Get current timezone as `Region/City`

In this comment by Stéphane Chazelas, he said:

timedatectl is a systemd thing that queries timedated over dbus and timedated derives the name of the timezone (like Europe/London) by doing a readlink() on /etc/localtime. If /etc/localtime is not a symlink, then that name cannot be derived as those timezone definition files don't contain that information.

Based on this and tonioc's comment, I put together the following:

#!/bin/bash
set -euo pipefail

if filename=$(readlink /etc/localtime); then
    # /etc/localtime is a symlink as expected
    timezone=${filename#*zoneinfo/}
    if [[ $timezone = "$filename" || ! $timezone =~ ^[^/]+/[^/]+$ ]]; then
        # not pointing to expected location or not Region/City
        >&2 echo "$filename points to an unexpected location"
        exit 1
    fi
    echo "$timezone"
else  # compare files by contents
    # https://stackoverflow.com/questions/12521114/getting-the-canonical-time-zone-name-in-shell-script#comment88637393_12523283
    find /usr/share/zoneinfo -type f ! -regex ".*/Etc/.*" -exec \
        cmp -s {} /etc/localtime \; -print | sed -e 's@.*/zoneinfo/@@' | head -n1
fi

For the time zone, you can use geolocation:

$ curl https://ipapi.co/timezone
America/Chicago

Or:

$ curl http://ip-api.com/line?fields=timezone
America/Chicago

http://wiki.archlinux.org/index.php/time#Time_zone