In this post I will discuss a relatively straightforward command: cat. The cat command is used to concatenate files. So if I had three files, I could do something like

cat file1.txt file2.txt file3.txt

...and this would display the concatenated content of all these files to standard output.

A Common Beginner's Mistake

Do not use cat with just a single file, piping to another program. You should instead just use that single file as input to the program you were piping to.

For example, rather than doing

# Looking for where I talk about feelings in my_file.txt
cat my_file.txt | grep "feelings"

...you should just do,

# Looking for where I talk about feelings in my_file.txt
grep "feelings" my_file.txt

There are efficiency reasons for doing things this way, but most importantly you don't want to look silly in front of scoffing neckbeards.

Some better uses of cat

There are lots of places where it is good to have cat around, particularly where plain text files are being used—cat probably won't be too useful manipulating images, dealing with audio files, etc.

However, dealing with little chunks of human readable text (such as in log files, short scripts, etc.) cat often comes in handy.

Entering short amounts of text really quickly

If you don't wanna open an editor, you can do

cat >> foo.txt # Append text fo foo.txt
cat > bar.txt  # Redirect (and replace) text fo bar.txt

Reading a bunch of stuff in less

To view/read mixed files on one screen, you can do something like,

cat file1.txt file2.txt file3.py | less

This could be useful for looking at a bunch of mixed/scattered information in a directory at once, creating an on-the-fly "custom view".

Organizing with cat in mind

In this post, we looked at cat which is not a terribly complicated command.

If you can remember what cat does and also know when it is not so useful, this is great! One less thing to look up!

Knowing that you have cat beside you and that you can likewise expect it to available in nearly any UNIX environment you may find yourself working in, you can create files knowing you might use cat with them.

Suppose, for instance, you keep track of groceries to buy on in a directory on your computer: shopping/. You might have a series of files:

• vegetables.txt
• meat.txt
• grains.txt

When you run out of lettuce, you might do echo "lettuce" >> vegetables.txt to quickly add an item to your TODO list.

You can view various combinations of these lists using,

cat vegetables.txt meat.txt | less

You can also use your shell's globbing functions with commands like ¹

cat *.txt # hitting tab will autocomplete filenames

What you have with cat is a kind of duct tape to smash files together in many ways; this allows you to create "dynamic views" of textual content. Combined with tools like less (used for viewing text files quickly) the possibilities are endless.