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13 Linux Cat Command Examples To Manage (Display,Sort,Create etc) Files
In Linux operating system most of configuration files, logs even shell scripts are in text file format. There is why we have numbers of text editors in Linux. When you just need to see the content of those files, you can use a simple command named cat.
From cat manual page it says
cat is a command that concatenate files and print on the standard output
Cat is built-in command in Linux. I believe that all of Linux distribution has this Cat command by default. Let’s start to use how to use it.
1. View the content of file
The easiest way to use cat is just type ‘cat file_name’.
# cat /etc/issue
CentOS release 5.10 (Final)
Kernel \r on an \m
2. Put the line number on the fly
When reading a configuration file it may that you have a long configuration file. It will be easier to if you can put line numbers on the fly. Use -n parameter to fulfill this purpose.
# cat -n /etc/ntp.conf
1 # Permit time synchronization our time resource but do not
2 # permit the source to query or modify the service on this system
3 restrict default kod nomodify notrap nopeer noquery
4 restrict -6 default kod nomodify notrap nopeer noquery
5
6 # Permit all access over the loopback interface. This could be
7 # tightened as well, but to do so would effect some of the
8 # administration functions
9 restrict 127.0.0.1
10 restrict -6 ::1
3. Number non-blank output lines
Similar with -n parameter, -b parameter will give you numbers on the fly. The difference is -b parameter will only number non-blank lines.
#cat -n /etc/ntp.conf
1 # Permit time synchronization our time resource but do not
2 # permit the source to query or modify the service on this system
3 restrict default kod nomodify notrap nopeer noquery
4 restrict -6 default kod nomodify notrap nopeer noquery
5 # Permit all access over the loopback interface. This could be
6 # tightened as well, but to do so would effect some of the
7 # administration functions
8 restrict 127.0.0.1
9 restrict -6 ::1
4. Show tabs
You may need to know where are the tabs on your text file. If you do, you can use -T parameter. Tab will be represented by ^I characters.
# cat -T /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1^I^Ilocalhost.localdomain localhost
::1^I^Ilocalhost6.localdomain6 localhost6
5. Show the end of lines
-E parameter will display $ at end of each line. Here’s the example :
# cat -E /etc/hosts
# Do not remove the following line, or various programs$
# that require network functionality will fail.$
127.0.0.1 localhost.localdomain localhost$
::1 localhost6.localdomain6 localhost6$
6. Show All
If you want to combine between -T and -E, you can use -A parameter.
# cat -A /etc/hosts
# Do not remove the following line, or various programs$
# that require network functionality will fail.$
127.0.0.1^I^Ilocalhost.localdomain localhost$
::1^I^Ilocalhost6.localdomain6 localhost6$
7. View the content per page
When your file can not fit in your screen, you can combine cat with another command to make it displayed per page. Use the pipe ( | ) sign to combine it.
# cat /proc/meminfo | less
# cat /proc/meminfo | more
The difference between less and more is that you can do scroll up and scroll down on less command using PageUp and PageDown buttons. While you can only do scroll down on more command using spacebar.
8. View the contents of 2 files
Let’s say we have 2 text files named linux and desktop located in /root folder. Each of files contains : Linux : ubuntu, centos, redhat, mint and slackware Desktop : gnome kde, xfce, enlightment, and cinnamon When you want to view more than 1 file simultaneously, you do like this :
# cat /root/linux /root/desktop
ubuntu
centos
redhat
mint
slackware
gnome
kde
xfce
enlightment
cinnamon
9. Sorting file
Again, you can combine cat using another command to make a custom output. If you want to sort it, you can combine it with sort command. Here’s the example :
# cat /root/linux | sort
centos
mint
redhat
slackware
Ubuntu
10. Redirect standard output
You can also redirect the output to screen or to another file. Just use > sign (greater-than symbol) if you want to redirect the output into a file.
# cat /root/linux > /root/linuxdistro
The above command will create a linuxdistro file that has the same content with /root/linux file.
11. Creating a new file
There are some ways to create a text file in Linux. One of them is using cat command.
# cat > operating_system
Unix
Linux
Windows
MacOS
When you type cat > operating system, it will create a file which named operating_system. Then you will see a blank line below the cat command. You can type the content that you want. We typed Unix, Linux, Windows and MacOS for example. When you’re done, press Ctrl-D to save the content and exit from cat command. You will see a file named operating_system is created in the current folder with the content that you add before.
12. Append the content of file
If you use the > sign twice, it means that the content of the first file will be added to the second file. Let’s see the example :
# cat /root/linux >> /root/desktop
# cat /root/desktop
The first cat command will add the content of /root/linux to /root/desktop file The second command will show you this output :
ubuntu
centos
redhat
mint
slackware
gnome
kde
xfce
enlightment
cinnamon
13. Redirect standard input
You can also redirect standard input using < sign (less-than symbol).
# cat < /root/linux
The above command means that /root/linux will be an input for cat command. The output will be like this :
ubuntu
centos
redhat
mint
slackware
To make it clearer, let’s see another example :
# cat < /root/linux | sort > linux-sort
That command will be read : sort content from /root/linux file then print the output to linux-sort file
So the output of linux-sort file will be like this :
centos
mint
redhat
slackware
ubuntu
That’s some examples of cat command on day-to-day operation. Of course you can explore more detail from cat manual page and experiment with Input-Output redirection operator.
via: http://linoxide.com/linux-command/13-cat-command-examples/