HowTo: Use cat Command In Linux / UNIX

by on February 9, 2010 · 15 comments· LAST UPDATED February 9, 2011

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How do I use cat command in Linux? How do I use cat command in UNIX? How can I use cat command in UNIX or Linux shell scripts?

The cat command is considered as one of the most frequently used commands on Linux or UNIX like operating systems.

It can be used for the following purposes under UNIX or Linux:

  • Display text files on screen.
  • Copy text files.
  • Combine text files.
  • Create new text files.

cat command Syntax

The syntax is as follows:

 
cat filename
cat options filename
cat file1 file2
cat file1 file2 > newcombinedfile
 

Displaying The Contents of Files

To read or read the contents of files, enter:
$ cat /etc/passwd
The above command will display the contents of a file named /etc/passwd. By default cat will send output to the monitor screen. But, you can redirect from the screen to another command or file using redirection operator as follows:
$ cat /etc/passwd > /tmp/test.txt
In the above example, the output from cat command is written to /tmp/text.txt file instead of being displayed on the monitor screen. You can view /tmp/text.txt using cat command itself:
$ cat /tmp/test.txt

Concatenate files

Concatenation means putting multiple file contents together. The original file or files are not modified or deleted. In this example, cat will concatenate copies of the contents of the three files /etc/hosts, /etc/resolv.conf, and /etc/fstab:
$ cat /etc/hosts /etc/resolv.conf /etc/fstab
You can redirect the output as follows using shell standard output redirection:
$ cat /etc/hosts /etc/resolv.conf /etc/fstab > /tmp/outputs.txt
$ cat /tmp/outputs.txt

You can also use a pipe to filter data. In this example send output of cat to the less command using a shell pipe as the file is too large for all of the text to fit on the screen at a time:
$ cat /etc/passwd | less

How Do I Create a File?

You can use cat command for file creation. To create a file called foo.txt, enter:
$ cat > foo.txt
Sample outputs:

This is a test.

To save and exit press the CONTROL and d keys (CTRL+D). Please note that if a file named foo.txt already exists, it will be overwritten. You can append the output to the same file using >> operator:
$ cat >> bar.txt
The existing bar.txt file is preserved, and any new text is added to the end of the existing file called bar.txt. To save and exit press the CONTROL and d keys (CTRL+D).

How Do I Copy File?

The cat command can also be used to create a new file and transfer to it the data from an existing file. To make copy of
$ cat oldfile.txt > newfile.txt
To output file1's contents, then standard input, then file2's contents, enter:
$ cat file1 - file2
A hyphen indicates that input is taken from the keyboard. In this example, to create a new file file2 that consists of text typed in from the keyboard followed by the contents of file1, enter:
$ cat - file1 > file2

cat command options

To number non-blank output lines, enter (only works with GNU cat command version):
$ cat -b /etc/passwd
Sample outputs:

     1	root:x:0:0:root:/root:/bin/bash
     2	daemon:x:1:1:daemon:/usr/sbin:/bin/sh
     3	bin:x:2:2:bin:/bin:/bin/sh
     4	sys:x:3:3:sys:/dev:/bin/sh
     5	sync:x:4:65534:sync:/bin:/bin/sync
     6	games:x:5:60:games:/usr/games:/bin/sh
     7	man:x:6:12:man:/var/cache/man:/bin/sh
     8	lp:x:7:7:lp:/var/spool/lpd:/bin/sh
     9	mail:x:8:8:mail:/var/mail:/bin/sh
    10	news:x:9:9:news:/var/spool/news:/bin/sh

To number all output lines, enter (GNU cat version only):
$ cat -n /etc/passwd
To squeeze multiple adjacent blank lines, enter (GNU cat version only):
$ cat -s /etc/passwd
To display all nonprinting characters as if they were visible, except for tabs and the end of line character, enter (GNU cat version only):
$ cat -v filename

cat Command Abuse

The main purpose of cat is to catenate files. If it's only one file, concatenating it with nothing at all is a waste of time, and costs you a process. For example,
$ cat /proc/cpuinfo | grep model
Can be used as follows:
$ grep model /proc/cpuinfo
Another example,
cat filename | sed -e 'commands' -e 'commands2'
Can be used as follows which is cheaper:
sed sed -e 'commands' -e 'commands2' filename
See useless use of cat command for more information.

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{ 15 comments… read them below or add one }

1 jason September 20, 2011 at 11:51 am

Hi I used the following command
$cat file
test1
$cat file2
test3
$cat file1 – file2
test1
test2
test2 – It got displayed twice and now its like never ending and I have to press Ctrl+c to close it and the out put of file3 never got displayed….Can you help me out with this command ?

Reply

2 jason September 20, 2011 at 12:02 pm

I did Ctrl+D and got
test1
test2
test2
test3
but why i am getting two test2 in the output ?

Reply

3 inba February 14, 2014 at 11:13 am

1 step: to creat new file (vim file1.txt) and save cmand(:wq)
$ cat > foo.txt
2 step: to creat anoter new file (vim file2.txt) and then open new file if any words or commands type and to save that file(:wq)
$ cat >> bar.txt
3 step: $ cat file1.txt > file2.txt
$ cat oldfile.txt > newfile.txt
4 step: $ cat file1.txt – file2.txt

Reply

4 onne April 20, 2012 at 1:25 pm

great article !

Reply

5 mann saini June 25, 2012 at 3:05 pm

it should be like this

$cat > test1
Love
Ctrl +D

$cat > Test2
You
Ctrl+D

$Cat Test1 Test2 > Test3

now Test3 must display like this

$Cat Test3

Love
You

Thanks
mann

Reply

6 jewel.saha July 10, 2013 at 3:17 am

Exactly !!!!

Reply

7 shahin January 20, 2013 at 5:28 pm

how i can cat an error help plzz!!!

Reply

8 sunil February 19, 2013 at 8:01 am

well i check it work or not

Reply

9 raza May 22, 2013 at 6:50 am

this is really awesome article .

Reply

10 Britt July 1, 2013 at 3:22 am

What does this command do? ls | grep cat*.* > listdir.txt

Reply

11 durga September 2, 2013 at 5:48 am

what is cat X in unix

Reply

12 James October 7, 2013 at 9:48 pm

I’m in a Linux intro class, having trouble with syntax and getting help. I entered:
cat > numbers
6
74
7111
42
172
2
11
42
42
30125
ctrl + d
This puts the output into a file. Now I have to find a command that consolidates the last 2 “42′s” into a single piece of input. It’s nothing but a training exercise, but it’s bothering me that I can’t find it or get help from the instructor.
Any help is appreciated.

Reply

13 prabhat January 17, 2014 at 1:35 pm

james: u can use uniq command.
just write
uniq

Reply

14 prabhat January 17, 2014 at 1:36 pm

uniq filename

Reply

15 lurvas777 April 4, 2014 at 8:41 pm

Tanks a ton!! This article cleared things up and have helped me a lot!
What’s great is the abuse section, from which I learned even more! (about being cautious with big files and actually considering what the command does)

Keep up the great work :D

Reply

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