A Weird Imagination

Making :w work everywhere

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The problem

After using Vim as my primary editor for a while, I find myself trying to use vi-style keyboard shortcuts in other contexts. Usually resulting in :w in middle of whatever I was writing as saving is a natural thing to do when pausing.

There's a few ways to fix this:

  1. Get used to the fact that not everything supports vi-style keyboard shortcuts.
  2. Change the keyboard shortcuts of the programs I do use.
  3. Use Vim for everything.

The first option is suboptimal because vi-style keyboard shortcuts are very useful. Luckily, in many cases there's ways to get them.

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Which command will be run?

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The problem

While trying to develop a modification to the Pelican source, I was unexpectedly having my installed version of Pelican get run instead of the local version:

$ which pelican
/usr/local/bin/pelican
$ command -v pelican
/usr/bin/pelican

For some reason, which was pointing to the executable I was expecting bash to run, but the Bash builtin command was telling me that bash was running the installed version instead.

The solution

Use the hash builtin to clear bash's cache of the location of pelican:

$ hash -d pelican
$ command -v pelican
/usr/local/bin/pelican

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Reverse sequence for tr

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.

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Better hash-based colors

The problem

Yesterday, I proposed using a hash function to choose colors:

ps1_color="32;38;5;$((0x$(hostname | md5sum | cut -f1 -d' ' | tr -d '\n' | tail -c2)))"

I did not discuss two issues with this approach:

  1. Colors may be too similar to other colors, such that they are not useful.
  2. Some colors may be undesirable altogether, particularly very dark ones may be too similar to a black terminal background color.

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Hash-based hostname colors

Random color selection

In my post about hostname-based prompt colors, I suggested a fallback color scheme that was obviously wrong in order to remind you to set a color for that host:

alice@unknown:~$ 

This carried with it an implicit assumption: you care what color each host is assigned. You may instead be happy to assign a random color to each host. We could use shuf to generate a random color:

ps1_color="32;38;5;$(shuf -i 0-255 -n 1)"

The problem with this solution is the goal of the recoloring the prompt was not simply to make it more colorful, but for that color to have meaning. We want the color to always be the same for each login to a given host.

One way to accomplish this would be to use that code to randomly generate colors, but save the results in a table like the one used before for manually-chosen colors. But it turns out we can do better.

Hash-based color selection

Hash functions have a useful property called determinism, which means that hashing the same value will always get the same result. The consequence is that we can use a hash function like it's a lookup table of random numbers shared among all of our computers:

ps1_color="32;38;5;$(($(hostname | sum | cut -f1 -d' ' | sed s/^0*//) % 256))"

The $((...)) syntax is bash's replacement for expr which is less portable but easier to use. Here we use it to make sure the hash value we compute is a number between 0 and 255. [sum][sum] computes a hash of its input, in this case the result of hostname. Its output is not just a number so cut selects out the number and sed gets rid of any leading zeros so it isn't misinterpreted as octal.

The idea of using sum was suggested by a friend after reading my previous post on the topic.

But this turns out to not work great for hosts with similar names like rob.example.com and orb.example.com:

alice@rob:~$ 
alice@orb:~$ 

Similar colors on hosts with very different names would not be so bad, but because of how sum works, it will tend to give similar results on similar strings (although less often than I expected; it took some effort to find such an example).

Better hash functions

While this is not a security-critical application, here cryptographic hash functions solve the problem. Cryptographic hash functions guarantee (in theory) that knowing that two inputs are similar tells you nothing about their hash values. In other words, the output of cryptographic hash functions are indistinguishable from random and, in fact, they can be used to build pseudorandom generators like Linux's /dev/urandom.

The cryptographic hash function utilities output hex instead of decimal, so they aren't quite a drop-in replacement for sum:

ps1_color="32;38;5;$((0x$(hostname | md5sum | cut -f1 -d' ' | tr -d '\n' | tail -c2)))"

Here we use cut and tr to select just the hex string of the hash. tail's -c option specifies the number of bytes to read from the end, where 2 bytes corresponds to 2 hex digits, which can have a value of 0 to 255, so the modulo operation is not needed. Instead the 0x prefix inside $((...)) interprets the string as a hex number and outputs it as a decimal number.

This code uses the md5sum utility to compute an MD5 hash of the hostname. This is recommended because md5sum is likely to be available on all hosts. Do be aware that MD5 is insecure and it is only okay to use here because coloring the prompt is not a security-critical application.

sha1sum and sha256sum are also likely available on modern systems and work as drop-in replacements for md5sum in the above command should you wish to use a different hash. Additionally, you could also get different values out of the hash by adding a salt:

salt="Some string."
ps1_color="32;38;5;$((0x$( (echo "$salt"; hostname) | sha256sum | cut -f1 -d' ' | tr -d '\n' | tail -c2)))"

Type your SSH passphrase less often

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The problem

SSH public key authenication can make SSH much more convenient because you do not need to type a password for every SSH connect. Instead, in common usage, the first time you connect, GNOME Keyring, KWallet, or your desktop environment's equivalent will pop up and offer to keep your decrypted private key securely in memory. Those programs will remember your key until the next time you reboot your computer (or possibly until you log out completely and log back in).

