A Weird Imagination

Read-only filesystem errors

Linux has a tendency to give very unhelpful error messages when it is unable to create a file. I previously blogged about a few different reasons Linux might report a disk is full, but all of the reasons included the disk actually not having space for more files. Yet another reason to get similar errors is if the partition is mounted readonly (ro):

$ mount | grep -F /usr
/dev/sdc2 on /usr type ext4 (ro,nodev,noatime,data=ordered)

mount without any options lists all of the mounted partitions along with their mount options.

Many programs will show a helpful error message:

$ touch test
touch: cannot touch ‘test’: Read-only file system

But some others won't:

rtorrent: Could not lock session directory: "./session/", held by "<error>".

That error is normally caused by ./session/rtorrent.lock not being writable due to being held by another process, but in this case it's not writable due to the filesystem being readonly. rtorrent doesn't distinguish the two.

For that reason, when running into weird behavior from a program on Linux, it's a good idea to check that the directories the program might try to write to are actually writable.

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.