Podcasts are RSS feeds

$ wget http://feeds.libsyn.com/330110/rss

On Monday 2nd, we briefly discussed podcasts as the antithesis of radio. That radio is live whereas podcasts are prerecorded and that modes of engaging with podcasts and radio differ. Podcasts are built upon Really Simple Syndication (RSS). In other words, in terms of code, there's little difference between the XML for an blog feed and the XML for a podcast feed. The following command combines a regular expression with the grep command to retrieve a list of some of the opening XML tags in RSS feed for the Call Me Mother podcast.

$ grep -E "<[[:alpha:]]+" rss

This command prints results such as

<pubDate>Fri, 02 Apr 2021 15:25:34 GMT</pubDate>

<title>Stephen Whittle</title>

and

<link>https://www.novel.audio</link>

These tags also appear in RSS feeds for written blogs. However, the command also prints results such as

<itunes:explicit>yes</itunes:explicit>

and

<acast:showId>62b087ec4f1d1f0014025b79</acast:showId>

Editing

What I want to retrieve is a list of the tags only. At the moment, I'm not interested in what's in between the tags. Ideally I'd like to use built in commands to generate a text which contains an outline of tags along the lines of the following listing.

1 <tag>
2 <subtag>
3 </subtag>
4 </tag>

First Attempt

Can this be achieved by making two files, open.txt and close.txt? The first file should contain all the opening tags and the latter all the closing tags.

$ grep -Eon "<[[:alpha:]]+>" rss > open.txt
$ grep -Eon "</[[:alpha:]]+>" rss > close.txt

It should then be possible to cat both files and sort the result by line number producing the desired outcome.

$ cat open.txt close.txt | sort -n > sorted.txt && cat sorted.txt | less

Second Attempt

Whilst reading through the output of the first attempt (sorted.txt), I discovered that

the </guid> tags had no correlating opening tag
That the closing tags were sorted before the opening tags

Problem 1: Some information was missing

I had already deliberately omitted acast and itunes tags. In doing so I worked on the assumption that there were no other relevant, colon-separated tags in the rss file. Fortunately, retrieving the additional data was a quick fix:

$ grep -Eon "<[[:alpha:]]+>|<[[:alpha:]]+ [^z-A]*>" rss > open.txt

Figuring out a way to sort the file such that the closing tags followed the opening tags was another matter. The general outline was there, but if I could sort out the details this could become a template for a podcast RSS generator. And that could potentially be useful in relation to Worm's sonic archive.

Problem 2: Adding whitespace with `sed`

The following command prepends a new line to each instance of '</' in the rss file.

sed 's/<\\//\n<\\//' rss >rss-out;

Now it's time to write the open.txt and close.txt files.

$ grep -Eon "<[[:alpha:]]+>|<[[:alpha:]]+ [^z-A]*>" rss-out > open.txt
$ grep -Eon "</[[:alpha:]]+>>" rss-out > close.txt

Placing a newline before each closing tag increases the line number on which each closing tag appears. Now it's time to concatenate open.txt and close.txt and view the output.

$ cat open.txt close.txt | sort -n > sorted.txt && cat sorted.txt | less

Third Attempt

On closer inspection of the output of sorted.txt I noticed a further lack of information. Various closing anchor tags had no corresponding opening tag. I therefore had to write a regular expression capable of matching the additional data in these tags. This might make the output of sorted.txt a little less readable, Although there may be a way to tidy up this information. For completeness, it would also make sense to include the acast and itunes tags.

