First i needed to get ruby from homebrew

Using gem install bundler jekyll didn’t work for me because I was getting a permission error,

ERROR:  While executing gem ... (Gem::FilePermissionError)
	You don't have write permissions for the /Library/Ruby/Gems/2.3.0 directory.

I did brew install ruby and brew install rbenv ruby-build

Also saw this message

(venv3) $ brew install ruby
==> Installing dependencies for ruby: libyaml, openssl@1.1 and readline
==> Installing ruby dependency: libyaml
==> Downloading https://homebrew.bintray.com/bottles/libyaml-0.2.2.mojave.bottle.tar.gz
######################################################################## 100.0%
==> Pouring libyaml-0.2.2.mojave.bottle.tar.gz
🍺  /usr/local/cellar/libyaml/0.2.2: 9 files, 291.6kb
==> installing ruby dependency: openssl@1.1
==> downloading https://homebrew.bintray.com/bottles/openssl@1.1-1.1.1d.mojave.bottle.tar.gz
==> downloading from https://akamai.bintray.com/10/104ef018b7bb8fcc49f57e5a60359a28a02d480d85a959e6141394b0571cbb28?__gda__=exp=1571
######################################################################## 100.0%
==> pouring openssl@1.1-1.1.1d.mojave.bottle.tar.gz
==> caveats
a ca file has been bootstrapped using certificates from the system
keychain. to add additional certificates, place .pem files in
  /usr/local/etc/openssl@1.1/certs

and run
  /usr/local/opt/openssl@1.1/bin/c_rehash

openssl@1.1 is keg-only, which means it was not symlinked into /usr/local,
because openssl/libressl is provided by macos so don't link an incompatible version.

if you need to have openssl@1.1 first in your path run:
  echo 'export path="/usr/local/opt/openssl@1.1/bin:$path"' >> ~/.bash_profile

for compilers to find openssl@1.1 you may need to set:
  export ldflags="-l/usr/local/opt/openssl@1.1/lib"
  export cppflags="-i/usr/local/opt/openssl@1.1/include"

for pkg-config to find openssl@1.1 you may need to set:
  export pkg_config_path="/usr/local/opt/openssl@1.1/lib/pkgconfig"

==> summary
🍺  /usr/local/cellar/openssl@1.1/1.1.1d: 7,983 files, 17.9mb
==> installing ruby dependency: readline
==> downloading https://homebrew.bintray.com/bottles/readline-8.0.1.mojave.bottle.tar.gz
==> downloading from https://akamai.bintray.com/3c/3c754391e9d243835811d128771ca0f1a565024100fd2c2871534353d46aaf0e?__gda__=exp=1571
######################################################################## 100.0%
==> pouring readline-8.0.1.mojave.bottle.tar.gz
==> caveats
readline is keg-only, which means it was not symlinked into /usr/local,
because macos provides the bsd libedit library, which shadows libreadline.
in order to prevent conflicts when programs look for libreadline we are
defaulting this gnu readline installation to keg-only.

for compilers to find readline you may need to set:
  export ldflags="-l/usr/local/opt/readline/lib"
  export cppflags="-i/usr/local/opt/readline/include"

for pkg-config to find readline you may need to set:
  export pkg_config_path="/usr/local/opt/readline/lib/pkgconfig"

==> summary
🍺  /usr/local/cellar/readline/8.0.1: 48 files, 1.5mb
==> installing ruby
==> downloading https://homebrew.bintray.com/bottles/ruby-2.6.5.mojave.bottle.tar.gz
==> downloading from https://akamai.bintray.com/e3/e30782df0575e8df1bc3c23da701644919099cc31f6b05b163cde847d116414c?__gda__=exp=1571
######################################################################## 100.0%
==> pouring ruby-2.6.5.mojave.bottle.tar.gz
==> caveats
by default, binaries installed by gem will be placed into:
  /usr/local/lib/ruby/gems/2.6.0/bin

you may want to add this to your path.

ruby is keg-only, which means it was not symlinked into /usr/local,
because macos already provides this software and installing another version in
parallel can cause all kinds of trouble.

if you need to have ruby first in your path run:
  echo 'export path="/usr/local/opt/ruby/bin:$path"' >> ~/.bash_profile

for compilers to find ruby you may need to set:
  export ldflags="-l/usr/local/opt/ruby/lib"
  export cppflags="-i/usr/local/opt/ruby/include"

for pkg-config to find ruby you may need to set:
  export pkg_config_path="/usr/local/opt/ruby/lib/pkgconfig"

