HowTo: Use cat Command In Linux / UNIX

Posted on in Categories , last updated February 21, 2017

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/test.txt file instead of being displayed on the monitor screen. You can view /tmp/test.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.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

25 comment

  1. 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 ?

    1. 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

  2. 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.

  3. 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

  4. Hi every one , i am new in using linux commands please help me out which command i can use
    Either the foo process, the process was launched from a console. I want to get the hands on the console without killing foo.
    What are the actions i can take:

    [A] c then press CTRL & foo
    [B] then press CTRL z foo &
    [C] do CTRL z then bg foo
    [D] then press CTRL c foo

  5. Thank you for the site and the great info you provide. Don’t take this wrong but, I gotta tell ya… the light gray text on a white background is tough on this old mans eyes.

  6. When i concatenate 2 files, first line of 2nd file is appended with last line of first file. How to make it to display as a separate line?

    Sample1.dat
    —————-
    1234|GM America|400
    2345|Apple|500
    3456|Coke|600

    Sample2.dat
    ABC|GM America|400
    DEF|Apple|500
    GHI|Coke|600

    When i run,
    cat Sample1.dat Sample2.dat > Sample3.dat

    Output is looking like below,
    1234|GM America|400
    2345|Apple|500
    3456|Coke|600ABC|GM America|400
    DEF|Apple|500
    GHI|Coke|600

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