What are the rules for valid identifiers (e.g. functions, vars, etc) in Bash?

Question

What are the syntax rules for identifiers, especially function and variable names, in Bash?

I wrote a Bash script and tested it on various versions of Bash on Ubuntu

Answer 1

Command identifiers and variable names have different syntaxes. A variable name is restricted to alphanumeric characters and underscore, not starting with a digit. A command name, on the other hand, can be just about anything which doesn't contain bash metacharacters (and even then, they can be quoted).

In bash, function names can be command names, as long as they would be parsed as a WORD without quotes. (Except that, for some reason, they cannot be integers.) However, that is a bash extension. If the target machine is using some other shell (such as dash), it might not work, since the Posix standard shell grammar only allows "NAME" in the function definition form (and also prohibits the use of reserved words).