Add more custom validations

[DannyBen]

Checklist

• Improve validation error message
• Add file_exists validation
  • and test in fixtures
• Add dir_exists validation
  • and test in fixtures
• Add not_empty validation
  • and test in fixtures - needs Allow empty string as argument (replacing #122) #135
• Remove support for array validations

Closes #132

[DannyBen]

In addition - I am also wondering if at all we need to allow validate to receive an array. I cannot find a single use case where this is useful.

[wolfgang42]

In addition - I am also wondering if at all we need to allow validate to receive an array. I cannot find a single use case where this is useful.

I left an example in the original issue, of [is_readable, is writable], but I agree that this is a vague use case for me too. Maybe to simplify, at least for now, these can simply be defined as custom validations:

validate_scratch_file() {
    validate_is_file "$1" && validate_readable "$1" && validate_writable "$1"
}

[DannyBen]

I left an example in the original issue, of [is_readable, is writable]

Well, supporting an array is not a big deal and is already implemented, it just adds some confusion to the docs, and I am not sure it is needed. We can keep it for now but it is on the chopping block.

[wolfgang42]

If it's on the chopping block I'd say take it out; it can always be added back in later if someone has a need or comes up with a clearer way to do it.

[DannyBen]

Ready to be merged from my standpoint.