Encourage dot (`.`) over `source`

[mcandre]

Though checkbashisms does not report this, I believe source is a bashism; POSIX sh (and even modern descendants like ash/dash) only recognizes the dot (.) command.

Could shellcheck get an optional rule for this, disabled by default? Something like:

In hello.sh line 1:
  source world.sh
  ^-- Replace source with dot (.) for POSIX compatibility

[mcandre]

Note: Except for csh/tcsh, which are non-POSIX compliant. csh and tcsh only recognize source, not dot (.).

[koalaman]

When specifying #!/bin/sh or -s sh, you get

source foo
^-- SC2039: #!/bin/sh was specified, but 'source' in place of '.' is not standard.