Re: [PATCH] t4014: shell portability fix

[Jeff King <peff@peff.net>]

On Tue, May 31, 2016 at 11:31:40PM -0400, Jeff King wrote:

> I wondered if we could implement our own "env" in the shell, but it's a
> little non-trivial, because:
> [...]
> Here's what I came up with, which does seem to work. It's pretty gnarly,
> though.

Here's a revised version that drops the shellquoting by using extra
layers of "eval" indirection. You may have heard of 3-star C
programmers. I think I just became a 2-dollar shell programmer.

So this is gross, but I think it actually _is_ portable, with the
exception of the "is it exported" check.

-- >8 --
# is there a simpler way, even when the contents are "unset"?
is_unset () {
	eval "test -z \"\${$1}\" && test unset = \"\${$1-unset}\""
}

# probably not portable; also, possible without sub-program?
is_exported () {
	export -p | grep "^declare -x $1="
}

# set var named by $1 to contents of $2
set_var () {
	eval "$1=\$2"
}

# set var named by $1 to contents of var named by $2
copy_var () {
	eval "set_var \$1 \"\$$2\""
}

# just a syntactic convenience
add_to () {
	eval "$1=\"\$$1
		\$2\""
}

fake_env () {
	fake_env_restore_=
	while test $# -gt 0
	do
		case "$1" in
		*=*)
			# this whole thing is not safe when the var name has
			# spaces or other meta-characters, but since the names
			# all come from our test scripts, that should be OK
			fake_env_var_=${1%%=*}
			fake_env_orig_=fake_env_orig_$fake_env_var_
			copy_var $fake_env_orig_ $fake_env_var_
			fake_env_val_=${1#*=}
			shift

			# unset value and clear export flag...
			add_to fake_env_restore_ "unset $fake_env_var_"
			# ...and then restore what was there, if anything
			if ! is_unset "$fake_env_var_"
			then
				add_to fake_env_restore_ \
					"copy_var $fake_env_var_ $fake_env_orig_"
				if is_exported "$fake_env_var_"
				then
					add_to fake_env_restore_ \
						"export $fake_env_var_"
				fi
			fi
			# and then clean up our temp variable
			add_to fake_env_restore_ "unset $fake_env_orig_"

			set_var $fake_env_var_ "$fake_env_val_"
			eval "export $fake_env_var_"
			;;
		*)
			"$@"
			fake_env_ret_=$?
			eval "$fake_env_restore_"
			return $fake_env_ret_
			;;
		esac
	done
}

# simple exercise
foo() {
	echo >&2 "$1: bar=$bar"
}

bar="horrible \"\\' mess"
foo before
fake_env bar="temporary $bar" foo during
foo after