turns out env doesn't like $, changed to #

README.md

···
 
 A plain argument such as `arg` is a required argument, which will be passed to the script as a positional argument.
 
-An argument starting with `$` such as `$arg` is an optional argument, which will be passed to the script as an environment variable like `$ARGSH_ARG`, where all `-` get converted to `_`.
+An argument starting with `#` such as `#arg` is an optional argument, which will be passed to the script as an environment variable like `$ARGSH_ARG`, where all `-` get converted to `_`.
 
 An argument starting with `?` such as `?arg` is a flag, which will be passed to the script as an environment variable like `$ARGSH_ARG`, where all `-` get converted to `_`, with the value set to `1`, the value is irrelevant, just check if the variable is empty.
 

src/main.rs

···
 			let mut pos: u8 = 0;
 
 			for arg in args_raw.split(';') {
-				if let Some(arg) = arg.strip_prefix('$') {
+				if let Some(arg) = arg.strip_prefix('#') {
 					if is_valid_name(arg) {
 						args.push(Arg::Optional(arg.to_owned()));
 					} else {

test

···
-#!./target/debug/argsh bash|required;also-required;?flag;$optional;$second-optional
+#!./target/debug/argsh bash|required;also-required;?flag;#optional;#second-optional
 
 echo "--required on pos 1: $1"
 echo "--also-required on pos 2: $2"