Install jekyll as --user

gem install --user jekyll

add something like

# jekyll (gem install --user jekyll)
PATH="$HOME/.gem/ruby/2.3.0/bin:$PATH"

to ~/.bashrc. To test

which jekyll # should return something like:
/home/user/.gem/ruby/2.3.0/bin/jekyl

When it complains about missing stuff

gem install --user jekyll-sitemap

https://jekyllrb.com/docs/installation/

jekyll

p.s. To get ruby stuff, use something like

sudo apt install jekyll
sudo apt remove jekyll
about / github / jekyll

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