$ grep -Eon "<[[:alpha:]]+>|<[[:alpha:]]+ [^z-A]*>|<[[:alpha:]]+ [[:alpha:]]+([=\":/'.;_ ]|[[:alnum:]])*>|<itunes:[[:alpha:]]+>|<acast:[[:alpha:]]+>" rss-out > open.txt
$ grep -Eon "</[[:alpha:]]+>|</itunes:[[:alpha:]]+>|</acast:[[:alpha:]]+>" rss-out > close.txt

Then, as above

$ cat open.txt close.txt | sort -n > sorted.txt && cat sorted.txt | less

Fourth Attempt

I refined the regular expression. It now matches every opening and closing tag. Also, it's necessary to pass the global option to the sed substitution command. The second sed command prepends whitespace to every opening tag. This improves the structure of the output

sed 's/<\\//\n<\\//g' rss >rss-out
sed "s/<\([[:alpha:]][^>]*>\)/\n<\1/g" rss-out > rss-out-out
grep -Eon "<[[:alpha:]]([^>]*>)" rss-out-out > open.txt
grep -Eon "</([^>]*>)" rss-out-out > close.txt
cat open.txt close.txt | sort -n > sorted.txt
grep -Eo "<([^>]*>)" sorted.txt | less > skeleton.xml
chromium sorted.xml

The webpage complains about unmatched horizontal rule tags. I deleted all of these using the query replace function in Emacs. The webpage subsequently complained about <br> and <br /> tags, so I deleted all of these too. The result is what I wanted. It views well in the browser; I am able to collapse tags for an abbreviated overview of the document.

Fleshing out the skeleton

item tags are the primary constituent parts of the channel tag in skeleton.xml. However, the channel tag also contains several other tags. I have decided to design the podcast generator around this feature. This raises questions about the possible options the command might take. How does the user provide input? There are three aspects to the overall design.

creating a system for generating items
Creating a system for generating channel information
Creating a system for adding items to a document containing channel information.

Wishlist

Create a system for updating channel information.
skeleton –add foo.mp3 bar.xml

A closer look at items

Each Item tag is made up of 15 tags. Many of these belong to the itunes schema.

<itunes:title>

<itunes:duration>

<itunes:explicit>

<itunes:episodeType>

<itunes:season>

<itunes:episode>

<itunes:image href="https://assets.pippa.io/shows/62b087ec4f1d1f0014025b79/show-cover.jpg"/>

<itunes:summary>

Others belong to the acast schema.

<acast:episodeId>

<acast:showId>

<acast:episodeUrl>

<acast:settings>

And then there's the rest.

<title>

<pubDate>

<enclosure url="https://sphinx.acast.com/p/open/s/62b087ec4f1d1f0014025b79/e/62f2226a61f23900137394a3/media.mp3" length="56168488" type="audio/mpeg"/>

<guid isPermaLink="false">

<description>

<link>

I intend only to make a basic podcast generator, so I'm not going to focus on the itunes and acast schemas.

Draft one of script

General Outline

The following shell script defines the function skeleton. It takes a directory as an argument, asks the user for some information and writes an xml file.

