sudo gem install wp2txt
When I executed, an error occurred when installing something called nokogiri.
As a result of a lot of research and various trials, when I executed the following command posted on the official website, it was solved safely. It was a long battle ...
sudo apt-get install build-essential patch ruby-dev zlib1g-dev liblzma-dev
gem install nokogiri
https://nokogiri.org/tutorials/installing_nokogiri.html