refactoring

[davidchambers]

Commit message:

This commit is intended to improve readability by:

• introducing an abort function for writing an error message to standard error then exiting with exit code 1;

• storing options in an associative array rather than in separate variables; and

• validating command-line arguments against the aforementioned associative array (to avoid enumerating all the options).

This is the last invasive change I will make before releasing v3.0.0. :)

[Avaq]

Looks good! Just a question:

[Avaq]

I can't find the ${parameter:value}-syntax in this page. Only parameter:[-=+?]value. What does this do?

[davidchambers]

It is substring extraction (i.e. string slicing). See ${string:position} and ${string:position:length}. In this case we're just stripping the leading hyphens.

[Avaq]

Ahhh! :)

[davidchambers]

-n ${options[${BASH_REMATCH[1]}]+x} requires an explanation.

BASH_REMATCH:

An array variable whose members are assigned by the ‘=~’ binary operator to the [[ conditional command (see Conditional Constructs). The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression. This variable is read-only.

So, if option is --branch, ${BASH_REMATCH[1]} will be branch.

We then wish to determine whether branch is a key of options. The naive approach is to test the result of the lookup with -n, but this assumes the null string will never be used as a value. The thing we actually want to determine is whether the key exists. For this we can use parameter expansion:

declare -A arr=([foo]=FOO [bar]='')

echo "foo: ${arr[foo]+x}" # foo: x
echo "bar: ${arr[bar]+x}" # bar: x
echo "baz: ${arr[baz]+x}" # baz:

So, ${options[${BASH_REMATCH[1]}]+x} will evaluate to x if the specified option exists; the null string otherwise. We then use -n to differentiate these two cases.