Abbreviate working directory in bash PS1

abbrev-pwd.bash

# Copyright 2018 Google LLC.
# SPDX-License-Identifier: Apache-2.0

_abbrev_pwd () {
    local dir="$PWD"

    if [ -n "$HOME" ] && ([ "$dir" = "$HOME" ] || [[ "$dir" == $HOME/* ]]); then
        echo -n "~"
        dir="${dir#$HOME}"
    fi

    local -a dir_components
    mapfile -d / -t dir_components <<< "$dir"
    local n="${#dir_components[@]}"
    for ((i=0; i<n-1; i++)); do
        local first_letter="${dir_components[i]::1}"
        if [ "$first_letter" = "." ]; then
            printf %s/ "${dir_components[i]::2}"
        else
            printf %s/ "$first_letter"
        fi
    done
    echo -n "${dir_components[n - 1]}"
}