This is the most straightforward way to allow sourcing scripts, as shellcheck
[SC1090] only allows `source` files that are in the same invocation, I believe
unless `-x` is also specified.
[SC1090]: https://github.com/koalaman/shellcheck/wiki/SC1090
Does not need bash to run; standard posix sh suffices.
Apply shellcheck to:
* files with shbang lines that indicate a shell (sh, bash, ash, bsh, ksh, zsh)
* typical home directory shell startup files:
for posix sh & all shells: .profile
for bash: .bashrc, .bash_aliases, .bash_completion, .bash_login, .bash_logout, .bash_profile
for ksh: .profile
for zsh: .zprofile, .zlogin, .zlogout, .zshenv, .zshrc
* typical system-wide shell startup files:
for posix sh & all shells: /etc/profile
for bash: /etc/bashrc, /etc/bash_profile
for ksh:
for zsh: /etc/zprofile, /etc/zlogin, /etc/zlogout, /etc/zshenv, /etc/zshrc
* files with suffixes: .bash, .zsh, .ksh, & .sh