is-nix-up-to-date.sh

#!/bin/sh

set -eu

CHAN=17.09
DELAY=675
SENTINAL=/var/lib/is-my-channel-up-to-date/out-of-date

fetch_local_version() {
    nixos-version --hash
}

fetch_channel_version() {
    curl "https://channels.nix.gsc.io/nixos-$CHAN/latest" \
        | cut -d' ' -f1
}
PREV_VERSION_LOCAL="bogus"
VERSION_LOCAL="bogus"
PREV_VERSION_CHAN="bogus"
VERSION_CHAN="bogus"


while true; do
    VERSION_LOCAL=$(fetch_local_version)
    if [ "$VERSION_LOCAL" != "$PREV_VERSION_LOCAL" ]; then
        printf "Local changed from %s to %s\n" \
               "$PREV_VERSION_LOCAL" \
               "$VERSION_LOCAL" >&2
    fi
    PREV_VERSION_LOCAL=$VERSION_LOCAL

    VERSION_CHAN=$(fetch_channel_version)
    if [ "$VERSION_CHAN" != "$PREV_VERSION_CHAN" ]; then
        printf "Channel changed from %s to %s\n" \
               "$PREV_VERSION_CHAN" \
               "$VERSION_CHAN" >&2
    fi
    PREV_VERSION_CHAN=$VERSION_CHAN

    if [ "$VERSION_CHAN" == "$VERSION_LOCAL" ]; then
        if [ -f "$SENTINAL" ]; then
            rm "$SENTINAL"
        fi
    else
        if [ ! -f "$SENTINAL" ]; then
            touch "$SENTINAL"
        fi
    fi

    sleep "$DELAY"
done