friendly interactive shell

fish

File:Fishlogo.png
File:Friendlyinteractiveshell.png The friendly interactive shell
Original author(s)	Axel Liljencrantz
Developer(s)	Axel Liljencrantz
Initial release	2005
Stable release	1.23.1 / 8 March 2009
Operating system	Unix-like
Type	Unix shell
License	GNU General Public License
Website	fishshell.org

The friendly interactive shell (fish) is a Unix shell which focuses on interactive use, discoverability, and user friendliness. The design goal of fish is to give the user a rich set of powerful features in a way that is easy to discover, remember, and use.

Released in 2005 under the terms of the GNU General Public License, fish is free software.

Highlights

fish features a user-friendly and powerful tab-completion, including descriptions of every completion, tab-completion of strings with wildcards, and many command specific completions. It also features an extensive and discoverable help system. A special help command gives access to all the fish documentation in the user's web browser.

Syntax

The fish syntax is slightly different from other shell script languages. These changes were made to make the language more powerful as well as to make the language small and easy to learn. One obvious difference between fish and other command-line interpreters like bash is that the contents of a variable is not subject to token separation, meaning that there is rarely a need to enclose variable dereferences within quotes.

# Variable assignment, set the variable 'foo' to the 
# value 'bar'.  Fish doesn't use the = operator, since 
# it is inherently whitespace sensitive.  Also, the set 
# command easily extends to work with arrays, scoping, etc.
> set foo bar
> echo $foo
bar
 
# Command substitution, assign the output of the command 
# 'pwd' into the variable 'wd'.  Fish doesn't use `` 
# since they can't be nested and look too much like ' '. 
# Don't use $() since $ is only used for variable 
# expansion in fish.
> set wd (pwd)
> echo $wd
~

# Array variables. 'A' becomes an array with 5 values:
> set A 3 5 7 9 12
# Array slicing. 'B' becomes the first two elements of 'A':
> set B $A[1 2]
> echo $B
3 5
# You can index with other arrays and even command 
# substitution output:
> echo $A[(seq 3)]
3 5 7
# Erase the third and fifth elements of 'A'
> set -e A[$B]
> echo $A
3 5 9

# for-loop, convert jpegs to pngs
> for i in *.jpg
      convert $i (basename $i .jpg).png
  end
# Semicolons work like newlines:
> for i in *.jpg; convert $i (basename $i .jpg).png; end
# but the multi-line form is comfortable to use because 
# fish supports multi-line history and editing.

# while-loop, read lines /etc/passwd and output the fifth 
# colon-separated field from the file. This should be
# the user description.
> cat /etc/passwd | while read line
      set arr (echo $line|tr : \n)
      echo $arr[5]
  end

One important difference between fish and other shells is the lack of subshells. Many tasks like pipelines, functions and loops are implemented using so called subshells in other languages. Subshells are simply child programs that run a few commands for the shell and then exit. Unfortunately, changes made inside a subshell do not have any effect in the main shell, meaning that actions such as variable assignments and the use of many builtin functions do not work as expected. Fish never forks off so-called subshells; all builtins are always fully functional.

# This will not work in most other shells, since the 'read' builtin
# will run in its own subshell. fish and zsh work as expected.
> cat *.txt | read line

Helpful error messages

Error messages in fish are designed to actually tell the user what went wrong and what can be done about it.

> foo=bar
fish: Unknown command “foo=bar”. Did you mean “set VARIABLE VALUE”? 
For information on setting variable values, see the help section on 
the set command by typing “help set”.

> echo ${foo}bar
fish: Did you mean {$VARIABLE}? The '$' character begins a variable 
name. A bracket, which directly followed a '$', is not allowed as a 
part of a variable name, and variable names may not be zero characters 
long. To learn more about variable expansion in fish, type “help 
expand-variable”.

> echo $(pwd)
fish: Did you mean (COMMAND)? In fish, the '$' character is only used 
for accessing variables. To learn more about command substitution in 
fish, type “help expand-command-substitution”.

Universal variables

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

# This will make emacs the default text editor. The '-U' tells fish to 
# make this a universal variable.
> set -U EDITOR emacs

# This command will make the current working directory part of the fish 
# prompt turn blue on all running fish instances. 
> set -U fish_color_cwd blue

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.

See also

• Comparison of command shells

External links

• Project home page
• fish on SourceForge.net
• An introductory article about fish
• Fish: the friendly interactive shell (an in-depth look at fish) at Ars Technica