Managing files 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.

Today, in this article we will be discussing the most popular commands called head, tail and cat, most of us already aware of such commands, but very few of us implement it when needed.

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 Tecmint-Team' > 1 
# echo 'Keep connected' > 2 
# echo 'Share your thought' > 3 
# echo 'connect us tecmint.com@gmail.com' > 4
# cat 1 2 3 4 > 5
# cat 5 

Hi Tecmint-Team 
Keep connected 
Share your thought 
connect us tecmint.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 > tecmint.txt

Tecmint 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 Avishek 
Here i am writing this post 
Hope your are enjoying 
end
# cat test.txt 

I am Avishek 
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 avi.txt

I am a Programmer by birth and Admin by profession
# cat avi.txt > avi1.txt
# cat avi1.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

For more examples of cat command usage, refer to the 13 cat Command Usage

That’s all for now. I’ll be here again with another Interesting Article, worth Knowing. Till then stay tuned and connected to Tecmint. Don’t forget to provide us with your valuable feedback in our comment section.

Using the free Command in Linux

This article provides some useful examples of “free” commands with options, that might be useful for you to better utilize memory that you have.

1. Display System Memory

Free command used to check the used and available space of physical memory and swap memory inKB. See the command in action below.

# free

             total       used       free     shared    buffers     cached
Mem:       1021628     912548     109080          0     120368     655548
-/+ buffers/cache:     136632     884996
Swap:      4194296          0    4194296

2. Display Memory in Bytes

Free command with option -b, display the size of memory in Bytes.

# free -b

             total       used       free     shared    buffers     cached
Mem:    1046147072  934420480  111726592          0  123256832  671281152
-/+ buffers/cache:  139882496  906264576
Swap:   4294959104          0 4294959104

3. Display Memory in Kilo Bytes

Free command with option -k, display the size of memory in (KB) Kilobytes.

# free -k

             total       used       free     shared    buffers     cached
Mem:       1021628     912520     109108          0     120368     655548
-/+ buffers/cache:     136604     885024
Swap:      4194296          0    4194296

4. Display Memory in Megabytes

To see the size of the memory in (MB) Megabytes use option as -m.

# free -m

             total       used       free     shared    buffers     cached
Mem:           997        891        106          0        117        640
-/+ buffers/cache:        133        864
Swap:         4095          0       4095

5. Display Memory in Gigabytes

Using -g option with free command, would display the size of the memory in GB(Gigabytes).

# free -g
             total       used       free     shared    buffers     cached
Mem:             0          0          0          0          0          0
-/+ buffers/cache:          0          0
Swap:            3          0          3

6. Display Total Line

Free command with -t option, will list the total line at the end.

# free -t

            total       used       free     shared    buffers     cached
Mem:       1021628     912520     109108          0     120368     655548
-/+ buffers/cache:     136604     885024
Swap:      4194296          0    4194296
Total: 5215924 912520 4303404

7. Disable Display of Buffer Adjusted Line

By default the free command display “buffer adjusted” line, to disable this line use option as -o.

# free -o

            total       used       free     shared    buffers     cached
Mem:       1021628     912520     109108          0     120368     655548
Swap:      4194296          0    4194296

8. Dispaly Memory Status for Regular Intervals

The -s option with number, used to update free command at regular intervals. For example, the below command will update free command every 5 seconds.

# free -s 5

             total       used       free     shared    buffers     cached
Mem:       1021628     912368     109260          0     120368     655548
-/+ buffers/cache:     136452     885176
Swap:      4194296          0    4194296

9. Show Low and High Memory Statistics

The -l switch displays detailed high and low memory size statistics.

# free -l

             total       used       free     shared    buffers     cached
Mem:       1021628     912368     109260          0     120368     655548
Low:        890036     789064     100972
High:       131592     123304       8288
-/+ buffers/cache:     136452     885176
Swap:      4194296          0    4194296

10. Check Free Version

The -V option, display free command version information.

# free -V

procps version 3.2.8