What Is the CAT Command in Linux and How to Use It

The cat command is Linux’s most basic way of combining files together. It’s short for conCATenation, but there’s more to this than just copy-pasting one file next to the other.

Basic Format for the CAT Command

This is how you normally use the cat command:

cat file1 file2 file3 > output

The order for appending files next to each other is based on the order you wrote the files within the command. In this example, when you open up the “output” file, the contents of “file1” will be followed by “file2” then “file3” and so on.

You can add as many files as you like as long as your filesystem can support the output’s eventual size.

Also, like in all Linux terminal commands, you will need to include the file extension when you add them in. For example, if you were working with JSON files and planning to combine two of these together, you’ll need to type it like this:

cat file1.json file2.json > combined_file.json

CAT without the Output Portion

For cat, the output part is optional. Removing this part prints out the contents of each file into the terminal.

You can also do this on single files. Do remember that you won’t be able to edit the content here like you would with DOC or TXT files on a text editor. The cat command becomes more like a text file viewer than an editor.

CAT Command Flags

There are six flags you can use whenever you use CAT on Linux:

• -n enumerate every line starting from 1.
• -b enumerate only the lines that contain characters.
• -s squeeze multiple consecutive empty lines into one empty line.
• -e mark the end of each line.
• -q do not display a message if CAT can not find an input file.
• -t display tab characters as ^I.

We’ll go through how each one works and how you can use them.

Using the -n Flag

The -n flag enumerates the contents of a file per line and prints it on the terminal. It does not edit the file itself.

cat -n file1.txt

But you can still save the output (with enumeration) by doing this:

cat -n file1.txt > file2.txt

That enumerates the content of “file1.txt” and saves it to “file2.txt”.

You can also enumerate multiple files together. The cat command will continue counting after the end of the first file. If the first file ends with 33, the next file will start at 34 and so on.

Tip: You can also make use of diff command to find the difference between files first before combining them together.

Using the -b Flag

The problem with the -n flag is that it also enumerates empty lines. If you had a file with plenty of whitespace for layout or readability, you’ll need the -b flag instead.

cat -b file1.txt

The format for the -b flag works the same as that of the -n flag. Also, if you use the two of these together, the -b flag will override the -n flag no matter how you order them.

Using the -s Flag

The -s flag is another handy cat command flag for when you have plenty of white space in your files. It works by reducing all repeating whitespace into a single one, which eases readability.

cat -s file2.txt

This flag looks for at least 2 consecutive empty lines before merging them. If your file contains a single empty line between two filled ones, it’ll skip over that and look for the next one.

Using the -e Flag

The -e flag finds the end of each line and adds a $ at the end of each line.

cat -e file1.txt

This flag makes it easier to work with your text files when you’re using regular expressions or regex. You can have an extra script or program read the contents of the saved file and check the ends using regex.

Do remember that the -e flag also adds a $ at the end of each empty line, so you might want to use this alongside -s to make thing look cleaner.

Using the -t Flag

The -t flag is similar to the -e flag, but looks for the “tab character” instead and replaces it with a ^I. What’s great with this flag is that it knows how to differentiate “tab” from the “double space” character.

cat -t file1.txt > file2.txt

This means that if you use the -n or -b flags alongside the -t flag, it’ll read the the space characters as they are and leave them be. The same goes if you add a ton of spaces to mimic a “tab” character.

Linux CAT Command Uses

The cat command should only be used when you’re trying to combine text files line by line. Don’t expect the cat command to take two lines of text and combine them into a single line.

On the other hand, you’ll see a lot of use for this when you’re trying to clean up text files for other scripts to work on. The -e, -t, -b, and -s flags are all useful enough to save you time from coding your own script for whatever these flags already do.

It also helps make it easier to view text files compared to having to use the Nano editor all the time.

But if you’re trying to do something it’s not meant for, then it might be time to take a look at other ways to concatenate files on Linux.

Frequently Asked Questions

Terenz Jomar Dela Cruz

Terenz is a hobbyist roboticist trying to build the most awesome robot the world has ever seen. He could have done that already if he wasn't so busy burning through LEDs as a second hobby.