Implementing shell environment

[magicant]

This is a tracking issue for implementation of shell environment.

Items

• System-managed attributes
  • File descriptors *
  • Working directory *
  • File creation mask (umask) * b9881be
  • Limits (ulimit) * c751c80
  • Spent times (times) baf273d
  • Real/effective/saved-set user/group
• System-managed attributes behind application-managed ones
  • Traps * Implementing semantics about traps #109
  • Jobs * Manage jobs in JobSet #153 Refactor JobSet #154
    • Current and previous jobs f548ab9
    • Parsing job IDs 8d78200
• Application-managed attributes
  • Parameters and variables *
    • Local/global variables
    • Positional parameters Positional parameters in function calls #107
    • Special parameters
      • $?
      • $$ 6c3d04d
      • $0 83d54d9
    • Shell variables #217
  • Functions *
  • Built-ins Built-in environment #33
  • Options * Shell options and set built-in #148
    • Mutually exclusive options
  • Aliases *
  • Execution context stack
  • Controlling terminal file descriptor 1bb5a7d bb7a3ff
  • Command history
• Attributes that possibly do not have to be in the environment because they're not copied when forking a subshell.
  • Files on disks
  • Line-editing configuration (key bindings and completion settings)

Items marked * are defined in POSIX.

Possible optimizations

• Virtual subshell: executing a synchronous subshell in the current process rather than in a child process

[magicant]

It may be helpful to have a set of internally used file descriptors as one of application-managed attributes. Such FDs can be redirections' saved FD, pipes for sending here-document contents, interactive shell's tty, etc.
Such FDs are invisible from the user, so they must move to other unused FDs when the user performs redirections on them. We can use Rc<Cell<Fd>> to keep track of where these FDs moved.

[magicant]

The CLOEXEC flag indicates whether an FD is internal, so we don't necessarily need an FD set.
We don't have to think about FDs moving as long as we don't support overwriting internal FDs.