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
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