friendly interactive shell

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Fish shell logo ascii.png
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; 6 months ago (2015-07-12)[2]
Operating system Unix-like
Type Unix shell
License GPL v2[3]

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.


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 exent 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]


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
# 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
# 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]

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[edit]

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

> 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[edit]

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[edit]

Version 2 adds:

  • Autosuggestions
  • 256 terminal colors
  • Web-based configuration
  • Improved performance (by having more builtins).

bash/fish Translation table[edit]

Feature bash syntax fish syntax Comment
variable expansion
Variables do not explode if unquoted
history completion Ctrl+R implicit
history substitution
sudo !! 
deliberately omitted Not discoverable
deliberately omitted Deemed useless
command substitution
process substitution
(expression | psub)
command, not syntax
variable assignment
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)
argument vector
argument vector
(count $argv)
argument vector
set argv $argv[2..-1]
array representation in environment variables
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 
   set -xl LANG C.UTF-8
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
"$(printf \\e)"
Fish interprets unquoted escape sequences. Bash can use its printf builtin to achieve the same, but requires escaping (or quoting) the backslash when writing the actual escape sequence to be interpreted by printf.

See also[edit]


  1. ^ "fish shell team members". Retrieved 2013-05-21. 
  2. ^ Release Notes for fish 2.2.0
  3. ^ License for fish
  4. ^ Linux Weekly News. Fish - A user-friendly shell. Retrieved 2010-03-24.
  5. ^ 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.

External links[edit]