feat: kb article for fish

knowledge base/fish.md

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+# fish
+
+The friendly interactive shell.
+
+1. [TL;DR](#tldr)
+1. [Configuration](#configuration)
+1. [Further readings](#further-readings)
+1. [Sources](#sources)
+
+## TL;DR
+
+```sh
+# Install the shell.
+apt install 'fish'
+brew install 'fish'
+zypper install 'fish'
+
+# Start `fish` sessions.
+fish
+fish -Pil
+
+# Change one's default shell to `fish`.
+chsh -s (which fish)      # from `fish`
+chsh -s "$(which fish)"   # from `{ba,z}sh`
+
+# Open the web-based configuration interface.
+fish_config
+fish_config browse
+```
+
+## Configuration
+
+Shell configuration file: `~/.config/fish/config.fish`.<br/>
+`.fish` scripts in `~/.config/fish/conf.d/` are automatically executed **before** `config.fish`.
+
+Configuration files are read at startup of every session, whether the shell is interactive and/or login.<br/>
+Use `status --is-interactive` and `status --is-login` to discriminate between interactive/login shells, respectively:
+
+```sh
+if status --is-login
+  fish_add_path ~/bin
+end
+```
+
+`fish` offers a web-based configuration interface. Open it executing `fish_config`.
+
+## Further readings
+
+- [Website]
+- [`bash`][bash]
+- [`zsh`][zsh]
+
+## Sources
+
+All the references in the [further readings] section, plus the following:
+
+- [Documentation]
+- [Github]
+
+<!--
+  References
+  -->
+
+<!-- In-article sections -->
+[further readings]: #further-readings
+
+<!-- Knowledge base -->
+[bash]: bash.md
+[zsh]: zsh.md
+
+<!-- Files -->
+<!-- Upstream -->
+[documentation]: https://fishshell.com/docs/current/
+[github]: https://github.com/fish-shell/fish-shell
+[website]: https://fishshell.com/
+
+<!-- Others -->