#!/usr/bin/fish
function skeleton -d "Generate an RSS channel for a podcast" -a directory
    set -l showid (uuidgen);
    set -l options (fish_opt -s h -l help);
    argparse --name='skeleton' 'h/help' -- $argv;
    mkdir -p /tmp/skeleton/{$showid}/e/;
    set -l channelfile /tmp/skeleton/{$showid}/rss.xml;
    # write the channel data
    echo '<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">' > $channelfile;
    echo '<channel>' >> $channelfile;
    echo '<ttl>60</ttl>' >> $channelfile;
    echo '<generator>skeleton</generator>' >> $channelfile;
    read -l channelname -P "Channel Title: ";
    echo '<title>'$channelname'</title>' >> $channelfile;
    echo '<link>https://hub.xpub.nl/chopchop/worm/</link>' >> $channelfile;
    echo '<atom:link href="hub.xpub.nl/chopchop/worm/'$showid'" ref="self" type="application/rss+xml"/>' >> $channelfile;
    echo '<language>en</language>' >> $channelfile;
    read -l channeldesc -P "Channel Description: ";
    echo '<description><![CDATA['$channeldesc']]></description>' >> $channelfile;
    echo '<image><url>https://hub.xpub.nl/chopchop/worm/'$showid'/image.jpg</url>' >> $channelfile;
    echo '<link>https://hub.xpub.nl/chopchop/worm/</link>' >> $channelfile;
    echo '<title>'$channelname'</title>' >> $channelfile;
    echo '</image>' >> $channelfile;
    # write the item data
    for file in (ls $argv);
        set -l guid (sha256sum {$argv}/{$file} | grep -Eo "[[:alnum:]]{64}");
        mkdir -p /tmp/skeleton/{$showid}/e/{$guid};
        ln -s {$argv}/{$file} /tmp/skeleton/{$showid}/e/{$guid}/{$file};
        set i (math $i + 1);
        set -l itemfile (printf '/tmp/skeleton/%s/e/%s/item-%i' $showid $guid $i);
        echo "<item>" > $itemfile;
        read -l title -P "Item $i Title: ";
        read -l desc -P "Item $i Description: ";
        echo "<title>"$title"</title>" >> $itemfile;
        echo "<pubDate>"(date)"</pubDate>" >> $itemfile;
        echo '<enclosure url="https://hub.xpub.nl/chopchop/worm/'$showid'/e/'$guid'/'$file'" length="'(soxi -D {$argv}/{$file})'" type="'(file -b --mime-type {$argv}/{$file})'"/>' >> $itemfile;
        sed -i 's/\\/\\/[^.]*\(\\/[^\\/]*.mp3\)/\1/' $itemfile;
        echo '<guid isPermaLink="false">'$guid'</guid>' >> $itemfile;
        set -l link (grep -Eo "https://[^.].mp3" $itemfile);
        echo '<link>'$link'</link>' >> $itemfile;
        echo '<description><![CDATA['$desc']]></description>' >> $itemfile;
        echo '</item>' >> $itemfile;
    end
    # concatenate items in reverse order and append to the channel file
    for item in (ls -t /tmp/skeleton/*/e/*/item-*);
        cat $item >> $channelfile;
    end
    # close the rss and channel tags
    echo '</channel>' >> $channelfile;
    echo '</rss>' >> $channelfile;
end

Analysis of Draft One

Lines 1 - 6

#!/usr/bin/fish
function skeleton -d "Generate an RSS channel for a podcast" -a directory
    set -l showid (uuidgen);
    set -l options (fish_opt -s h -l help);
    argparse --name='skeleton' 'h/help' -- $argv;
    mkdir -p /tmp/skeleton/{$showid}/e/;

The function skeleton is given a description and takes a directory as an argument. Next two local variables are set showid and options. The former has the value of evaluating the command uuidgen. The latter allows for the user to pass -h or --help to skeleton which will display help information. I need to figure out how to enable more options than 'help' alone. argparse is a command which defines how the skeleton command takes both options and arguments. The script next calls the command mkdir to create a temporary file structure. The showid variable expands to the value which was set in line 3.

Improvements
```
#!/usr/bin/fish
function skeleton -d "Generate an RSS channel for a podcast" -a channel
    set -l options (fish_opt -s h -l help);
    set options $options (fish_opt --short=g --long=generate --required-val); 
    set options $options (fish_opt --short=a --long=add --required-val --multiple-vals);
    argparse $options -- $argv;
    or return
    if set -ql _flag_help
        echo "skeleton [ -g DIR | -a FILE | -h ] CHANNEL
        -h --help                Display this text
        -g --genenerate DIR      Generate an RSS feed for DIR
        -a --add FILE            Add an item to a channel

        Skeleton is a command line tool for generating and writing RSS feeds for podcasts."
        return 0
    end
    if set _flag_generate and set _flag_add;
        echo 'ERROR: skeleton cannot add and generate simultaneously'
        return 1
    end

    if set -ql _flag_generate
        # echo $_flag_generate
    end
    if set -ql _flag_add
        # echo $_flag_add
    end
end
```
I have adjusted the code such that different flags create different ways of interacting with the command. I have added three flags, -h, -g and -a, in full --help, --generate and --add. These are stored in the options variable. The add flag can be passed to skeleton multiple times. Furthermore, skeleton now takes a channel as an argument. This is more flexible than taking a directory as an argument. I have made use of if statements to make the function do things under certain conditions. The first if statement relates to the help flag. If that flag is passed to skeleton, then text detailing how to use the command is displayed. The second if statement prevents the user from adding items to rss feeds and generating rss feeds simultaneously. --add and --generate become mutually exclusive options. Next, the variable showid is declared. It retains the value of evaluating uuidgen. It will be necessary to implement this variable in such a way that add can make sensible use of it. I plan to use awk to create a file which keeps track of generated channels and their showid. It will then be possible to use grep to retrieve the showid for the channel when adding items to channels. The remaining if statements control what the programme does when the generate and add flags are passed to skeleton. Some lines have been omitted because it makes sense to place them elsewhere following these changes.

