The problem#

When doing an incremental backup, any moved file on the source filesystem usually results in recopying the file to the destination filesystem. For a large file this can both be slow and possibly waste space if the destination keeps around deleted files (e.g. ZFS holding on to old snapshots). If both sides are ZFS, then you can get zfs send/recv to handle all of the details efficiently. But if only the source filesystem is ZFS or the ZFS datasets are not at the same granularity on both sides, that doesn't apply.

zfs diff gives the information about file moves from a snapshot, but its output format is a little awkward for scripting.

The solution#

Download the script I wrote, zfs-diff-move.sh and run it like

zfs-diff-move.sh /path/ /tank/dataset/ tank/dataset@base @new

The following is an abbreviated version of it:

#!/bin/bash
zfs diff -H "$3" "$4" | grep '^R' | while read -r line
do
  get_path() {
    path="$(echo -e "$(echo "$line" | cut -d$'\t' "-f$3")")"
    echo "${path/#$2/$1}"
  }

  from="$(get_path "$1" "$2" 2)"
  to="$(get_path "$1" "$2" 3)"
  mkdir -vp -- "$(dirname "$to")"
  mv -vn -- "$from" "$to" || echo "Unable to move $from"
done

The details#

Dice in shell#

Today I was borrowing a board game from the lending library at Emerald City Comicon and it was missing its dice. We could have gotten some physical dice somewhere, but instead we decided to use the materials we had on hand. The people I was playing with agreed that we did not want to drain our phone batteries by using a dice app on our phones, but I had a laptop with me. So I wrote a dice app for the shell:

while true
do
    reset
    seq 1 6 | shuf -n1
    seq 1 6 | shuf -n1
    read
done

This rolls two six-sided dice every time you hit enter and clears the screen before showing the result using reset.

The problem#

I used to have an occasionally unreliable internet connection. I wanted logs of exactly how unreliable it was and an easy way to have notice when it was back up.

The solution#

Use cron to check online status once a minute and write the result to a file. An easy way to check is to confirm that google.com will reply to a ping (this does give a false negative in the unlikely event that Google is down).

To run a script every minute, put a file in /etc/cron.d containing the line

* * * * * root /root/bin/online-check

where /root/bin/online-check is the following script:

#!/bin/sh

# Check if computer is online by attempting to ping google.com.
PING_RESULT="`ping -c 2 google.com 2>/dev/null`"
if [ $? -eq 0 ] && ! echo "$PING_RESULT" | grep -F '64 bytes from 192.168.' >/dev/null 2>/dev/null
then
    ONLINE="online"
else
    ONLINE="offline"
fi
echo "`date '+%Y-%m-%d %T%z'` $ONLINE" >> /var/log/online.log

The details and pretty printing#