## knob.bash: primitives for kernel settings files (sysfs, procfs).

## write_knob writes a value into a kernel settings file.
## These files often read back in a different format than they accept
## (`lru_gen/enabled` takes `y` and reads back a hex bitmask),
## so a change is reported when the read-back moved, not by comparing it to VALUE.
## The write happens every time; these files reset at boot anyway, and rewriting a value is harmless.
##
## - @param $1 absolute path of the settings file.
## - @param $2 value to write.
write_knob() {
	if [ "$#" -ne 2 ]; then
		_pave_error "usage: write_knob PATH VALUE"
		return 1
	fi
	local path=$1 value=$2 before after
	_pave_need_abs "$path" || return 1
	before=$(<"$path") || return 1
	printf '%s\n' "$value" >"$path" || return 1
	after=$(<"$path") || return 1
	if [ "$before" != "$after" ]; then
		changed "$path: $before -> $after (wrote $value)"
	fi
}