friendly interactive shell

fish

The friendly interactive shell
Original author(s) Axel Liljencrantz
Developer(s) ridiculousfish, siteshwar, JanKanis[1]
Initial release 13 February 2005
Stable release 2.2.0 / 12 July 2015 (2015-07-12)[2]
Operating system Unix-like
Type Unix shell
License GPL v2[3]
Website fishshell.com

The friendly interactive shell (fish) is a Unix shell that attempts to be more interactive and user-friendly than former shells. 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.[4] fish is considered an "exotic shell", in that its syntax derives from neither the Bourne shell (ksh, bash, zsh) nor the C shell (csh, tcsh). Also unlike previous shells, which do not have certain features enabled by default as a means of saving system resources, fish includes all features enabled by default.

Highlights

fish has "search as you type" automatic suggestions based on history and current directory. This is essentially like bash's Ctrl+R history search, but because it is always on instead of being a separate mode, the user gets continuous feedback while writing the command line, and can select suggestions with the arrow keys, or as in bash, press Tab ↹ for a tab completion instead. Tab-completion is featureful, expanding file paths (with wildcards and brace expansion), variables, and many command specific completions. Command specific completions, including options with descriptions, can to some extent be generated from the commands' manpages.

Fish has few syntactic rules, preferring 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.[5]

Syntax

The syntax resembles a POSIX compatible shell (such as bash), but deviates in important ways where the creators believe the POSIX shell was badly designed.[6]

# 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 many other shells, since the 'read' builtin
# will run in its own subshell.  in bash, the right side of the pipe can't have any side effects.  In ksh, the below command works, but the left side can't have any side effects. In fish and zsh, both sides can have side effects.
> 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.[7]

> 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 user's 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

Version 2 adds:

bash/fish Translation table

Feature bash syntax fish syntax Comment
variable expansion 
"$var" 
$var
Variables do not explode if unquoted
history completion Ctrl+R implicit
history substitution 
sudo !!
deliberately omitted Not discoverable
subshell 
(expression)
deliberately omitted Deemed useless
command substitution 
"$(expression)" 
(expression)
process substitution 
<(expression)
 
(expression | psub)
 command, not syntax
variable assignment 
var=value 
set var value
export variable 
export var 
set -x var
Options discoverable via tab completion
declare local variable 
local var 
set -l var
remove variable 
unset var 
set -e var
array initialization 
var=( a b c ) 
set var a b c
Every variable is an array
array iteration 
for i in "${var[@]}"; do echo "$i"; done

for i in $var; echo $i; end
argument vector
(all arguments) 
"$@" 
$argv
argument vector
(indexing) 
"$1" 
$argv[1]
argument vector
(count) 
$#
 
(count $argv)
argument vector
(shift) 
shift

set argv $argv[2..-1]
array representation in environment variables 
PATH="$PATH:$HOME/.local/bin"

set PATH $PATH $HOME/.local/bin
fish assumes colon as array delimiter for translating variables to and from the environment. This aligns with many common variables, like $PATH and $LS_COLORS.
export and run 
LANG=C.UTF-8 python3 
 begin
   set -xl LANG C.UTF-8
   python3
 end
env LANG=C.UTF-8 python3 works in any shell, as env is a separate program.
logical operators 
[[ ! $var = "" ]] && echo nonempty || echo empty

not test "$var" = ""
and echo nonempty
or echo empty
In bash, [[ is special syntax, not a command, that allows for not quoting the variable. In fish, the bare unquoted variable is an array of zero or more arguments, but in quotes serializes to exactly one argument, as expected by the test command.
escape sequence interpretation 
$'\e'

\e
Fish interprets unquoted escape sequences. The printf command also interprets escape sequences, which in both fish and bash is a builtin that is compatible with GNU printf; printf '\e' produces the same result in both shells.[8]

See also

References

  1. ↑ "fish shell team members". GitHub.com. Retrieved 2013-05-21.
  2. ↑ fishshell.com Release Notes for fish 2.2.0
  3. ↑ fishshell.com License for fish
  4. ↑ Linux Weekly News. Fish - A user-friendly shell. Retrieved 2010-03-24.
  5. ↑ Linux.com. CLI Magic: Enhancing the shell with fish. Retrieved 2010-03-24.
  6. ↑ Paul, Ryan. "An in-depth look at fish: the friendly interactive shell". Ars Technica. Retrieved 10 March 2015. the Posix syntax has several missing or badly implemented features, including variable scoping, arrays, and functions. For this reason, fish strays from the Posix syntax in several important places.
  7. ↑ Handle With Linux. Afraid of the command line? Try fish. Archived from the original on 2012-07-19.
  8. ↑ "printf does not support \e". fish issues. 11 Jul 2013. Retrieved 24 March 2016.

External links

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