Fish (Unix shell)

Fish (stylized in lowercase) is a Unix shell with a focus on interactivity and usability. Fish is designed to be feature-rich, rather than highly configurable. Fish is considered an exotic shell since it does not adhere to POSIX shell standards, at the discretion of its maintainers.

Highlights
Fish has search as you type automatic suggestions based on history and current directory. This functions like Bash's history search, but is always on, giving the user continuous feedback while writing the command line. Users can select suggestions with the arrow keys, or as in Bash, press for a tab completion instead. Tab-completion is feature-rich and has expanding file paths (with wildcards and brace expansion), variables, and many command specific completions. Command-specific completions, including options with descriptions, which can to some extent be generated from the commands' man pages.

The creator of fish preferred to make features as commands rather than syntax. This makes features discoverable in terms of commands with options and help texts. Functions can also carry a human readable description. A special help command gives access to all the fish documentation in the user's web browser.

Syntax
The syntax resembles a POSIX compatible shell (such as Bash), but deviates in many ways

No implicit subshell
Some language constructs, like pipelines, functions and loops, have been implemented using so called subshells in other shell languages. Subshells are child programs that run a few commands for the shell and then exit. This implementation detail typically has the side effect that any state changes made in the subshell, such as variable assignments, do not propagate to the main shell. Fish never forks off so-called subshells; all builtins are always fully functional.

Variable assignment example
This Bash example doesn't do what it seems: because the loop body is a subshell, the update to  is not persistent.

Workaround:

Fish example:

Universal variables
Fish has a feature known as universal variables, which allows a user to permanently assign a value to a variable across all the user's running fish shells. The variable value is remembered across logouts and reboots, and updates are immediately propagated to all running shells.

Other features

 * Advanced tab completion.
 * Syntax highlighting with extensive error checking.
 * Support for the X clipboard.
 * Smart terminal handling based on terminfo.
 * Searchable command history.
 * Web-based configuration.