==> summary
🍺  /usr/local/cellar/ruby/2.6.5: 19,390 files, 31.1mb

And rbenv

(venv3) $ brew install rbenv ruby-build
==> Installing dependencies for rbenv: ruby-build
==> Installing rbenv dependency: ruby-build
==> Downloading https://github.com/rbenv/ruby-build/archive/v20191004.tar.gz
==> Downloading from https://codeload.github.com/rbenv/ruby-build/tar.gz/v20191004
######################################################################## 100.0%
==> ./install.sh
🍺  /usr/local/Cellar/ruby-build/20191004: 459 files, 229.8KB, built in 7 seconds
==> Installing rbenv
==> Downloading https://homebrew.bintray.com/bottles/rbenv-1.1.2.mojave.bottle.tar.gz
######################################################################## 100.0%
==> Pouring rbenv-1.1.2.mojave.bottle.tar.gz
🍺  /usr/local/Cellar/rbenv/1.1.2: 36 files, 65KB
(venv3) $ which ruby
/usr/bin/ruby
(venv3) $ .  /Users/blahmichal/.bash_profile

I did some of those things

I put the new brew /usr/local into my .bash_profile updating my PATH ..

And I retried the jekyll instllation…

$ which ruby
/usr/local/opt/ruby/bin/ruby
$ 
$ which gem
/usr/local/opt/ruby/bin/gem
$ 
$ gem install bundler jekyll
Fetching bundler-2.0.2.gem
bundler's executable "bundle" conflicts with /usr/local/lib/ruby/gems/2.6.0/bin/bundle
Overwrite the executable? [yN]  y
bundler's executable "bundler" conflicts with /usr/local/lib/ruby/gems/2.6.0/bin/bundler
Overwrite the executable? [yN]  y
Successfully installed bundler-2.0.2
Parsing documentation for bundler-2.0.2
Installing ri documentation for bundler-2.0.2
Done installing documentation for bundler after 4 seconds
Fetching http_parser.rb-0.6.0.gem
Fetching em-websocket-0.5.1.gem
Fetching public_suffix-4.0.1.gem
Fetching addressable-2.7.0.gem
Fetching colorator-1.1.0.gem
Fetching eventmachine-1.2.7.gem
Fetching concurrent-ruby-1.1.5.gem
Fetching i18n-1.7.0.gem
Fetching ffi-1.11.1.gem
Fetching sassc-2.2.1.gem
Fetching jekyll-sass-converter-2.0.1.gem
Fetching rb-fsevent-0.10.3.gem
Fetching rb-inotify-0.10.0.gem
Fetching listen-3.2.0.gem
Fetching jekyll-watch-2.2.1.gem
Fetching kramdown-2.1.0.gem
Fetching kramdown-parser-gfm-1.1.0.gem
Fetching liquid-4.0.3.gem
Fetching mercenary-0.3.6.gem
Fetching forwardable-extended-2.6.0.gem
Fetching pathutil-0.16.2.gem
Fetching rouge-3.12.0.gem
Fetching safe_yaml-1.0.5.gem
Fetching unicode-display_width-1.6.0.gem
Fetching jekyll-4.0.0.gem
Fetching terminal-table-1.8.0.gem
Successfully installed public_suffix-4.0.1
Successfully installed addressable-2.7.0
Successfully installed colorator-1.1.0
Building native extensions. This could take a while...
Successfully installed http_parser.rb-0.6.0
Building native extensions. This could take a while...
Successfully installed eventmachine-1.2.7
Successfully installed em-websocket-0.5.1
Successfully installed concurrent-ruby-1.1.5

HEADS UP! i18n 1.1 changed fallbacks to exclude default locale.
But that may break your application.

Please check your Rails app for 'config.i18n.fallbacks = true'.
If you're using I18n (>= 1.1.0) and Rails (< 5.2.2), this should be
'config.i18n.fallbacks = [I18n.default_locale]'.
If not, fallbacks will be broken in your app by I18n 1.1.x.

For more info see:
https://github.com/svenfuchs/i18n/releases/tag/v1.1.0

Successfully installed i18n-1.7.0
Building native extensions. This could take a while...
Successfully installed ffi-1.11.1
Building native extensions. This could take a while...
Successfully installed sassc-2.2.1
Successfully installed jekyll-sass-converter-2.0.1
Successfully installed rb-fsevent-0.10.3
Successfully installed rb-inotify-0.10.0
Successfully installed listen-3.2.0
Successfully installed jekyll-watch-2.2.1
Successfully installed kramdown-2.1.0
Successfully installed kramdown-parser-gfm-1.1.0
Successfully installed liquid-4.0.3
Successfully installed mercenary-0.3.6
Successfully installed forwardable-extended-2.6.0
Successfully installed pathutil-0.16.2
Successfully installed rouge-3.12.0
Successfully installed safe_yaml-1.0.5
Successfully installed unicode-display_width-1.6.0
Successfully installed terminal-table-1.8.0
-------------------------------------------------------------------------------------
Jekyll 4.0 comes with some major changes, notably:

