In this article, we are going to find out the handy use of cat commands with their examples in Linux.
The cat (short for “concatenate“) command is one of the most frequently used commands in Linux/Unix-like operating systems. cat command allows us to create single or multiple files, view content of a file, concatenate files and redirect output in terminal or files.
General Syntax of Cat Command
$ cat [OPTION] [FILE]...
1. Display Contents of File
The below example will show the contents of /etc/passwd file.
# cat /etc/passwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin narad:x:500:500::/home/narad:/bin/bash
2. View Contents of Multiple Files in terminal
In below example, it will display the contents of the test and test1 file in the terminal.
# cat test test1 Hello everybody Hi world,
3. Create a File with Cat Command
We will create a file called test2 file with the below command.
# cat >test2
# cat test2 hello everyone, how do you do?
4. Use Cat Command with More & Less Options
If a file having a large number of content that won’t fit in the output terminal and the screen scrolls up very fast, we can use parameters more and less with the cat command as shown below.
# cat song.txt | more # cat song.txt | less
5. Display Line Numbers in File
With the -n option you could see the line numbers of a file song.txt in the output terminal.
# cat -n song.txt 1 "Heal The World" 2 There's A Place In 3 Your Heart 4 And I Know That It Is Love 5 And This Place Could 6 Be Much 7 Brighter Than Tomorrow 8 And If You Really Try 9 You'll Find There's No Need 10 To Cry 11 In This Place You'll Feel 12 There's No Hurt Or Sorrow
6. Display $ at the End of File
In the below, you can see with the -e option that ‘$‘ is shows at the end of the line and also in space showing ‘$‘ if there is any gap between paragraphs. This option is useful to squeeze multiple lines into a single line.
# cat -e test hello everyone, how do you do?$ $ Hey, am fine.$ How's your training going on?$ $
7. Display Tab Separated Lines in File
In the below output, we could see TAB space is filled up with the ‘^I‘ characters.
# cat -T test hello ^Ieveryone, how do you do? Hey, ^Iam fine. ^I^IHow's your training ^Igoing on? Let's do ^Isome practice in Linux.
8. Display Multiple Files at Once
In the below example we have three files test, test1, and test2, and able to view the contents of those files as shown above. We need to separate each file with ; (semicolon).
# cat test; cat test1; cat test2 This is a test file This is the test1 file. This is test2 file.
9. Use Standard Output with Redirection Operator
We can redirect the standard output of a file into a new file else existing file with a ‘>‘ (greater than) symbol. Careful, existing contents of the test1 will be overwritten by the contents of the test file.
# cat test > test1
10. Appending Standard Output with Redirection Operator
Appends in existing file with ‘>>‘ (double greater than) symbol. Here, the contents of the test file will be appended at the end of the test1 file.
# cat test >> test1
11. Redirecting Standard Input with Redirection Operator
When you use the redirect with standard input ‘<‘ (less than symbol), it uses file name test2 as input for command and output will be shown in a terminal.
# cat < test2 This is test2 file.
12. Redirecting Multiple Files Contain in a Single File
This will create a file called test3 and all output will be redirected in a newly created file.
# cat test test1 test2 > test3
13. Sorting Contents of Multiple Files in a Single File
This will create a file test4 and the output of the cat command is piped to sort and the result will be redirected to a newly created file.
# cat test test1 test2 test3 | sort > test4
This article shows the basic commands that may help you to explore the cat commands.
To know more about memory usage on centos server click here.