Lines 7 - 15

set -l channelfile /tmp/skeleton/{$showid}/rss.xml;
# write the channel data
echo '<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">' > $channelfile;
echo '<channel>' >> $channelfile;
echo '<ttl>60</ttl>' >> $channelfile;
echo '<generator>skeleton</generator>' >> $channelfile;

Previously, the programme took a directory as an argument. Now it takes a channel as an argument. Taking a directory as an argument resembles the generate option. Consequently, much of the code from before can be placed within a newly written if statement. This extends to, with some adjustments, some of the lines which were omitted above.

Improvements
```
set -l showid (sha256sum {$_flag_generate} | grep -Eo "[[:alnum:]]{64}");
mkdir -p /tmp/skeleton/{$showid};
```
Due to the resemblance described above the remainder of the script will be placed inside the third if statement. Some changes can be made to the way in which the value of showid is determined. Rather than assigning a random id with uuidgen the sha256sum of the value of _flag_generate, a directory, is used. The grep command ensures that only the sha256sum is assigned to the value of showid. The full result of calling the command sha256sum on the value of _flag_generator contains extra information that needs to be filtered out. The code goes on to create a temporary directory using the value of the showid variable.
Keeping track of things for later

Somehow, when using the add flag, skeleton will need to retrieve the value of showid for a given channel. This will ensure coherent and consistent output. I'm going to write a file which pairs the showid with the value of flag_generate. The code for the add flag will consult this file when it sets the value of showid.
```
echo $_flag_generate $showid >> /tmp/skeleton/index
```

Further improvements

set -l channelfile /tmp/skeleton/{$showid}/rss.xml;
# write the channel data
echo '<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">' > $channelfile;
echo '<channel>' >> $channelfile;
echo '<ttl>60</ttl>' >> $channelfile;
echo '<generator>skeleton</generator>' >> $channelfile;

A channelfile is then set. Redirecting the output of echo to the value of channelfile creates the file and stores the output there.

read -l channelname -P "Channel Title: ";
echo '<title>'$channelname'</title>' >> $channelfile;
echo '<link>https://hub.xpub.nl/chopchop/worm/</link>' >> $channelfile;

Next, the user is prompted to provide a title for the channel. The input is stored in the variable channelname. echo is used to encase the value of channelname within within xml title tags. This line of code is appended to the channelfile.

Line 16

The following line of code implicitly suggests a location for the overall output of the skeleton command. It currently reads

echo '<atom:link href="hub.xpub.nl/chopchop/worm/'$showid'" ref="self" type="application/rss+xml"/>' >> $channelfile;

This suggests that it will eventually be necessary to create the directory /media/worm/radio/$showid on chopchop. Doing so would intervene in the current structure of radio worm's archive and I am hesitant about doing so for two reasons:

Space
Risk Management

Space What is the output of the skeleton command when it takes a directory as an argument?

