timezones

#!/bin/sh

set -eu

conf_file="$HOME/.config/timezones"

usage() {
    basename="$(basename "$0")"
    cat >&2 <<EOM
usage: $basename [Options]

Display the time in several time zones. The zones will be taken from \$ZONES,
if set, or from ~/.config/timezones.

Options will be passed through to date(1). See tzselect(1) or
/usr/share/zoneinfo for lists of timezones.

Examples:

    ZONES='UTC Japan/Tokyo' $basename

    $basename +%c

    $basename -d '+3 hours' '+%F %R'

EOM
}

if [ $# -ge 1 ] && [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
    usage
    exit
fi

ZONES="${ZONES:-}"

if [ -z "$ZONES" ]; then
    if [ -e "$conf_file" ]; then
        ZONES="$(cat "$conf_file")"
    else
        usage
        exit 1
    fi
fi

if which gdate >/dev/null; then
    date=gdate
else
    date=date
fi

for tz in $ZONES; do
    if [ ! -e "/usr/share/zoneinfo/$tz" ]; then
        echo >&2 "Warning: not sure whether $tz is a valid timezone"
    fi
    output="$(TZ="$tz" "$date" "$@")"
    echo "$output	$tz"
done