How to Use the Linux cat Command (and When Not To)

Cat is one of the most basic commands you’ll encounter on Linux. It may seem mysterious at first, but it is really rather easy to use.

Here’s how to use and when not to use the cat command in Linux.

What Is cat?

cat is an ancient tool that dates back to the first Unix implementation. The name may seem weird, but it is an abbreviation for “concatenate.” It’s a fancy word for putting things together. In this sense, “stuff” refers to files. cat is a program for joining several text files together. The majority of users use it to print files to the screen or into another file.

The GNU implementation is included in the coreutils package, which is included by default on most Linux distributions, although there is also a BusyBox version that is prevalent on minimal or live distros.

Using cat With Standard I/O

cat works with normal input and output. This implies that you may utilize pipelines to use its output with another application and take text input from the keyboard or another program using I/O redirection:

cat | some_program

Remember that many other Linux programs allow standard I/O as well, so you may not need to use cat in pipelines at all. More to come on this later.

It also takes command-line inputs in the form of files. You may use the following command to print a whole file to the terminal:

cat file.txt

You can also print multiple files using cat:

cat file1 file2

This is the “concatenate” part of cat.

Cat will wait for you to write something and click Enter before echoing anything you wrote back at the terminal until you press Ctrl + D. You may also use the redirection operator to direct the text into a file:

cat > some_file

Useless Use of cat

Because cat is such a simple tool, it’s easy to get carried away with its use with pipelines. Randal Schwartz, widely known for his Perl books, invented the Useless Use of Cat (UUOC) “award” (as seen on Era Eriksson’s home page) and distributed it on Usenet on occasion.

The essential need for a UUOC is that you use cat to pipe output to a program that already accepts standard input or files as arguments. Because so many Unix and Linux programs do this function, using cat to pipe standard output or a single file is a waste of time. To avoid “losing” this prize, you should verify the manual page for every command you want to use and ensure that it supports standard input.

If you’re going to look at large files, you should use a pager like Most or a text editor.

cat is a simple command, but like many other simple Linux utilities, it’s useful for printing text files and passing output to other applications. If you want more color in your terminal, you may try lolcat, a filter-based application that creates rainbow-colored text in your terminal.

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