/tmp/skeleton/
/tmp/skeleton/$showid/
/tmp/skeleton/$showid/rss.xml
/tmp/skeleton/$showid/e/
/tmp/skeleton/$showid/e/$guid/
/tmp/skeleton/$showid/e/$guid/item-n
/tmp/skeleton/$showid/e/$guid/symbolic-link-to.mp3

The implication of line 16 is that eventually these temporary files are moved to /media/worm/radio/ which corresponds to the public url https://hub.xpub.nl/chopchop/worm/. Adding files and directories to this location will take up space on the harddrive; perhaps more space than there is available.

Risk Management The shell is powerful and there's no undo function. Consequently, caution and care must be exercised when executing shell commands on files. Whereas chopchop has a backup of Radio Worm's sonic archive it would be time consuming and undesirable to have to recopy data from the original hard drive in the event of data loss. Whilst this is unlikely as long as the rm command is not used, things can still go wrong. For this reason, it might be better to situate the final, non-temporary location of the files at a distance from Worm's archive itself. Perhaps in the home folder of a user called 'podcasts', or on a separate hard drive.

Improvements

In light of the above, it's evident that line sixteen is bound up with the final location of the generated files. I'll eventually place the rss.xml file in ~/public_html/podcasts/$showid/. Therefore line 16 can be changed to
```
echo '<atom:link href="hub.xpub.nl/chopchop/river/public_html/podcasts/'$showid'" ref="self" type="application/rss+xml"/>' >> $channelfile;
```

Line 17

I have set the channel language to English. This could be changed or made into a user defined variable. However, I will not make these changes here.

echo '<language>en</language>' >> $channelfile;

lines 18 - 23

The following lines do not need adjusting.

read -l channeldesc -P "Channel Description: ";
echo '<description><![CDATA['$channeldesc']]></description>' >> $channelfile;
echo '<image><url>https://hub.xpub.nl/chopchop/worm/'$showid'/image.jpg</url>' >> $channelfile;
echo '<link>https://hub.xpub.nl/chopchop/worm/</link>' >> $channelfile;
echo '<title>'$channelname'</title>' >> $channelfile;
echo '</image>' >> $channelfile;

Lines 24 - 43

The following for loop acts upon the result of calling ls on the value of argv. Currently, the value of argv is supposed to be a directory full of .mp3 files. The for loop uses the name of each file to write item data in an xml format for each mp3 file in a given directory.

# write the item data
for file in (ls $argv);

Each mp3 file is assigned a guid which is equivalent to the sha256sum of the mp3 file. This was Thijs' suggestion. Piping the result of calling sha256sum on the full path of the mp3 file through a grep command is required to retrieve the sha256sum alone. Notably, $argv is used here to prefix the full path to the value of the mp3 file. I intend to change the value of argv to a channel and consequently the way in which the value of guid is set will have to change.

set -l guid (sha256sum {$argv}/{$file} | grep -Eo "[[:alnum:]]{64}");

In any case, the value of guid is utilised

mkdir -p /tmp/skeleton/{$showid}/e/{$guid};
ln -s {$argv}/{$file} /tmp/skeleton/{$showid}/e/{$guid}/{$file};
set i (math $i + 1);
set -l itemfile (printf '/tmp/skeleton/%s/e/%s/item-%i' $showid $guid $i);
echo "<item>" > $itemfile;
read -l title -P "Item $i Title: ";
read -l desc -P "Item $i Description: ";
echo "<title>"$title"</title>" >> $itemfile;
echo "<pubDate>"(date)"</pubDate>" >> $itemfile;
echo '<enclosure url="https://hub.xpub.nl/chopchop/worm/'$showid'/e/'$guid'/'$file'" length="'(soxi -D {$argv}/{$file})'" type="'(file -b --mime-type {$argv}/{$file})'"/>' >> $itemfile;
sed -i 's/\\/\\/[^.]*\(\\/[^\\/]*.mp3\)/\1/' $itemfile;
echo '<guid isPermaLink="false">'$guid'</guid>' >> $itemfile;
set -l link (grep -Eo "https://[^.].mp3" $itemfile);
echo '<link>'$link'</link>' >> $itemfile;
echo '<description><![CDATA['$desc']]></description>' >> $itemfile;
echo '</item>' >> $itemfile;
end

