71 TCP_DENIED/407 1771 GET http://download.newnext.me/spark.bin? - NONE/- text/html

1390288226.198      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

1390288226.210   1182 172.16.20.44 TCP_MISS/200 70872 GET http://mahavat.gov.in/Mahavat/index.jsp pg DIRECT/61.16.223.197 text/html

1390288226.284     70 172.16.20.44 TCP_MISS/304 269 GET http://mahavat.gov.in/Mahavat/i/i-19.gif pg DIRECT/61.16.223.197 -

1390288226.362    570 172.16.176.139 TCP_MISS/200 694 GET http://p4-gayr4vyqxh7oa-3ekrqzjikvrczq44-if-v6exp3-v4.metric.gstatic.com/v6exp3/redir.html pg 

1390288226.402      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

1390288226.437    145 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html

1390288226.445      0 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html

1390288226.605      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

1390288226.808      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

If more than one file is provided, tail will print the last ten lines of each file as shown below.

# tail access.log error.log

==> access.log <== 1390288226.042      0 172.16.18.71 TCP_DENIED/407 1771 GET http://download.newnext.me/spark.bin? - NONE/- text/html 1390288226.198      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html 1390288226.210   1182 172.16.20.44 TCP_MISS/200 70872 GET http://mahavat.gov.in/Mahavat/index.jsp pg DIRECT/61.16.223.197 text/html 1390288226.284     70 172.16.20.44 TCP_MISS/304 269 GET http://mahavat.gov.in/Mahavat/i/i-19.gif pg DIRECT/61.16.223.197 - 1390288226.362    570 172.16.176.139 TCP_MISS/200 694 GET http://p4-gayr4vyqxh7oa-3ekrqzjikvrczq44-if-v6exp3-v4.metric.gstatic.com/v6exp3/redir.html pg  1390288226.402      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html 1390288226.437    145 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html 1390288226.445      0 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html 1390288226.605      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html 1390288226.808      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html ==> error_log <==

[Sun Mar 30 03:16:03 2014] [notice] Digest: generating secret for digest authentication ...

[Sun Mar 30 03:16:03 2014] [notice] Digest: done

[Sun Mar 30 03:16:03 2014] [notice] Apache/2.2.15 (Unix) DAV/2 PHP/5.3.3 mod_ssl/2.2.15 OpenSSL/1.0.0-fips configured -- resuming normal operations

Similarly, you can also print the last few lines using the ‘-n‘ option as shown below.

# tail -5 access.log

1390288226.402      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

1390288226.437    145 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html

1390288226.445      0 172.16.18.53 TCP_DENIED/407 1723 OPTIONS http://172.16.25.252/ - NONE/- text/html

1390288226.605      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

1390288226.808      0 172.16.16.55 TCP_DENIED/407 1753 CONNECT ent-shasta-rrs.symantec.com:443 - NONE/- text/html

You can also print the number of characters using ‘-c’ argument as shown below.

# tail -c5 access.log

ymantec.com:443 - NONE/- text/html

3. cat Command

The ‘cat‘ command is most widely used, universal tool. It copies standard input to standard output. The command supports scrolling, if text file doesn’t fit the current screen.

The basic syntax of cat command is:

# cat [options] [filenames] [-] [filenames]

The most frequent use of cat is to read the contents of files. All that is required to open a file for reading is to type cat followed by a space and the file name.

# cat /etc/passwd 

root:x:0:0:root:/root:/bin/bash 

daemon:x:1:1:daemon:/usr/sbin:/bin/sh 

bin:x:2:2:bin:/bin:/bin/sh 

sys:x:3:3:sys:/dev:/bin/sh 

sync:x:4:65534:sync:/bin:/bin/sync 

games:x:5:60:games:/usr/games:/bin/sh 

man:x:6:12:man:/var/cache/man:/bin/sh 

lp:x:7:7:lp:/var/spool/lpd:/bin/sh 

…

The cat command also used to concatenate number of files together.

# echo 'Hi Manish-Team' > 1 

# echo 'Keep connected' > 2 

# echo 'Share your thought' > 3 

# echo 'connect us manish.com@gmail.com' > 4

# cat 1 2 3 4 > 5

# cat 5 

Hi Manish-Team 

Keep connected 

Share your thought 

connect us manish.com@gmail.com

It can be also used to create files as well. It is achieved by executing cat followed by the output redirection operator and the file name to be created.

# cat > manish.txt

Manish is the only website fully dedicated to Linux.

We can have custom end maker for ‘cat’ command. Here it is implemented.

# cat > test.txt << end 

I am Manish 

Here i am writing this post 

Hope your are enjoying 

end

# cat test.txt 

I am Manish 

Here i am writing this post 

Hope your are enjoying

Never underestimate the power of  ‘cat’ command and can be useful for copying files.

# cat manish.txt

I am a Programmer by birth and Admin by profession

# cat manish.txt > manish1.txt

# cat manish1.txt

I am a Programmer by birth and Admin by profession

Now what’s the opposite of cat? Yeah it’s ‘tac‘. ‘tac‘ is a command under Linux. It is better to show an example of ‘tac’ than to talk anything about it.

Create a text file with the names of all the month, such that one word appears on a line.

# cat month

January

February

March

April

May

June

July

August

September

October

November

December

# tac month

December

November

October

September

August

July

June

May

April

March

February

January