Internet Radio to Podcast with Shell Tools
It may seem strange that I'm calling a scripting language from another scripting language. The point is that I'm using each to do the things it's best at. Bash is designed to execute commands, and it's really easy to start a background process, find out its process ID and kill it again. On the other hand, trying to add an XML entry in Bash using the more basic string-handling tools, such as sed and grep, would have been, well, exactly the kind of thing that drove Larry Wall to write Perl in the first place.
Now that we have a script, we make the file executable and run it:
chmod +x catchthewolf ./catchthewolf
which results in a properly tagged MP3 file and a new entry in the wolfrss.xml RSS feed. When testing, you can uncomment the 30-second test line to make sure everything's working properly, but be sure to comment it back out before trying to catch a show.
Now all that's left is to get our computer to run this thing at 5AM on Saturday. That's done by using the system's cron utility—invoke crontab -e— and adding an entry like this:
MAILTO=phil # Testing: mail script output to me # Catch hour of the wolf 5AM Saturdays 59 4 * * sat /home/phil/catchthewolf
crontab's editor is most likely to be set to vi-style commands, so you have to use i to start typing and <Esc>:wq to save-and-exit. When you're done, you should see this message:
crontab: installing new crontab
which says you're all set. Check man 5 crontab for more information on how to make jobs repeat every day, once a month or whatever. You also want to make sure your user name is in the file /etc/cron.allow—the list of who can run jobs on the system's scheduler. If you're running on a remote system, verify with the administrators that you're allowed to run cron jobs.
To see the resulting podcast, point your RSS-aware software at the XML file the script creates. In Firefox, use Bookmarks→Manage Bookmarks→Add Live Bookmark, and remember to enter the URL starting with file:// and not the filename itself.
By taking two programs already on the hard drive, downloading two Perl modules and writing a few lines of shell script, we have assembled a homebrew Webcast recording system that saves our favorite programs for us to listen to whenever we choose. It also lets us know what it has done by popping up live bookmarks in Firefox and automatically transfers the recordings to our MP3 player. Some scripts for capturing other Internet radio shows will be available on the Linux Journal FTP site (see the on-line Resources). Now I just have to remember to delete the older files before my hard drive fills up with leftover Webcasts.
Thanks to Anne Troop, Jen Hamilton and Chris Riley for their many shell-scripting hints over the years; to Anne's friend Janeen Pisciotta for finding “Hour of the Wolf” for us in the first place; and to LJ Editor in Chief Don Marti for the cool podcast idea.
When streaming radio first came out, it often was transmitted in proprietary data formats, making it tough for Linux users to listen. Now most streams are MP3, but there still may be something in a different format that you want to capture, such as BBC Radio's RealPlayer streams—see the on-line Resources for a link. Assuming that it's something MPlayer can handle, we simply can rearrange our process a bit. Tell MPlayer to write audio data to the disk in the form of a WAV file and then encode it using lame for MP3 or oggenc for ogg files. Be aware, though, that lame is not included with Fedora, again due to patent issues.
The audio capture commands then would look like:
# Use mplayer to capture the stream # at $STREAM to the file $FILE /usr/local/bin/mplayer -really-quiet -cache 500 \ -ao pcm:file="$FILE.wav" -playlist $STREAM & # the & turns the capture into a background job sleep $DURATION # wait for the show to be over kill $! # kill the stream capture # Encode to .ogg, quality 2, and tag the file oggenc -q 2 -t $TITLE -a $AUTHOR -l $ALBUM \ -n "1/1" -G "Radio" -R 16000 -o $FILE $FILE.wav rm $FILE.wav # Remove the raw audio data file
followed by the original call to the Perl script. No need to use id3v2 here, as both the lame and oggenc encoders insert tags as part of the encoding process. We wind up with the same result as capturing an MP3 stream directly. But because of the intermediate WAV file's large size, we need much more disk space during the actual capture process. The optional -R 16000 specifies the sample rate of the captured WAV file—this is needed only if MPlayer does not correctly detect the speed of the incoming audio stream and your captured MP3 sounds like whale song or chipmunks. You probably want to comment out the rm command until you're sure the encoding is working the way you want it to and remove the WAV files manually until then.
Webinar: 8 Signs You’re Beyond Cron
11am CDT, April 29th