unwrap

#!/usr/bin/env sh

# Run a command from a wrapper. The goal is to allow wrapper scripts located in
# $PATH to `exec` the "real" command without ending up in an infinite loop.

# Written by a tired ayekat in a train on a cold Wednesday night in November
# 2017.

set -e
set -u

RUNNAME="$(basename "$0")"
E_SUCCESS=0
E_USER=1
E_INTERNAL=2
E_COMMAND=127

which()
(
	test $# -gt 0 || return 1
	cmd="$1"; shift
	if [ $# -gt 0 ]; then
		ignored="$1"
		shift
	fi
	test $# -eq 0 || return 1

	IFS=:
	for p in $PATH; do
		cmdpath=$p/$cmd
		if ! cmdpath_resolved=$(readlink -f "$cmdpath"); then
			continue
		fi
		case "$ignored" in (*:$cmdpath:*|*:$cmdpath_resolved:*)
			continue
		esac
		if [ -x "$cmdpath" ]; then
			echo "$cmdpath"
			return 0
		fi
	done
	return 127
)

die()
{
	retval=$(($1 + 0)); shift
	if [ $retval -eq $E_INTERNAL ]; then
		printf 'Internal error: ' >&2
	fi
	# shellcheck disable=SC2059
	printf "$@" >&2
	echo >&2
	if [ $retval -eq $E_USER ]; then
		printf 'Run with -h for more help\n' >&2
	fi
	exit $retval
}

help()
{
	cat <<- EOF
	Usage: $RUNNAME [OPTION...] COMMAND ...

	Options:
	  -h           Display this help message and exit.
	  -i FILEPATH  In addition to ignoring the wrapper script location,
	               also ignore FILEPATH when searching for commands
	               (this option may be passed multiple times)
	  -t           Test if the command could be unwrapped (exit with 1 if not)
	  -v           Verbose: print path of unwrapped executable
	EOF
}

# Options:
ignored=':'
test=false
verbose=false
while getopts :hi:tv opt; do
	case "$opt" in
		(h) help; exit $E_SUCCESS ;;
		(i) ignored="$ignored$OPTARG:" ;;
		(t) test=true ;;
		(v) verbose=true ;;
		(:) die $E_USER 'Missing argument for -%s' "$OPTARG" ;;
		('?') die $E_USER 'Unknown option: -%s' "$OPTARG" ;;
		(*) die $E_INTERNAL 'Unhandled option: -%s' "$OPTARG" ;;
	esac
done
shift $((OPTIND - 1))

# Command:
test $# -gt 0 || die $E_USER 'Please specify a command'
cmd="$1"
shift
case "$cmd" in (*/*)
	die $E_USER 'Please specify a command, not a path'
esac

# Get wrapper script path:
if ! wpath="$(which "$cmd" "$ignored")"; then
	die $E_COMMAND 'command not found: %s' "$cmd"
fi

# Get "real" path:
if ! rpath="$(which "$cmd" ":$wpath$ignored")"; then
	if $test; then
		exit 1
	fi
	die $E_COMMAND 'wrapper script not unwrappable: %s' "$wpath"
fi
if $verbose; then
	printf '%s\n' "$rpath"
fi

# Execute real executable:
if $test; then
	exit 0
fi
exec "$rpath" "$@"