• Find the path of fish using which fish

  dilawar@halwa ~> which fish
  /usr/local/bin/fish
  

• Append this path to file /etc/shells

• Execute chsh -s /usr/local/bin/fish

References

• https://stackoverflow.com/a/26321141
• https://gist.github.com/gagarine/cf3f65f9be6aa0e105b184376f765262