4 ms·

They avoid an unnecessary invocation of the cat executable. Instead, they open a file descriptor and pass that. Tiny difference but there you go.

latexr 3y ago
To add, searching for “useless use of cat” will yield several results for those interested in learning more. Other examples include “useless use of echo” and “useless use of ls *”.
- vram22 3y ago
  Yes. Even the Wikipedia page on cat has a section about that, titled eponymously. https://en.m.wikipedia.org/wiki/Cat_(Unix) https://en.m.wikipedia.org/wiki/Cat_(Unix)
adrianmonk 3y ago
Not just that, but also all the bytes have to go through an extra pipe. Presumably they're copied an extra time because of this. When you run "cmd < file", the command reads from stdin, which pulls directly from the file. When you do "cat file | cmd", "cat" opens the file, reads from there, and writes to a pipe. Then "cmd" reads from its stdin, which is a pipe.
- chlorion 3y ago
  GNU cat will use the copy_file_range syscall when possible! copy_file_range allows a user land program to copy data between two files without doing any user space work. Instead of reading data into a buffer and writing it back out to the destination, the kernel will somehow manage to move the data for you. I think this will prevent any extra copies from occurring in situations where it can be used. https://man.archlinux.org/man/copy_file_range.2 https://man.archlinux.org/man/copy_file_range.2 https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat.c#n507 https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat...
omginternets 3y ago
>They avoid an unnecessary invocation of the cat executable. And ... ?
wpm 3y ago
I teach shell scripting. Cat invocations are cheap and help learners understand and keep clear where input is coming from, and where it is going. There are no awards or benefits to reducing the number of lines, commands invoked, or finding the shortest possible way to perform a task in a script. There are plenty of detriments to reading and understanding though when we try to obfuscate this to save 1ms of execution time on a script that is going to execute near instantaneously anyways. In short, I straight up don't care.
- revscat 3y ago
  I 100% agree with you. My only defense of OP is that `<` is something tends to be forgotten. Like everyone else in this thread I go to `cat` first for things like this. But sometimes I forget that even `<` exists, and the callout is a nice reminder.

latexr 3y ago

To add, searching for “useless use of cat” will yield several results for those interested in learning more. Other examples include “useless use of echo” and “useless use of ls *”.

vram22 3y ago
Yes. Even the Wikipedia page on cat has a section about that, titled eponymously. https://en.m.wikipedia.org/wiki/Cat_(Unix) https://en.m.wikipedia.org/wiki/Cat_(Unix)

adrianmonk 3y ago

Not just that, but also all the bytes have to go through an extra pipe. Presumably they're copied an extra time because of this. When you run "cmd < file", the command reads from stdin, which pulls directly from the file. When you do "cat file | cmd", "cat" opens the file, reads from there, and writes to a pipe. Then "cmd" reads from its stdin, which is a pipe.

chlorion 3y ago
GNU cat will use the copy_file_range syscall when possible! copy_file_range allows a user land program to copy data between two files without doing any user space work. Instead of reading data into a buffer and writing it back out to the destination, the kernel will somehow manage to move the data for you. I think this will prevent any extra copies from occurring in situations where it can be used. https://man.archlinux.org/man/copy_file_range.2 https://man.archlinux.org/man/copy_file_range.2 https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat.c#n507 https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/cat...

omginternets 3y ago

>They avoid an unnecessary invocation of the cat executable. And ... ?

wpm 3y ago

I teach shell scripting. Cat invocations are cheap and help learners understand and keep clear where input is coming from, and where it is going. There are no awards or benefits to reducing the number of lines, commands invoked, or finding the shortest possible way to perform a task in a script. There are plenty of detriments to reading and understanding though when we try to obfuscate this to save 1ms of execution time on a script that is going to execute near instantaneously anyways. In short, I straight up don't care.

revscat 3y ago
I 100% agree with you. My only defense of OP is that `<` is something tends to be forgotten. Like everyone else in this thread I go to `cat` first for things like this. But sometimes I forget that even `<` exists, and the callout is a nice reminder.