fling

#!/bin/bash
set -euo pipefail
cd "$( dirname "${BASH_SOURCE[0]}" )"

##
## Emits a line slice of command names found in the given dir.
##
## The pattern for command filenames is "cmd.*.sh".
##
## Commands may also declare flags, help text, and and other
## similar autocomplete-helpful information in matching files
## named "cmd.*.flingcfg"; to get this info for a command,
## use `TODO`.
##
## Command names are sanitychecked to print cleanly without
## escaping.  You may safely use them in standard for-loops
## with normal IFS, etc.  Uncooperative names are simply
## excluded from the return.
##
## The special command "cmd._.sh" will be skipped by this list.
##
function Fling_GetCommandsInDir {
	dir=$1

	while IFS= read -r -u3 -d $'\0' subcommand; do
		if [ "$(printf %q "$subcommand")" != "$subcommand" ]; then continue; fi
		subcommand="${subcommand#\./cmd\.}"
		subcommand="${subcommand%\.sh}"
		if [ "$subcommand" == "_" ]; then continue; fi
		printf "%s\n" $subcommand
	done 3< <(
		cd "$dir" && find . -maxdepth 1 -type f -executable -name 'cmd.*.sh' -print0
	)
}

##
## Checks that a subcommand exists, printing a sensible error if not.
##
## Use before `Fling_InvokeCommand` if you want to handle non-existence separately.
##
function Fling_CheckCommand {
	dir="$1"
	cmd="$2"
	shift 2 || true

	## args validation
	if [ "$(printf %q "$cmd")" != "$cmd" ]; then
		>&2 printf "cannot possibly be a subcommand: too many strange chars!\n"
		return 1
	fi

	## check it exists
	if [ ! -x "$dir/cmd.$cmd.sh" ]; then
		>&2 printf "no such command \"%q\".\n" "$cmd"
		return 1
	fi
	return 0
}


##
## Invokes a subcommand.  Varargs expected, and will be passed on.
##
## The subcommand script will be invoked with CWD set to its
## own parent (subcommands are expected to load any library
## files relative to themselves with a simple `source ./foo.shlib`).
## The $FLING_BASE environment variable is provided to
## refer back to the root dir of the project.
##
## All the same rules for command name sanity are checked as
## they are in `Fling_GetCommandsInDir`.
##
function Fling_InvokeCommand {
	dir="$1"
	cmd="$2"
	shift 2 || true

	## args validation
	Fling_CheckCommand "$dir" "$cmd"

	## go
	export FLING_BASE=${FLING_BASE:-$PWD}
	(cd "$dir" && ./cmd.$cmd.sh "$@")
}

##
## Emits the usage text (short form, e.g. the thing you get
## if you didn't give a command).
##
function Fling_EmitUsage {
	>&2 printf "usage:\n\n"
	>&2 printf "  fling <SUBCOMMAND>\n"
	>&2 printf "  fling help\n"
	>&2 printf "\n"

	>&2 printf "subcommands:\n\n"
	subcommands="$(Fling_GetCommandsInDir "fling.d")"
	for subcommand in ${subcommands[@]}; do
		>&2 printf "  - %s\n" $subcommand
	done
	>&2 printf "\n"
}

##
## Main!
##
function Fling_Main {
	cmd=${1:-}
	shift || true

	case "$cmd" in
	'')
		if Fling_CheckCommand "fling.d" "_" 2>/dev/null; then
			Fling_InvokeCommand "fling.d" "_"
		else
			>&2 printf "fling v0.0.00 -- a command launcher.\n\n"
			Fling_EmitUsage
			exit 1
		fi
		;;
	help)
		>&2 printf "fling v0.0.00 -- a command launcher.\n\n"
		Fling_EmitUsage
		exit 1
		;;
	*)
		## try to find a matching subcommand, and call it.
		Fling_CheckCommand "fling.d" "$cmd" || {
			## If not found, print usage.
			>&2 printf "\n"
			Fling_EmitUsage
			exit 1
		}
		Fling_InvokeCommand "fling.d" "$cmd" "$@"
		;;
	esac
}
Fling_Main "$@"