A pure bash implementation of the `tree` utility, for when the `tree` binary isn't available. Avoids subshells or subprocesses to improve performance.

tree.bash

#!/usr/bin/env bash

# A fallback, when GNU tree isn't installed.
# Example:

#     >>> mkdir -p {A1,A2}/{B1,B2/C1}
#     >>> touch A2/B3
#     >>> my-tree
#     .
#     |-- A1
#     |   |-- B1
#     |   \-- B2
#     |       \-- C1
#     \-- A2
#         |-- B1
#         |-- B2
#         |   \-- C1
#         \-- B3
#     
#     8 directories, 1 files
#     >>> 
#
# Avoids subshells and subprocesses to give better speed.


set -e -E -u
shopt -s nullglob # * -> empty in empty directory                 
shopt -s dotglob  # * -> include .hidden files but not ../parent. 

NUMFILES=0
NUMDIRS=-1 # top doesn't count

tree_do () {
    local INDENT="$1" PREFIX="$2" ROOT="$3" POS="$4"
    case "$POS" in 
        nonlast) local ADDINDENT='|   ';;
        last)    local ADDINDENT='    ';;
        top)     local ADDINDENT='';;
        *)
            echo "Invalid value POS=$POS"
            exit 1
            ;;
    esac 
    local NEWINDENT="$INDENT$ADDINDENT"
    if ! [[ -d "$ROOT" ]]; then
        printf "%s\n" "$INDENT$PREFIX$ROOT"
        : $((++NUMFILES))
    else
        : $((++NUMDIRS))
        printf "%s\n" "$INDENT$PREFIX$ROOT"
        local OLDPWD="$PWD"
        cd "$ROOT" || return 0
        local FILE="" NEXTFILE
        for NEXTFILE in *; do
            # Print for not-last file.
            if [[ -n "$FILE" ]]; then
                tree_do "$NEWINDENT" "|-- " "$FILE" nonlast
            fi
            FILE="$NEXTFILE"
        done
        # Print for last file - if any.
        if [[ -n "$FILE" ]]; then
            tree_do "$NEWINDENT" "\-- " "$FILE" last
        fi
        cd "$OLDPWD"
    fi
}

tree_do "" "" "${1:-.}" top
echo ''
echo "$NUMDIRS directories, $NUMFILES files"