Use cat Command In Linux / UNIX

 Cat command of the most frequently used commands on Linux OS.

we can use for lots of purposes under Linux:

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

Cat command

cat filename
cat options filename
cat file1 file2
cat file1 file2 > file1&2

Displaying The Contents of Files

To read the contents of files

$ 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

Multiple file in single file . The original file or files are not modified or deleted. Example, cat will copies 2 or 3 or more than 3 files of the contents in single file . Let the three files /etc/hosts, /etc/resolv.conf, and /etc/fstab:
$ cat /etc/hosts /etc/resolv.conf /etc/fstabwe 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.txtwe 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?

we can use cat command for file creation. To create a file called foo.txt, $ 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,
$ 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,
$ 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

    
Enjoy 

Ravi ranjan kr singh

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