But those are tied to your desktop environment. If you are not at a GUI, either using a computer in text-mode using a console or connecting over SSH, then you do not have access to those programs.

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256 color hostnames

The problem

When using multiple terminals on different hosts, it can sometimes be confusing to remember which host you are on. The hostname appears in the command prompt, but it's easy to skim past that if you are not paying attention.

One solution that works pretty well for me is recoloring the prompt based on what host I am on. This is in fact why I researched how to get 256 colors terminals working in the first place: in order to have enough colors to be able to make a good choice for each host I use frequently.

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256 color terminals

The problem

By default, terminals on Linux only use 8 colors (or 16 if setup to use bright variants instead of bold text). Everything else on a modern computer uses 24-bit color, allowing for millions of colors. More colors in the terminal would allow for better syntax highlighting and color output of various commands to be more readable.

In practice, while a few terminals support full 24-bit RGB color (at least Konsole does), it is not widespread enough to be used much. On the other hand, most terminals support 256 colors, which is significantly better than just 8.

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Setting up rTorrent

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rTorrent is a text-based BitTorrent client, which makes it convenient to leave running in a screen or tmux session, so you don't have to leave a terminal window open and you can access it remotely over ssh. It also has an API for web frontends if you don't like text.

Basic setup

You can set it up to automatically start and stop downloads based on placing .torrent files into a watch/ directory by putting the following in your ~/.rtorrent.rc:

# Default session directory. Make sure you don't run multiple instance
# of rtorrent using the same session directory. Perhaps using a
# relative path?
session = ./session

# Watch a directory for new torrents, and stop those that have been
# deleted.
schedule = watch_directory,5,5,load_start=./watch/*.torrent

Those settings also use a session directory to keep track of torrents across runs of rTorrent, which is useful if you have a lot of torrents and want to be able to restart rTorrent, say, after rebooting your computer. Note rTorrent will complain if the session directory doesn't already exist, so your first run will look like

$ screen
$ mkdir session watch
$ rtorrent

That configuration uses relative paths for watch/ and session/ so you can have multiple instances of rTorrent in different directories.

magnet: links

In additional to .torrent files, BitTorrent also supports magnet: links as a way to join a torrent without needing a file. There is built-in support for magnet: links in rTorrent, but it requires a little extra work to make clicking one in a web browser start the download in rTorrent. Here's a script for doing so along with instructions for having your web browser use it to handle magnet: links. I modified it to handle multiple watch/ directories:

#!/bin/bash

DEFAULT_WATCH='/path/to/your/watch'
if [[ $# -ge 2 ]]
then
    WATCH="$2"
else
    if [[ -z "$DISPLAY" ]]
    then
        WATCH="$DEFAULT_WATCH"
    else
        WATCH=$(zenity --file-selection --directory --title="Select rtorrent watch directory" --filename="$DEFAULT_WATCH")
        [[ "$(basename "$WATCH")" = watch ]] || exit;
    fi
fi
cd "$WATCH"
[[ $1 =~ xt=urn:btih:([^&/]+) ]] || exit;
echo "d10:magnet-uri${#1}:${1}e" > "meta-${BASH_REMATCH[1]}.torrent"

This script uses bash because it uses the bash-only =~ operator for regular expression matching.

This script has a hard-coded default directory to use, but supports either specifying a different directory as the second argument or will use zenity to show a dialog asking the user to select a watch/ directory. zenity is quite useful for easily adding interactivity to shell scripts, especially for something like a directory chooser which doesn't work as well in text.

Child process not in ps?

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A buggy program

Consider the following (contrived) program1 which starts a background process to create a file and then waits while the background process is still running before checking to see if the file exists:

#!/bin/sh

# Make sure file doesn't exist.
rm -f file

# Create file in a background process.
touch file &
# While there is a touch process running...
while ps -C "touch" > /dev/null
do
    # ... wait one second for it to complete.
    sleep 1
done
# Check if file was created.
if [ -f file ]
then
    echo "Of course it worked."
else
    echo "Huh? File wasn't created."
    # Wait for background tasks to complete.
    wait
    if [ -f file ]
    then
        echo "Now it's there!"
    else
        echo "File never created."
    fi
fi

# Clean up.
rm -f file

Naturally, it will always output "Of course it worked.", right? Run it in a terminal yourself to confirm this. But I claimed this program is buggy; there's more going on.

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