Implementing shell language semantics (non-POSIX extensions)

[magicant]

This is a tracking issue for implementation of the non-portable part of the shell language semantics.
For the semantics required by POSIX, see #51.

• Word expansion
  • Tilde expansion
    • ~+
    • ~-
    • ~+n, ~-n
    • Report error in tilde expansion #193
  • Parameter expansion
    • Array variable expansion abf4976 Refactor expansion #140
    • Nested expansion
    • Index
    • Switch modifiers
      • Assignment to an array like ${a[1]:=foo}
    • Substitution modifiers #201
  • Arithmetic expansion (non-POSIX extensions) #261
  • Character expansion ($'...')
  • Brace expansion
• Command substitution
  • $(< filename)
• Command execution
  • Simple command
    • Command search cache (hash)
    • COMMAND_NOT_FOUND_HANDLER
  • Compound commands
    • Double brackets ([[ ... ]])
• Redirections
  • Sockets (/dev/tcp/..., /dev/udp/...)
  • Pipe redirection (>>|)
  • Here-string (<<<)
  • Process redirections (<(...), >(...))
• Signal handling
  • (Run traps while reading commands)
  • Implement missing tests

Possible optimizations

• Omit removing signal catching handlers when starting a subshell that is just execing
• The finally_exit flag: avoid unnecessary fork & wait if a command is known to be the last in the current shell execution environment.