rc/sh/minutemaid

#!/usr/bin/env dash
# compat: +ash +bash +dash -hush -ksh +mksh +oksh +osh +posh +yash +zsh

minutemaid() { ### @-
    ### check if the current minute is divisible by a given number,
    ### and optionally execute a command if it is. if a command is given,
    ### either execute the command and return its exit value,
    ### or execute nothing and return 0. if a command is omitted,
    ### either return 0 on the minute, or return 1.
    ###
    ### note that a "minute" is relative to the seconds since the epoch,
    ### not the minute of the hour. this ensures that commands will run
    ### roughly every N minutes, regardless of the minute hand on the clock.
    ###
    ### note that `minutemaid 1` will always return 0,
    ### and `minutemaid 1 command` will always execute the command,
    ### since every integral interval is evenly divisible by 1.
    ### `minutemaid 0`, and any negative interval, is an error.
    ###
    ### ```
    ### # crontab usage:
    ### * * * * * minutemaid 9 ~/work/do_my_bidding # runs every nine minutes
    ### # loop usage:
    ### until minutemaid 9; do sleep 5; done
    ### echo the wait is finally over; date
    ### # improper usage:
    ### while minutemaid 1; do sleep 1; done
    ### echo this will never happen
    ### ```

    local offset=0 name=
    while getopts 'o:h' name; do
        case $name in
        o)  offset="$OPTARG";;
        ?)  local fd=0 # FIXME: this also matches single-character filenames!
            [ $name = h ] && fd=1 || fd=2
            printf '%s\n' "usage: $0 [-o offset] {interval} [{command} [{args...}]]" >&$fd
            [ $name = h ] && return 0 || return 1;;
        esac
    done
    shift $((OPTIND-1))

    local interval="${1:?no interval specified}"
    shift

    if [ "$interval" -gt 0 ] 2>/dev/null; then
        :
    else
        printf "%s\n" "$0: interval must be a positive integer" >&2
        return 1
    fi

    local sec="$(date +%s)"
    local min="$((sec/60+offset))"
    local mod="$((min%interval))"
    if [ $# -gt 0 ]; then
        [ $mod -ne 0 ] || "$@"
    else
        [ $mod -eq 0 ] && return 0 || return 1
    fi
}

[ -n "${preload+-}" ] || minutemaid "$@"