cat command

cat command is one of the basic linux commands used to concatenate and write files. cat command can be used for the following major purposes

• View text files
• Create text files
• Append, Combine text files
• Copy a text file

 

Viewing a file

To view a file type cat followed by filename or path to the file name

Syntax:

cat filename
cat /path/to/file

For example, to view /etc/shells file which contains the full pathnames of valid login shells enter the command as

cat /etc/shells

Sample Output:

/bin/sh
/bin/bash
/sbin/nologin
/bin/dash

Creating a file

In order to create a text file use the following syntax

cat > filename

Usage:

cat > demo.txt

The cursor will go to the next line instead of the prompt. Now type the text to be entered.

Hello World !!!!
This is a demo file
[cntrl+D]

Once the entries are made go to a new line and press control key and  d keys  together to quit.  A new text file demo.txt will be created  containing the texts entered. Verify the file with cat command (i.e. cat demo.txt)

Combining Files

cat command can be used to combine two or more files to a single file

Usage:

cat file1.txt file2.txt file3.txt > file123.txt

In the above example  file1.txt file2.txt and file3.txt are 3 different text files and they are combined together using cat command  to create a new file file123.txt. Verify the file with cat command (i.e. cat file123.txt)

Appending Files 

cat command can be used append additional data to an already existing file. 

Syntax:

cat >> filename

The method is almost similar to that of creating a new file except we are using double greater-than symbol (>>) instead of greater-than symbol (>). To append data to the file enter
 

cat >> demo.txt

Type the text in the new line

To err is human.. to really foul up requires the root password
[cntrl+d]

The data will be appended.  If greater-than sign (>) is used instead of double greater-than sign (>>) the file will be overwritten. ie the new data will replace the existing data contained in the file. Verify the file with cat command (i.e. cat demo.txt)

Copy Files

cat command can be used to copy the contents of a file to another file

Syntax:

cat file1.txt > file2.txt

Here the contents of file1.txt is copied to file2.txt i.e. Output of cat file1.txt is re-directed to file2.txt


These are the cat command's most commonly used  methods. There are so many ways the commands can be used based on your requirements and that's the beauty of linux. For additional info and usages check the cat man page.