f=$(expr "$c" * 9 / 5 + 32)
Prefer rewriting to a modern style (see SC2003):
f=$((c * 9 / 5 + 32))
If you do not wish to do so, at least escape the glob characters when passing them to
f=$(expr "$c" \* 9 / 5 + 32)
expr is a command so
expr 2 * 2 will consider
* to mean "all files in the current directory". This causes the expression to fail to evaluate unless you are in an empty directory with the
nullglob options turned off.
Prefer rewriting it using the modern, POSIX standard arithmetic expansion
$((..)). If you do not wish to do so, you can escape any characters like
* to avoid the shell performing pathname expansion on them.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.