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Comment by teddyh | Hacker News Reader

Useless use of cat detected!

Rememeber, nearly all cases where you have:

  cat file | some_command and its args ...

you can rewrite it as:

  <file some_command and its args ...

and in some cases, such as this one, you can move the filename to the arglist as in:

  some_command and its args ... file

— Randal L. Schwartz (http://porkmail.org/era/unix/award.html#cat)

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hah, I knew someone would point that out (which is why I talked about it in the article).

I actually prefer useless cat because when you're prototyping a pipeline it's very awkward to use non-useless cat. You'll probably start off with something like this to observe the content of the file:

    cat something.txt

Using this doesn't work in bash:

    <something.txt

Then, continuing with useless cat to build on it you do

    cat something.txt | grep stuff

Which you can type easily from using 'up' in your terminal. But if you use non-useless cat you have to re-type the entire thing or move the cursor around:

    grep stuff < something.txt

With useless cat, you can keep adding things and check the result:

    cat something.txt | grep stuff | sed 's/"//g'

Or if you need to insert another filter before the last stage like this, you can just press "up" and insert it:

    cat something.txt | grep -v negmatch | grep stuff

I don't think there is any easily-typed equivalent workflow with non-useless cat.

If you’re working with a lot of data you probably want to pipe it into head anyway, initially, so

<file head -n50 | whatever

Can be the starting command. When you no longer need the head there, just get rid of “head |”.

Although I agree that the pointing out of “useless cat” is usually not particularly useful or constructive.

Using head when there's lots of data make sense, but I really don't see any advantage to avoiding useless cat. Useless cat is way faster to type and make additions to. I sort of get the feeling that 'useless cat' is really just a fun copypasta kinda like when people like post "I'd just like to interject for a moment. What you're referring to as Linux, is in fact, GNU/Linux, or as I've recently taken to calling it..."

It's still good to be aware about `useless cat`, to save some CPU and I/O, when converting one liners into scripts.

Rather than using head and worrying about the size of the file, it is easier to simply use "useless" cat then ctrl-c the stream of data that comes out.

What makes it useless? It’s functionally the same but makes it easier to author pipelines, which I think is a valid use.

  cat foo | bar

is useless use of cat, since it’s equivalent to

  < foo bar

which is both shorter and starts one less process. Why do you think that “cat” makes it “easier to author pipelines”?

1). It doesn’t work in all shells, so you have to think about your environment before you use it.

2). I often start with `bar < foo`, so if I need to add more arguments to `bar`, I need to always skip over the input.

3). If I just want to delete all processing and look at the input, I can’t just backspace away the processing because `< foo` is invalid.

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