`tree` util implemented in pure bash

tree.sh

#!/bin/bash

# all files (include dirs) under a dir
_allunder() {
    local f; for f in $1/* $1/.*; do
        case $f in
            "$1/*" | "$1/.*" | "$1/." | "$1/..") ;;
            *) printf "$f " ;;
        esac
    done
}

fo() {
    local files=(`_allunder $2`)
    local i; for i in ${!files[@]}; do
        local f=${files[i]}
        local j; for j in $1; do
            if (( $j )); then
                printf '│   '
            else
                printf '    '
            fi
        done
        if [[ $i = $[${#files[@]}-1] ]]; then
            printf '└── '
        else
            printf '├── '
        fi
        echo ${f##*/} # $i ${#files[@]}
        [[ -d $f ]] && {
            if [[ $i = $[${#files[@]}-1] ]]; then
                fo "$1 0" $f
            else
                fo "$1 1" $f
            fi
        }
    done
}

[[ $@ ]] || set -- .
for d in $@; do
    echo $d
    fo '' $d
done