    # concatenate items in reverse order and append to the channel file
    for item in (ls -t /tmp/skeleton/*/e/*/item-*);
        cat $item >> $channelfile;
    end
    # close the rss and channel tags
    echo '</channel>' >> $channelfile;
    echo '</rss>' >> $channelfile;
end

Draft two of script

#!/usr/bin/fish
function skelegen -d "Generate an RSS channel for a podcast" -a channel
    set -l options (fish_opt -s h -l help);
    set options $options (fish_opt --short=g --long=generate --required-val); 
    set options $options (fish_opt --short=a --long=add --required-val --multiple-vals);
    argparse $options -- $argv;
    or return
    if set -ql _flag_help
        echo "skelegen [ -g DIR | -a FILE | -h ] CHANNEL
        -h --help                Display this text
        -g --genenerate DIR      Generate an RSS feed for DIR
        -a --add FILE            Add an item to a channel

        Skelegen is a command line tool for generating and writing RSS feeds for podcasts."
        return 0
    end
    if set _flag_generate and set _flag_add;
        echo 'ERROR: skelegen cannot add and generate simultaneously'
        return 1
    end

    if set -ql _flag_generate
          set -l showid (sha256sum {$_flag_generate} | grep -Eo "[[:alnum:]]{64}");
          mkdir -p /tmp/skeleton/{$showid};
          echo $_flag_generate $showid >> /tmp/skeleton/index
          set -l channelfile /tmp/skeleton/{$showid}/rss.xml;
          # write the channel data
          echo '<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">' > $channelfile;
          echo '<channel>' >> $channelfile;
          echo '<ttl>60</ttl>' >> $channelfile;
          echo '<generator>skeleton</generator>' >> $channelfile;
          read -l channelname -P "Channel Title: ";
          echo '<title>'$channelname'</title>' >> $channelfile;
          echo '<link>https://hub.xpub.nl/chopchop/worm/</link>' >> $channelfile;
          echo '<atom:link href="hub.xpub.nl/chopchop/river/public_html/podcasts/'$showid'" ref="self" type="application/rss+xml"/>' >> $channelfile;
          echo '<language>en</language>' >> $channelfile;
          read -l channeldesc -P "Channel Description: ";
          echo '<description><![CDATA['$channeldesc']]></description>' >> $channelfile;
          echo '<image><url>https://hub.xpub.nl/chopchop/worm/'$showid'/image.jpg</url>' >> $channelfile;
          echo '<link>https://hub.xpub.nl/chopchop/worm/</link>' >> $channelfile;
          echo '<title>'$channelname'</title>' >> $channelfile;
          echo '</image>' >> $channelfile;
    end
    if set -ql _flag_add
        echo $_flag_add
    end
end

User:Riviera/Podcast rss

Contents

Podcasts are RSS feeds

Editing

First Attempt

Second Attempt

Problem 1: Some information was missing

Problem 2: Adding whitespace with `sed`

Third Attempt

Fourth Attempt

Fleshing out the skeleton

A closer look at items

Draft one of script

General Outline

Analysis of Draft One

Lines 1 - 6

Lines 7 - 15

Line 16

Line 17

lines 18 - 23

Lines 24 - 43

Draft two of script

Podcasts are RSS feeds

Editing

First Attempt

Second Attempt

Problem 1: Some information was missing

Problem 2: Adding whitespace with sed

Third Attempt

Fourth Attempt

Fleshing out the skeleton

A closer look at items

Draft one of script

General Outline

Analysis of Draft One

Lines 1 - 6

Lines 7 - 15

Line 16

Line 17

lines 18 - 23

Lines 24 - 43

Draft two of script

Problem 2: Adding whitespace with `sed`