The problem#

When writing a blog post like last time's, I often will be at least partway through the process before I realize it's interesting enough to write a post on. Then I need to somehow go back and reconstruct a narrative of the troubleshooting steps I performed.

The solution#

As long as I still have the terminals or screen sessions open, I can at least capture a snapshot of the scrollback history that's still available.

In Xfce Terminal (and likely similar in other terminals), I can save the history by going into the "Edit" menu and selecting "Select All" and then "Copy" (plain text) or "Copy as HTML" (to also capture styles) and pasting into a file or writing the clipboard to a file using xclip:

xclip -o -selection clipboard > some_file.html

In screen, the hardcopy command can be used to dump the full history to a file. Type Ctrl+a, : to get into command mode and run the command hardcopy -h to include the entire scrollback buffer. Unfortunately, this is plain text only.

If you use tmux instead of screen, the equivalent is the capture-pane command which does support saving styles if you provide the -e option. Note this encodes the styles so cating the file to a terminal will look right, not in HTML as Xfce Terminal does; you could convert it to HTML using aha.

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#

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.