How to Use the Cat Command in Linux

The cat command, short for “concatenate,” is a commonly-used tool in Linux, enabling users to view, create, and concentrate files, or redirect their output. In this post, we’ll explore how both new and experienced Linux users can leverage the cat command for various tasks.

The general syntax of the cat command is as follow:

$ cat [OPTION] [FILE]...

1. Use cat to create a new file and add content

cat > filename

Example:

The command cat > vegetables.txt takes the standard input and redirects it to a file named “vegetables.txt”. When you execute this command, your terminal doesn’t display any output, but instead waits for you to enter text from the keyboard.

Whatever you type will be written to “vegetables.txt”. To finish and save the file, you’ll need to press CTRL-D (or CTRL-Z on Windows systems using some terminal applications).

Here’s an example of how you might use it:

$ cat > vegetables.txt
Carrot
Broccoli
Spinach
^D

At this point, a file named “vegetables.txt” would be created with the following content:

Carrot
Broccoli
Spinach

2. Display file’s content with cat

cat filename

Example:

Assume the file contains a list of common vegetables; executing the command cat vegetables.txt would display the contents of the file in the terminal.

Carrots
Broccoli
Spinach
Tomatoes
Cucumbers
Peppers
Onions
Potatoes
Kale
Lettuce

3. Display content of multiple files with cat

cat filename_1 filename_2

Example:

Assuming the contents of the files fruits.txt and vegetables.txt are respectively as follows:

Apple
Banana
Cherry

Carrot
Lettuce
Tomato

The command cat fruits.txt vegetables.txt would produce:

Apple
Banana
Cherry
Carrot
Lettuce
Tomato

4. Display content with line numbering with cat

cat -n filename

Example:

If the contents of fruits.txt are the same as in the previous example:

Apple
Banana
Cherry

Then the command cat -n fruits.txt would produce:

     1	Apple
     2	Banana
     3	Cherry

5. Copy, replace, or replicate a file’s content using cat

cat filename new_filename

Example:

The command cat fruits.txt > new_fruits.txt will take the contents of fruits.txt and write them into a new file named new_fruits.txt. This command won’t display any output to the terminal, assuming it executes successfully.

If fruits.txt has the same content as before:

Apple
Banana
Cherry

After running the command, the content of new_fruits.txt will be:

Apple
Banana
Cherry

6. Merge multiple files’ content into one with cat

cat filename_1 filename_2 > filename_3

Example:

The command cat fruits.txt vegetable.txt > grocery.txt will concatenate the contents of fruits.txt and vegetable.txt, then redirect the output to a file called grocery.txt. If either of the input files doesn’t exist, an error message will be displayed in the terminal, but you won’t see the concatenated contents in the terminal because they are being redirected to grocery.txt.

Suppose fruits.txt contains:

Apple
Banana
Cherry

And vegetable.txt contains:

Carrot
Lettuce
Tomato

After running the command, the terminal won’t show any output (unless there’s an error), but the file grocery.txt will contain:

Apple
Banana
Cherry
Carrot
Lettuce
Tomato

If vegetable.txt was misspelled or doesn’t exist, you would see an error in the terminal similar to:

cat: vegetable.txt: No such file or directory

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