fish-shell/share/functions/eval.fish

function eval -S -d "Evaluate parameters as a command"
	# keep a copy of the previous $status and use restore_status
	# to preserve the status in case the block that is evaluated
	# does not modify the status itself.
	set -l status_copy $status
	function __fish_restore_status
		return $argv[1]
	end

	if not set -q argv[2]
		# like most builtins, we only check for -h/--help
		# if we only have a single argument
		switch "$argv[1]"
		case -h --help
			__fish_print_help eval
			return 0
		end
	end

	# If we are in an interactive shell, eval should enable full
	# job control since it should behave like the real code was
	# executed.  If we don't do this, commands that expect to be
	# used interactively, like less, wont work using eval.

	set -l mode
	if status --is-interactive-job-control
		set mode interactive
	else
		if status --is-full-job-control
			set mode full
		else
			set mode none
		end
	end
	if status --is-interactive
		status --job-control full
	end
	__fish_restore_status $status_copy

	# To eval 'foo', we construct a block "begin ; foo; end <&3 3<&-"
	# Note the redirections are also within the quotes.
	#
	# We then pipe this to 'source 3<&0’.
	#
	# You might expect that the dup2(3, stdin) should overwrite stdin,
	# and therefore prevent 'source' from reading the piped-in block. This doesn't happen
	# because when you pipe to a builtin, we don't overwrite stdin with the read end
	# of the block; instead we set a separate fd in a variable 'builtin_stdin', which is
	# what it reads from. So builtins are magic in that, in pipes, their stdin
	# is not fd 0.
	#
	# ‘source’ does not apply the redirections to itself. Instead it saves them and passes
	# them as block-level redirections to parser.eval(). Ultimately the eval’d code sees
	# the following redirections (in the following order):
	#    dup2 0 -> 3
	#    dup2 pipe -> 0
	#    dup2 3 -> 0
	# where the pipe is the pipe we get from piping ‘echo’ to ‘source’. Thus the redirection
	# effectively makes stdin fd0, instead of the thing that was piped to ‘source’
	echo "begin; $argv "\n" ;end <&3 3<&-" | source 3<&0
	set -l res $status

	status --job-control $mode
	return $res
end