List examples of GNU Coreutils date

date_usage.bash

#!/bin/bash

source_user_defined() {
	readonly cmds=(
		'date +%Y/%m/%d'
		'date +"%Y/%m/%d %T"'
		'date +"%Y/%-m/%-d (%a)"'
		'LC_ALL=C date +"%Y/%-m/%-d (%a)"'
		'LC_ALL=ja_JP.utf8 date +"%EY"'
		"date --date @$(date +%s)"
		'date --iso-8601'
		'date --iso-8601=hours'
		'date --iso-8601=minutes'
		'date --iso-8601=seconds'
		'date --iso-8601=ns'
		'date --rfc-email'
		'date --rfc-822'
		'date --rfc-2822'
		'date --rfc-3339=date'
		'date --rfc-3339=seconds'
		'date --rfc-3339=ns'
		'date --utc'
	)

	for s in "${cmds[@]}"; do
		printf '%s\t\033[32;1m| %s\033[m\n' "$(eval "$s")" "$s"
	done
}

source_date_help() {
	date --help |
		sed -n '/^ *%%/,/^ *%Z/p' | while read -r line; do
		format="${line/% */}"
		[[ $format == '%%' ]] && continue
		[[ $format == '%t' ]] && continue
		[[ $format == '%n' ]] && continue

		example="$(date "+$format")"
		description=${line#%[a-zA-Z:]* *}
		printf '%s\t\033[32;1m| date +%s  #%s\033[m\n' "$example" "$format" "$description"
	done
}

main() {
	{
		source_user_defined
		source_date_help
	} | column -t -s $'\t'
}

main