The problem#

Last time, I presented a script read_keypress.sh which would read any single keypress in a shell script. For printable characters, it's straightforward what this means, but for keys like left arrow (🠈), it's not obvious how to deal with them.

The solution#

keytest.sh demonstrates using read_keypress.sh to respond to keypresses including 🠈 and Ctrl+c:

#!/bin/bash

esc=$(printf '\e')
ctrlc=$(printf '\x03')

while true
do
  key="$(./read_keypress.sh)"
  case $key in
    q|"$ctrlc")
      echo "Quitting..."
      exit
      ;;
    "${esc}[D")
      echo "Moved left."
      ;;
    *)
      echo "Unexpected key: $key ($(echo -n "$key" | xxd))"
      ;;
  esac
done

The details#

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#

The problem#

If you take the word wizard, reverse the order of the letters and reverse the alphabet:

From: abcdefghijklmnopqrstuvwxyz
To:   ZYXWVUTSRQPONMLKJIHGFEDCBA

then you get the word wizard back, an observation made at least as early as 1972.

Now let's write a shell script to verify this so we can find other words with similar interesting properties. The obvious shell script to verify this

echo wizard | tr a-z z-a | rev

unfortunately fails with the error

tr: range-endpoints of 'z-a' are in reverse collating sequence order

The error is by design: it's not clear what a sequence in reverse order should mean, so POSIX actually requires that it not work.