I have come to very much like shellcheck and now automatically use it on any shell script I'm writing or revising (at least any scripts for work; personal ones I'm sometimes lazier with). Although some of its issues are a little bit picky, using it is good for me so I go along with them. Well, with one exception, because I cannot go along with Shellcheck's views on using egrep and fgrep.

If you've used Shellcheck on scripts, especially older ones, you have probably run into SC2196 and SC2197:

egrep is non-standard and deprecated. Use grep -E instead.
fgrep is non-standard and deprecated. Use grep -F instead.

Sorry, nope. Both egrep and fgrep are long standing programs and names for these versions of grep, and they will be present on any sane Unix until the end of time (and I don't mean 2038, the end of the 32-bit Unix epoch). I have a visceral and probably irrational reaction to the idea of using the POSIXism of 'grep -E' and 'grep -F' for them, and I have no intention of switching my scripts away from using fgrep and egrep.

(And grep already has enough flags to keep track of, including important ones for searching test files in GNU Grep.)

Fortunately modern versions of Shellcheck (including the one in Ubuntu 18.04) can disable these warnings for your entire script, as covered in the wiki's page on ignoring errors. Just put a line in the comment at the start of your script to turn them off:

#!/bin/sh
[...]
# Disable warnings about use of egrep and fgrep
# shellcheck disable=SC2196,SC2197

(I don't want to use a $HOME/.shellcheckrc for this, because I want our scripts to Shellcheck cleanly regardless of who runs Shellcheck.)