source: vendor/bash/3.1/examples/functions/which

#
# which - emulation of `which' as it appears in FreeBSD
#
# usage: which [-as] command [command...]
#

which()
{
        local aflag sflag ES a opt

        OPTIND=1
        while builtin getopts as opt ; do
                case "$opt" in
                a)      aflag=-a ;;
                s)      sflag=1 ;;
                ?)      echo "which: usage: which [-as] command [command ...]" >&2
                        exit 2 ;;
                esac
        done

        (( $OPTIND > 1 )) && shift $(( $OPTIND - 1 ))

        # without command arguments, exit with status 1
        ES=1

        # exit status is 0 if all commands are found, 1 if any are not found
        for command; do
                # if $command is a function, make sure we add -a so type
                # will look in $PATH after finding the function
                a=$aflag
                case "$(builtin type -t $command)" in
                "function")     a=-a;;
                esac

                if [ -n "$sflag" ]; then
                        builtin type -p $a $command >/dev/null 2>&1
                else
                        builtin type -p $a $command
                fi
                ES=$?
        done

        return $ES
}