SC2002

Useless cat. Consider cmd < file | .. or cmd file | .. instead.

This suggestion was enabled by default up to and including ShellCheck 0.10.0.

In later versions, it is optional. It must be explicitly enabled with a directive enable=useless-use-of-cat in a # shellcheck comment or .shellcheckrc

Problematic code:

cat file | tr ' ' _ | nl
cat file | while IFS= read -r i; do echo "${i%?}"; done

Correct code:

< file tr ' ' _ | nl  
while IFS= read -r i; do echo "${i%?}"; done < file

Rationale:

cat is a tool for con"cat"enating files. Reading a single file as input to a program is considered a Useless Use Of Cat (UUOC).

It's more efficient and less roundabout to simply use redirection. This is especially true for programs that can benefit from seekable input, like tail or tar.

Many tools also accept optional filenames, e.g. grep -q foo file instead of cat file | grep -q foo.

Exceptions

Pointing out UUOC is a long standing shell programming tradition, and removing them from a short-lived pipeline in a loop can speed it up by 2x. However, it's not necessarily a good use of time in practice, and rarely affects correctness. Ignore as you see fit.