  * Our `link` tag now comes with the `relative_url` filter incorporated into it.
	You should no longer prepend `{{ site.baseurl }}` to `{ % link foo.md % }`
	For further details: https://github.com/jekyll/jekyll/pull/6727

  * Our `post_url` tag now comes with the `relative_url` filter incorporated into it.
	You shouldn't prepend `{{ site.baseurl }}` to `{ % post_url 2019-03-27-hello % }`
	For further details: https://github.com/jekyll/jekyll/pull/7589

  * Support for deprecated configuration options has been removed. We will no longer
	output a warning and gracefully assign their values to the newer counterparts
	internally.
-------------------------------------------------------------------------------------
Successfully installed jekyll-4.0.0
Parsing documentation for public_suffix-4.0.1
Installing ri documentation for public_suffix-4.0.1
Parsing documentation for addressable-2.7.0
Installing ri documentation for addressable-2.7.0
Parsing documentation for colorator-1.1.0
Installing ri documentation for colorator-1.1.0
Parsing documentation for http_parser.rb-0.6.0
unknown encoding name "chunked\r\n\r\n25" for ext/ruby_http_parser/vendor/http-parser-java/tools/parse_tests.rb, skipping
Installing ri documentation for http_parser.rb-0.6.0
Parsing documentation for eventmachine-1.2.7
Installing ri documentation for eventmachine-1.2.7
Parsing documentation for em-websocket-0.5.1
Installing ri documentation for em-websocket-0.5.1
Parsing documentation for concurrent-ruby-1.1.5
Installing ri documentation for concurrent-ruby-1.1.5
Parsing documentation for i18n-1.7.0
Installing ri documentation for i18n-1.7.0
Parsing documentation for ffi-1.11.1
Installing ri documentation for ffi-1.11.1
Parsing documentation for sassc-2.2.1
Installing ri documentation for sassc-2.2.1
Parsing documentation for jekyll-sass-converter-2.0.1
Installing ri documentation for jekyll-sass-converter-2.0.1
Parsing documentation for rb-fsevent-0.10.3
Installing ri documentation for rb-fsevent-0.10.3
Parsing documentation for rb-inotify-0.10.0
Installing ri documentation for rb-inotify-0.10.0
Parsing documentation for listen-3.2.0
Installing ri documentation for listen-3.2.0
Parsing documentation for jekyll-watch-2.2.1
Installing ri documentation for jekyll-watch-2.2.1
Parsing documentation for kramdown-2.1.0
Installing ri documentation for kramdown-2.1.0
Parsing documentation for kramdown-parser-gfm-1.1.0
Installing ri documentation for kramdown-parser-gfm-1.1.0
Parsing documentation for liquid-4.0.3
Installing ri documentation for liquid-4.0.3
Parsing documentation for mercenary-0.3.6
Installing ri documentation for mercenary-0.3.6
Parsing documentation for forwardable-extended-2.6.0
Installing ri documentation for forwardable-extended-2.6.0
Parsing documentation for pathutil-0.16.2
Installing ri documentation for pathutil-0.16.2
Parsing documentation for rouge-3.12.0
Installing ri documentation for rouge-3.12.0
Parsing documentation for safe_yaml-1.0.5
Installing ri documentation for safe_yaml-1.0.5
Parsing documentation for unicode-display_width-1.6.0
Installing ri documentation for unicode-display_width-1.6.0
Parsing documentation for terminal-table-1.8.0
Installing ri documentation for terminal-table-1.8.0
Parsing documentation for jekyll-4.0.0
Installing ri documentation for jekyll-4.0.0
Done installing documentation for public_suffix, addressable, colorator, http_parser.rb, eventmachine, em-websocket, concurrent-ruby, i18n, ffi, sassc, jekyll-sass-converter, rb-fsevent, rb-inotify, listen, jekyll-watch, kramdown, kramdown-parser-gfm, liquid, mercenary, forwardable-extended, pathutil, rouge, safe_yaml, unicode-display_width, terminal-table, jekyll after 52 seconds
27 gems installed

So I guess that looks good.

Ran the new command..

