SC2002.md

Pattern: Unnecessary use of cat

Issue: -

Description

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 seek-able input, like tail or tar.

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

Example of incorrect code:

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

Example of correct code:

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

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.

Further Reading

• ShellCheck - SC2002