Add persist-changes script (#401)

persist-changes/README.md

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+# Persist changes
+
+Do you want to make some custom changes to your UDM which Unifi hasn't released yet?
+Would you like to make some changes that on EdgeOS would be achieved with config.something.yml?
+
+If you just make changes to the interfaces, Unifi will overwrite what you've done periodically/next time you change something.
+Luckily we can hook into Unifi's state file and perform the updates immediately afterwards.
+
+For example, [configuring two IP addresses on your WAN interface, so that you can get to your modem's configuration](https://community.ui.com/questions/Access-modem-connected-to-USG/db5986b8-26cb-4d66-a332-2ace81ac8c4f#answer/7da28d8d-25c8-4ca3-b455-c6eba836f034).
+
+## Installation
+
+1. [Enable on-boot-script](https://github.com/unifi-utilities/unifios-utilities/blob/main/on-boot-script/README.md)
+1. Copy `42-watch-for-changes.sh` to `/mnt/data/on_boot.d/`
+    * Check the `FILE` variable, it should point to a file that exists, it might be in `/data` or in `/mnt/data`
+1. Copy `on-state-change.sh` to `/mnt/data/scripts/`
+1. Edit `/mnt/data/scripts/on-state-change.sh` to your heart's content
+
+> Make sure that your script doesn't error in the likely case that it tries to execute an update which has already been made
+
+## Example: configuring two IP addresses on your WAN interface
+
+`/mnt/data/scripts/on-state-change.sh`
+
+```
+#!/bin/sh
+
+# give port9 this IP, allows access to router web interface
+ip addr add 192.168.0.2/24 dev eth8 || true
+```

persist-changes/on_boot.d/42-watch-for-changes.sh

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+#!/bin/sh
+
+EXECUTE01='/mnt/data/scripts/on-state-change.sh'
+FILE="/data/udapi-config/ubios-udapi-server/ubios-udapi-server.state"
+# /usr/bin/logger -t "${PROCESSNAME}" "$*"
+
+# run on boot aswell
+$EXECUTE01
+
+if [ "$1" = "DAEMON" ]; then
+    # is this necessary? Add other signals at will (TTIN TTOU INT STOP TSTP)
+    trap '' INT
+    cd /tmp
+    shift
+    ### daemonized section ######
+    # RUNNING=`ps aux | grep $CMD | grep -v grep | wc -l`
+    # echo $RUNNING
+    # if [ "$RUNNING" -lt 1 ]; then
+    LAST=`ls -l "$FILE"`
+    # echo $LAST
+    while true; do
+      sleep 1
+      NEW=`ls -l "$FILE"`
+      # echo $NEW
+      if [ "$NEW" != "$LAST" ]; then
+        DATE=`date`
+        echo "${DATE}: Executing ${EXECUTE01}"
+        $EXECUTE01
+        LAST="$NEW"
+      fi
+    done
+    # fi
+    #### end of daemonized section ####
+    exit 0
+fi
+
+export PATH=/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/sbin:/usr/local/bin
+umask 022
+# You can add nice and ionice before nohup but they might not be installed
+# nohup setsid $0 DAEMON $* 2>/var/log/mydaemon.err >/var/log/mydaemon.log &
+nohup setsid $0 DAEMON WATCH $* 2>/var/log/watch.err >/var/log/watch.log &

persist-changes/scripts/on-state-change.sh

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+#!/bin/sh
+
+# edit this file with your custom instructions...