Recommend local variables

[mcandre]

For new checks and feature suggestions

• shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

As a developer, I would like shellcheck to warn me when my scripts declare variables that are not local scope, so that I can catch subtle errors more quickly, rather than having scripts blow up in production.

Bash and bash-like shells provide syntax for declaring that a variable is locally scoped, rather than globally scoped. In general, it is a good idea to prefer local scope, to prevent colliding with variables that influence how different commands run (think VERBOSE, LIBS, etc). Note that some shells such as POSIX sh, unfortunately do not provide syntax to declare locally scoped variables, and so only scripts with the relevant shebangs / filename patterns should receive this rule.

Note that some variables are expected to be global or even environment scope (PATH, USER, CC). Such variables should not be locally scoped.

[matthewpersico]

I would suggest that ALL variables in ALLCAPS be excluded from this check. I would also suggest that this functionality be controlled with a command line switch and the default would be OFF, unless the command line switch is present.

[mgedmin]

Duplicate of #468?