Escaping characters

Created by: skylerberg

Like other shells we will need to allow the user to escape characters and strings. I would like to discuss the details here. Escaping is surprisingly involved in posix shells with four escaping mechanisms:

• Backslash: Escape the following character.
• Single quotes: Preserve literal value of all strings between quotes.
• Double quotes: Escape most characters between quotes with several exceptions
  • Dollar sign retains meaning.
  • Backtick retains meaning.
  • Backslash retains its meaning only when followed by a backtick, dollar sign, backslash, or newline. Otherwise it is preserved.
• Heredoc: Redirect standard input to read a multiline string provided at the command line. This string is escaped similar to double quotes. E.g., <<EOF. Zsh and bash actually implement escaping within heredocs in an incompatible way.
• Herestring: Not part of posix, but provided by Bash and other shells. It is like a one line heredoc, but at least in bash it provides no escaping. E.g., <<<some string.

The point of all this is to say that escaping is more complicated than meets the eye and I would like to discuss our approach to it here.