Nice thanks for the report. Will investigate.

@lch88 it does not seem we hardcode the Ruby path anywhere for this to be caused...wonder if it's a High Sierra update issue.

I've just tried to solve it, and It seems that the issue happens when upgrading from Sierra to High Sierra. System ruby version is upgraded but the version imprinted inside pod script doesn't change.
I'm not a ruby developer so not sure how that got there but reinstalling would solve it for now.
but feels like there should be some automatic migrator or guide or sort for users who are upgrading.

but the version imprinted inside pod script doesn't change.

Thats the script code:
https://github.com/CocoaPods/CocoaPods/blob/master/bin/pod

@lch88 do other ruby commands work for you, irrelevant to cocoapods?

@dnkoutso
Other ruby commands works fine. However I didn't try something installed via gem. So this might happens to other CLI tools or libraries installed via gem in Sierra, but I have nothing much to test on my Mac.
Here's the pod script which actually get executed currently.

$ cat `which pod`
#!/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
#
# This file was generated by RubyGems.
#
# The application 'cocoapods' is installed as part of a gem, and
# this file is here to facilitate running it.
#

require 'rubygems'

version = ">= 0.a"

if ARGV.first
  str = ARGV.first
  str = str.dup.force_encoding("BINARY") if str.respond_to? :force_encoding
  if str =~ /\A_(.*)_\z/ and Gem::Version.correct?($1) then
    version = $1
    ARGV.shift
  end
end

load Gem.bin_path('cocoapods', 'pod', version)

The first line was #!/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby before reinstallation. I guess this auto-generated file could be the culprit.

@orta or @segiddins any thoughts on this? I am reaching my limit on Ruby and rubygems knowledge.

Try reinstalling, gem install cocoapods.

@segiddins Yup. Reinstalling solves this and already did. Maybe I wasn't really clear enough up there 😅
It's perfectly okay to close this issue since it's resolved but I was just curious what could be done on CocoaPods side to prevent this sudden break or smooth it out.

@lch88 not sure. I will be closing but keeping an eye out for future reports. Thank you for trying it out on the latest macOS!

Nothing, that's a downside of RubyGems hardcoding in a path rather than using /usr/bin/env ruby

Which, just in case more people find this, is not a CocoaPods issue (and it might get fixed in later betas) - the re-install recommendation is really the only option here.

For re-install instructions check: #6898 (comment)