This is an optional rule, which means that it has a special "long name" and is not enabled by default. See the optional page for more details. In short, you have to enable it with the long name instead of the "SC" code like you would with a normal rule:

enable=useless-use-of-cat # SC2002

(Note that this check was not optional prior to version 0.11.0.)

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

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

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.

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.