Cat

cat is a quite useful Unix command. You may have seen cat used to display text in the terminal, e.g., cat something.txt would show the contents of something.txt. However, that isn’t really cat’s true purpose. Let’s check the Unix manual (man) pages:

$ man cat | head -n 5
CAT(1)    User Commands

NAME
       cat - concatenate files and print on the standard output

(we piped the output of man cat into head -n 5 to limit the output to 5 lines).

View an online copy of the cat man page.

So the real purpose of cat is to concatenate files and print them. Exciting stuff. But this might also make you wonder: what is the standard output? Standard output, or stdout for short, is the default output stream of a program. When you use printf or puts you are printing to stdout, and the stream is usually connected to your terminal, so you see text. That doesn’t mean that stdout has to print to your terminal though – it can be redirected elsewhere. We’ll worry about that later.

Concatenating files

We have established what cat does. Great. Let’s run it!

$ cat

... (nothing happens) ...

... (heat death of the universe eventually happens) ...

cat isn’t doing anything because we didn’t provide any files to concatenate. However, instead of reminding us this, cat is instead reading from another stream: standard input or stdin. It’s waiting for us to give it something to concatenate. We can start typing:

$ cat
hello world?
hello world?
hey, stop that!
hey, stop that!
stop copying me!
stop copying me!

And cat will do its best impression of your younger sibling during a 10-hour drive across country.

Here, we saw cat reading from stdin and writing to stdout.

Moving on, let’s have cat read from a file instead (while still outputting to stdout):

$ cat file.txt
This is what's inside file.txt!
Incredible, right?

I wonder if…

$ cat file.txt file.txt
This is what's inside file.txt!
Incredible, right?
This is what's inside file.txt!
Incredible, right?

Ok, so we can even concatenate a file with itself. Create a few text files, a.txt, b.txt, and c.txt like so (the <<EOM ... EOM syntax below is called a heredoc):

$ cat > a.txt <<EOM
Start writing a message here
'>' or 'heredoc>' might appear;
That's completely fine, keep writing.
Whatever you write will be written
to 'a.txt' -- we *redirected* the
output of cat to a.txt with the
'>' symbol above. When you are done
writing, you need to type 'EOM' on
its own line like this:
EOM

$ cat > b.txt <<EOM
Hello world!
EOM

$ cat > c.txt <<EOM
Great, one more file.
Just what we need.
EOM

$ cat > a.txt <<EOM
Overwriting 'a.txt' so we don't have
so much text to display below :-)
EOM

Finally, you can cat them:

$ cat a.txt b.txt c.txt
Overwriting 'a.txt' so we don't have
so much text to display below :-)
Hello world!
Great, one more file.
Just what we need.

$ cat c.txt a.txt b.txt
Great, one more file.
Just what we need.
Overwriting 'a.txt' so we don't have
so much text to display below :-)
Hello world!

So, the order of the command line arguments matters. One more useful thing we can do is add line numbers to the output with the -n flag:

$ cat -n /etc/passwd | head -n4
     1	root:x:0:0::/root:/bin/bash
     2	bin:x:1:1::/:/usr/bin/nologin
     3	daemon:x:2:2::/:/usr/bin/nologin
     4	mail:x:8:12::/var/spool/mail:/usr/bin/nologin

Great, a numbered list of the first 4 users in our system’s passwd file!

A bit more redirection

We can use what we’ve done so far to create a new file:

$ cat a.txt b.txt c.txt > d.txt
$ cat d.txt
Overwriting 'a.txt' so we don't have
so much text to display below :-)
Hello world!
Great, one more file.
Just what we need.

You’ll see this usage often – concatenate a few files and then write the result out to a new file. If you change > to >> then the file will be appended to rather than overwritten. So:

$ cat b.txt >> c.txt
$ cat c.txt
Great, one more file.
Just what we need.
Hello world!

The same applies to using the heredoc syntax.

Final thoughts

You can see the other features cat has by perusing the man pages: man cat.

One thing to be aware of: it’s generally not great practice to cat files of unknown origin. If the file has any special control characters that your terminal might interpret as commands, it can cause all kinds of trouble (especially if the file was provide by a malicious user). If you just want to view a file, less or more might be better options – they are pagers designed for viewing pages of text.