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

#
# An almost-ksh compatible `whence' command.  This is as hairy as it is 
# because of the desire to exactly mimic ksh.
# 
# This depends somewhat on knowing the format of the output of the bash
# `builtin type' command.
#
# Chet Ramey
# chet@ins.CWRU.Edu
#
whence()
{
        local vflag= path=

        if [ "$#" = "0" ] ; then
                echo "whence: argument expected"
                return 1
        fi
        case "$1" in
                -v) vflag=1
                    shift 1
                    ;;
                -*) echo "whence: bad option: $1"
                    return 1
                    ;;
                 *) ;;
        esac

        if [ "$#" = "0" ] ; then
                echo "whence: bad argument count"
                return 1
        fi

        for cmd
        do
                if [ "$vflag" ]  ; then
                        echo $(builtin type $cmd | sed 1q)
                else
                        path=$(builtin type -path $cmd)
                        if [ "$path" ] ; then
                                echo $path
                        else
                                case "$cmd" in
                                        /*) if [ -x "$cmd" ]; then
                                                echo "$cmd"
                                            fi
                                            ;;
                                         *) case "$(builtin type -type $cmd)" in
                                                "") ;;
                                                 *) echo "$cmd"
                                                    ;;
                                            esac
                                            ;;
                                esac
                        fi
                fi
        done
        return 0
}