As any Unix veteran will tell you, the cat utility is much abused. Here’s the common scenario:

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cat myfile | grep mypattern

Of course, this kind of abuse isn’t limited to the “pipe to grep” variety. Here’s the general form:

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cat <file> | <command> arg1 arg2 argN

Indeed, flaming other Unix users for the Useless Use of Cat has long been the ‘national sport’ of the Unix tribe, since Usenet days.

What’s wrong with this pattern?

One thing, certainly. Two things, probably.

1. cat is meant to concatenate two or more files together

If you are using cat with only one filename argument, then by definition you aren’t concatenating anything.

So, here’s the proper use of cat:

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cat blacklion.txt greenlion.txt redlion.txt bluelion.txt yellowlion.txt > VOLTRON.txt

2. many commands accept STDIN, but prefer a filename argument

This falls into a related anti-pattern of “Useless Use of Pipe”. Instead of:

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cat internet_memes.txt | grep -i "chuck norris"

Consider:

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grep -i "chuck norris" internet_memes.txt

Why is this such a common anti-pattern?

I still find myself occasionally falling into this pattern, despite two decades’ worth of keyboard time on Unix variants. I think this habit likely persists for two reasons:

1. The habit is learned early

Early bad habits are often hard to break. One of the first things a CLI user wants to do is to read a file, and until the use of pager commands becomes second nature, this will likely lead to:

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cat somefile

For small ASCII files, this may even be an appropriate use of cat. Usually though, this early habit leads to a second, more subtle, and persistent mental model.

2. cat is a conceptual pipeline source

When building up a complex filter, often cat starts things off:

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cat myfile | grep pattern | tr 'A-Z' 'a-z' > outfile

The thought process behind this command is: start with some text, then filter it thus, and filter it so, then write it to an output file. Simple. In some cases, I even give myself a pass on this if it makes my intention clearer. Usually, though, I try to fix the habit.

The Fix?

Refactoring. No, really. More on this next time.

Bonus: Useless use of wc

Often I will do the following to get a matching pattern count:

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cat file1 file2 | grep pattern | wc -l

So, first let’s clean up the UUoP:

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grep pattern file1 file2 | wc -l

Turns out, grep has a -c flag, which displays the count of matched lines, so:

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grep -c pattern file1 file2