Fish shell 2.1

Version 2.1 of the fish shell has been released, complete with a string of new features and fixes sure to hook users.

We first looked at fish all the way back in 2005. In keeping with longstanding tradition for shell-naming, fish is an abbreviation, for "friendly interactive shell." In the early days, fish's friendliness was visible through features like automatic syntax highlighting, feature-rich tab-completion behavior, and context-aware help. In subsequent years, it is clear that fish has had an influence on other shells, as older projects like Bash have added and enhanced their own takes on some of those features.

Naturally, fish itself has continued to evolve over the same time period, too. The project still advertises its friendlier implementations of well-known shell features, however—among them syntax highlighting and tab completion, plus command history, prompts, and even common built-in commands like cd. In addition, there are unique features like a web-based configuration tool (which runs on a local Python-powered HTTP server).

The fish source code is hosted at GitHub, while the project's web site offers binary packages for a wide range of Linux distributions, Windows, and Mac OS X. Version 2.1.0, the latest release, was released on October 28. The release notes page provides an overview of the changes in 2.1 and a handful of previous releases, though not for the entire history of the project.

Less is more

Many shells offer tab-completion for commands and filenames, so that users only need to type enough letters to uniquely identify what follows—and, when there are multiple possibilities, the shell usually writes them to the terminal window to help. Fish takes this concept considerably further. First, its tab-completion feature expands not only commands and filenames, but usernames, environment variables, job IDs, and process names. The filename-completion also supports strings with wildcard characters; so typing "b??t" would correctly bring up "bait" as a tab-completion match.

Second, fish includes several command-specific completions, allowing manpage completion for man and whatis, Makefile target completion for make, mount-point completion for mount, package-name completion for apt-get, rpm, and yum, hostname completion for ssh, and username completion for su. For other programs, fish offers tab-completion for each program's command-line switches and options. Each of the program-specific completions is tailor-made for the specific command; for example, ssh hostname completion draws on the list of hosts in the user's known_hosts file. It is also possible to write custom completion options for any command.

The 2.1 release adds another factor to fish's tab-completion effort: fuzzy matching of filenames and commands. The shell first attempts to find a prefix match (that is, "fo" matches "foo"), but if it does not find one it will look for substring matches in the middle and at the end of filenames, and, as a last resort, it will look for true fuzzy matches. Thus, typing "far" could match "foobar," if there are no better matches.

On the other hand, tab completion requires the user to actually hit the tab key; those who find even that effort to be too strenuous will be relieved that fish also makes a best-guess attempt at predicting what the user will type next, in the form of autosuggestions that appear after the text cursor while the user is typing. To accept an autosuggestion, one only needs to hit the right-arrow key on the keyboard. The upshot is predictive typing not unlike the text-prediction on mobile phones or in web browser location bars, and like those autosuggestions, fish provides suggestions that are weighted to the most-recently-typed input, rather than simple string matching. Furthermore, while tab completion works one token at a time, fish's autosuggestions can match entire commands, and are thus—potentially—faster.

Fish also features a number of other expansions, such as numeric ranges (e.g., [1..9]), sets (e.g., {a,b,d}), and process IDs (using the percent sign %). The PID expansion will match process names and expand them to the appropriate PID, so typing kill %jav will match a running java process, allowing the user to kill it without looking up the PID first. A change in fish 2.1 is that % by itself expands to the last command backgrounded, so fg % will put the last backgrounded process back into the foreground.

Going off script

Fish is scriptable, and in most cases uses familiar syntax, but here too it adds its own twist on the feature. There are some special environment variables, the most notable of which is status, which stores the exit status of the last process. In addition to 0 for success and 1 for an error, fish will attempt to provide more information with some specific values that begin with 12. An status value of 126, for example, means that the last command exited with an error because the filename provided was not executable; 127 means that no matching command or function was found. A process that exits with a signal will set status to 128N, where N is the signal number. In these cases, of course, the value of the status variable is treated like a string, which could trip up shell scripts if not handled properly.

Fish also treats the shell history differently than many of its competitors. First, there is a history variable that is an array of the previous commands; among other possibilities, that makes the history available to scripts. But the built-in history command is different, too. Simply hitting the up arrow will step back through previous commands (as it does in other shells), but typing a few letters and then hitting the up arrow performs a history search for the typed characters.

For example, typing make followed by the up arrow will let the user step back through all the previous matches against make. Holding down the Alt key while using the up and down arrows allows the user to search only for the word beneath the text cursor; that could be useful for narrowing down a search with a lot of matches or particularly long commands. Users can also delete items from the history, and can prevent any command from being recorded in the history (by preceding the command with a space character).

The fish project also prides itself on its extensive built-in help, which certainly could come in handy for those who find all of the differences with Bash and other shells daunting. The documentation is provided as both manpages and HTML, and all of fish's built-in commands support the -h switch.

Looking good

Fish's syntax highlighting is another area where the project attempts to provide more functionality than other shells. Although many standard Unix commands are available with their own color-coding (ls perhaps being the most popular), fish color-codes all commands as the user types. Commands, filenames, autosuggestions, tab-completion matches, quoted strings, parameters, comments, and error messages are all colored differently. Color overload is perhaps an understandable fear, but the color values are user-selectable.

The best way to modify the color values is to use fish's web-based configuration tool. The tool is launched with fish_config, and starts up a web server running on localhost:8000, which it opens using the user's default browser. The configuration page lets users graphically select colors (foreground and background) for all of the syntax options, as well as whether each option is underlined. Most of the syntax options are used in the command line and output, but some (such as the current working directory) are also used in the shell prompt.

The shell prompt itself is also configurable within the web configuration tool; version 2.1 includes 14 presets, some of which are tailored toward specific use cases (there are two Git-specific prompts, for example). A right-side prompt (i.e., a prompt-like informational area that sits in the right-hand side of the command line), which is a feature also found in Zsh, is also supported. The configuration tool also lets the user browse through any functions and aliases defined in the current session, view the history array, and look at the environment variables.

Fish also supports many of the same configuration options expected by shell users, such as setting the terminal window title or setting a greeting message. There are a few differences, of course; for example, the fish equivalent to .bashrc is ~/.config/fish/config.fish, so it may take a few minutes of looking through the documentation to get up to speed. But on the whole, fish does a good job of adding its layer of friendliness onto the standard shell experience without introducing major disruptions to how users work. True, people with a significant standing investment in Bash scripts may not find migrating them to fish (or any other shell) an appealing prospect, but for more casual shell usage, it could be quite a good catch.