friendly interactive shell

From Wikipedia, the free encyclopedia
Jump to: navigation, search
The friendly interactive shell
Original author(s) Axel Liljencrantz
Developer(s) ridiculousfish, siteshwar, JanKanis[1]
Initial release 13 February 2005
Stable release 2.1.1 / 26 September 2014; 4 months ago (2014-09-26)[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" (incremental search) history based completion, which is discoverable and practical, while not in the way for users used to tab completion. Independently of this, there is powerful tab-completion, of file paths (with wildcards), variables, and many command specific completions. Command specific completions, including options with descriptions, can to some exent be generated from the commands' manpages. It has an extensive and discoverable help system, with facilities for having help texts on functions, and its "everything is a command" approach to syntax means that what is often magic syntax in other shells, tends to be commands with their own help text in fish. A special help command gives access to all the fish documentation in the user's web browser.[5]

In contrast with other shells, the modern features of fish are enabled by default.


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.[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 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[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 Forget Ctrl+R Search as you type
history substitution
sudo !! 
deliberately omitted Not discoverable
deliberately omitted Deemed useless
command substitution
process substitution
(expression | psub)
Everything is a command, including psub
variable assignment
set var value 
Everything is a command, including set
export variable
export var 
set -x var 
Parameters to set are discoverable
declare local variable
local var 
set -l var 
Parameters to set are discoverable
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
Arrays are not a syntactic afterthought
argument vector
(all arguments)
Like any array
argument vector
(number of arguments)
(count $argv) 
Like any array
argument vector
Like any array
command path
export PATH="$PATH:$HOME/.local/bin"
set -x PATH $PATH $HOME/.local/bin
$PATH is emulated as an array (but remains a colon separated string to programs).
export and run
LANG=C.UTF-8 python3 
  set -xl LANG C.UTF-8
env LANG=C.UTF-8 python3 works in both shells, as env is a separate program.
logical operators
[[ ! $var = "" ]] && echo nonempty || echo empty
not test "$var" = ""
and echo nonempty
or echo empty
[[ is magical syntax in bash, not a command. In fish, the variable in the test must be quoted, in case it is the empty string.
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 2.1.1
  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. ^ Ars Technica. An in-depth look at fish, the friendly interactive shell. Retrieved 2010-03-24.
  7. ^ Handle With Linux. Afraid of the command line? Try fish. Archived from the original on 2012-07-19.

External links[edit]