var=grep -c pattern file
var=$(grep -c pattern file)
To assign the output of a command to a variable, use
$(command substitution). Just typing a command after the
= sign does not work.
This warning triggers generally for
var=value -flag and
var=value *glob*. See related warning SC2209 which matches
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.