$ jekyll new namoopsoo-github
Running bundle install in /blahdir/namoopsoo-github... 
  Bundler: Fetching gem metadata from https://rubygems.org/...........
  Bundler: Fetching gem metadata from https://rubygems.org/.
  Bundler: Resolving dependencies...
  Bundler: Using public_suffix 4.0.1
  Bundler: Using addressable 2.7.0
  Bundler: Using bundler 1.17.2
  Bundler: Using colorator 1.1.0
  Bundler: Using concurrent-ruby 1.1.5
  Bundler: Using eventmachine 1.2.7
  Bundler: Using http_parser.rb 0.6.0
  Bundler: Using em-websocket 0.5.1
  Bundler: Using ffi 1.11.1
  Bundler: Using forwardable-extended 2.6.0
  Bundler: Using i18n 1.7.0
  Bundler: Using sassc 2.2.1
  Bundler: Using jekyll-sass-converter 2.0.1
  Bundler: Using rb-fsevent 0.10.3
  Bundler: Using rb-inotify 0.10.0
  Bundler: Using listen 3.2.0
  Bundler: Using jekyll-watch 2.2.1
  Bundler: Using kramdown 2.1.0
  Bundler: Using kramdown-parser-gfm 1.1.0
  Bundler: Using liquid 4.0.3
  Bundler: Using mercenary 0.3.6
  Bundler: Using pathutil 0.16.2
  Bundler: Using rouge 3.12.0
  Bundler: Using safe_yaml 1.0.5
  Bundler: Using unicode-display_width 1.6.0
  Bundler: Using terminal-table 1.8.0
  Bundler: Using jekyll 4.0.0
  Bundler: Fetching jekyll-feed 0.12.1
  Bundler: Installing jekyll-feed 0.12.1
  Bundler: Fetching jekyll-seo-tag 2.6.1
  Bundler: Installing jekyll-seo-tag 2.6.1
  Bundler: Fetching minima 2.5.1
  Bundler: Installing minima 2.5.1
  Bundler: Bundle complete! 6 Gemfile dependencies, 30 gems now installed.
  Bundler: Use `bundle info [gemname]` to see where a bundled gem is installed.
New jekyll site installed in /blahdir/namoopsoo-github. 
$ ls
namoopsoo-github

But trying to host failed…

$ bundle exec jekyll serve
Traceback (most recent call last):
	2: from /usr/local/opt/ruby/bin/bundle:23:in `<main>'
	1: from /usr/local/Cellar/ruby/2.6.5/lib/ruby/2.6.0/rubygems.rb:303:in `activate_bin_path'
/usr/local/Cellar/ruby/2.6.5/lib/ruby/2.6.0/rubygems.rb:284:in `find_spec_for_exe': Could not find 'bundler' (1.17.2) required by your /blahdir/namoopsoo-github/Gemfile.lock. (Gem::GemNotFoundException)
To update to the latest version installed on your system, run `bundle update --bundler`.
To install the missing version, run `gem install bundler:1.17.2`
$ which bundle
/usr/local/opt/ruby/bin/bundle
$ 

Okay I did what it said..

$ which bundle
/usr/local/opt/ruby/bin/bundle
$ bundle update --bundler
Using public_suffix 4.0.1
Using addressable 2.7.0
Using bundler 2.0.2
Using colorator 1.1.0
Using concurrent-ruby 1.1.5
Using eventmachine 1.2.7
Using http_parser.rb 0.6.0
Using em-websocket 0.5.1
Using ffi 1.11.1
Using forwardable-extended 2.6.0
Using i18n 1.7.0
Using sassc 2.2.1
Using jekyll-sass-converter 2.0.1
Using rb-fsevent 0.10.3
Using rb-inotify 0.10.0
Using listen 3.2.0
Using jekyll-watch 2.2.1
Using kramdown 2.1.0
Using kramdown-parser-gfm 1.1.0
Using liquid 4.0.3
Using mercenary 0.3.6
Using pathutil 0.16.2
Using rouge 3.12.0
Using safe_yaml 1.0.5
Using unicode-display_width 1.6.0
Using terminal-table 1.8.0
Using jekyll 4.0.0
Using jekyll-feed 0.12.1
Using jekyll-seo-tag 2.6.1
Using minima 2.5.1
Bundle updated!
$ gem install bundler:1.17.2
Fetching bundler-1.17.2.gem
Successfully installed bundler-1.17.2
Parsing documentation for bundler-1.17.2
Installing ri documentation for bundler-1.17.2
Done installing documentation for bundler after 3 seconds
1 gem installed
$ 

Try to serve again… and nice this time worked

$ bundle exec jekyll serve
Configuration file: /blahdir/namoopsoo-github/_config.yml
			Source: /blahdir/namoopsoo-github
	   Destination: /blahdir/namoopsoo-github/_site
 Incremental build: disabled. Enable with --incremental
	  Generating... 
	   Jekyll Feed: Generating feed for posts
					done in 0.404 seconds.
 Auto-regeneration: enabled for '/blahdir/namoopsoo-github'
	Server address: http://127.0.0.1:4000/
  Server running... press ctrl-c to stop.