Bash: return all the characters between the nth occurence of two different strings within a string

Question

In a bash script (on Ubuntu 14.04) I'm running the command:

WP055="$(wget -qO - http://alerts.weather.gov/cap/wwaatmget.php?x=CAZ055&y=1)"

Within the WP055 variable string there will be an unknown number of '<title>' and '</title>' pairs. I need to search within each of these pairs for the string 'by NWS' which means this particular string contains the start and end time of the particular weather advisory. This found string (all the characters between the opening and closing title tags) is what I'm looking to capture into another variable so that I can drop it into an index.html file the script is building.

I was planning on looping through the WP055 variable x number of times analyzing the text within each pair of tags until I find the correct one.

I can't search WP055 for 'by NWS' because there may be more than one occurrence within WP055 (multiple advisories within the WP055 string).

(The above wget command will definitely have a 'by NWS' string within the 2nd title pair until March 07 at 3:00AM PST, when the current wind advisory will be cancelled.)

Answer 1

A little unpolished, but it seems to work:

WP055="$(wget -qO - http://alerts.weather.gov/cap/wwaatmget.php?x=CAZ055&y=1)"
remainder=${WP055#*<title>}
if [ "$WP055" = "$remainder" ]
then
        echo "No title found"
        exit
fi
while true
do
        this_title=${remainder%%</title>*}
        if [ "$remainder" = "$this_title" ]
        then
                echo "</title> not found"
                exit
        fi
        if [[ "$this_title" == *"by NWS"* ]]
        then
                echo "$this_title contains \"by NWS\""
                # You probably want to do something here, like return.
        fi
        new_remainder=${remainder#*<title>}
        if [ "$new_remainder" = "$remainder" ]
        then
                echo "No more titles"
                exit
        fi
        remainder=$new_remainder
done

remainder=${WP055#*<title>} is a form of parameter expansion that removes a matching prefix pattern.  Here, it sets remainder to

• The first title in the string (excluding the introductory <title>),
• the trailing </title>, and
• all the rest of the string after that (including all the subsequent titles).

If "$WP055" = "$remainder", that means that the shell didn't find <title> in the string.

this_title=${remainder%%</title>*} similarly sets this_title to be $remainder up to but not including the first </title>.

if [[ something1 == something2 ]], with the double brackets ([[ … ]]) and double equal sign (==), does a pattern match.  Everything else is repetition.

This might behave oddly on malformed input; i.e., text where <title> and </title> do not occur in alternating pairs.