Useless use of cat?

Question

This is probably in many FAQs - instead of using:

cat file | command

(which is called useless use of cat), correct way supposed to be:

command < file

In the 2nd, "correct" way - OS does not have to spawn an extra process.
Despite knowing that, I continued to use useless cat for 2 reasons.

1. more aesthetic - I like when data moves uniformly only from left to right. And it easier to replace cat with something else (gzcat, echo, ...), add a 2nd file or insert new filter (pv, mbuffer, grep ...).

2. I "felt" that it might be faster in some cases. Faster because there are 2 processes, 1st (cat) does the reading and the second does whatever. And they can run in parallel, which means sometimes faster execution.

Is my logic correct (for 2nd reason)?

Answer 1

I was not aware of the award until today when some rookie tried to pin the UUOC on me for one of my answers. It was a cat file.txt | grep foo | cut ... | cut .... I gave him a piece of my mind, and only after doing so visited the link he gave me referring to the origins of the award and the practice of doing so. Further searching led me to this question. Somewhat unfortunately despite conscious consideration, none of the answers included my rationale.

I had not meant to be defensive in responding to him. After all, in my younger years, I would have written the command as grep foo file.txt | cut ... | cut ... because whenever you do the frequent single greps you learn the placement of the file argument and it is ready knowledge that the first is the pattern and the later ones are file names.

It was a conscious choice to use cat when I answered the question, partly because of a reason of "good taste" (in the words of Linus Torvalds) but chiefly for a compelling reason of function.

The latter reason is more important so I will put it out first. When I offer a pipeline as a solution I expect it to be reusable. It is quite likely that a pipeline would be added at the end of or spliced into another pipeline. In that case having a file argument to grep screws up reusability, and quite possibly do so silently without an error message if the file argument exists. I. e. grep foo xyz | grep bar xyz | wc will give you how many lines in xyz contain bar while you are expecting the number of lines that contain both foo and bar. Having to change arguments to a command in a pipeline before using it is prone to errors. Add to it the possibility of silent failures and it becomes a particularly insidious practice.

The former reason is not unimportant either since a lot of "good taste" merely is an intuitive subconscious rationale for things like the silent failures above that you cannot think of right at the moment when some person in need of education says "but isn't that cat useless".

However, I will try to also make conscious the former "good taste" reason I mentioned. That reason has to do with the orthogonal design spirit of Unix. grep does not cut and ls does not grep. Therefore at the very least grep foo file1 file2 file3 goes against the design spirit. The orthogonal way of doing it is cat file1 file2 file3 | grep foo. Now, grep foo file1 is merely a special case of grep foo file1 file2 file3, and if you do not treat it the same you are at least using up brain clock cycles trying to avoid the useless cat award.

That leads us to the argument that grep foo file1 file2 file3 is concatenating, and cat concatenates so it is proper to cat file1 file2 file3 but because cat is not concatenating in cat file1 | grep foo therefore we are violating the spirit of both the cat and the almighty Unix. Well, if that were the case then Unix would need a different command to read the output of one file and spit it to stdout (not paginate it or anything just a pure spit to stdout). So you would have the situation where you say cat file1 file2 or you say dog file1 and conscientiously remember to avoid cat file1 to avoid getting the award, while also avoiding dog file1 file2 since hopefully the design of dog would throw an error if multiple files are specified.

Hopefully, at this point, you sympathize with the Unix designers for not including a separate command to spit a file to stdout, while also naming cat for concatenate rather than giving it some other name. <edit> removed incorrect comments on <, in fact, < is an efficient no-copy facility to spit a file to stdout which you can position at the beginning of a pipeline so the Unix designers did include something specifically for this </edit>

The next question is why is it important to have commands that merely spit a file or the concatenation of several files to stdout, without any further processing? One reason is to avoid having every single Unix command that operates on standard input to know how to parse at least one command line file argument and use it as input if it exists. The second reason is to avoid users having to remember: (a) where the filename arguments go; and (b) avoid the silent pipeline bug as mentioned above.

That brings us to why grep does have the extra logic. The rationale is to allow user-fluency for commands that are used frequently and on a stand-alone basis (rather than as a pipeline). It is a slight compromise of orthogonality for a significant gain in usability. Not all commands should be designed this way and commands that are not frequently used should completely avoid the extra logic of file arguments (remember extra logic leads to unnecessary fragility (the possibility of a bug)). The exception is to allow file arguments like in the case of grep. (By the way, note that ls has a completely different reason to not just accept but pretty much require file arguments)

Finally, what could have been done better is if such exceptional commands as grep (but not necessarily ls) generate an error if the standard input is also available when file arguments are specified.