Fish (Unix shell)

Fish (or friendly interactive shell- stylized in lowercase) is a Unix-like shell with a focus on interactivity and usability. Fish is designed to be feature-rich by default, rather than highly configurable. Fish is considered an exotic shell since it does not adhere to POSIX shell standards, at the discretion of its maintainers.

Fish

Version 3.3.1 of fish
Original author(s)	Axel Liljencrantz
Developer(s)	Fish-shell developers
Initial release	13 February 2005 (2005-02-13)
Stable release	3.7.1  / 19 March 2024 (19 March 2024)
Repository	github.com/fish-shell/fish-shell
Written in	Rust
Operating system	Unix-like
Type	Unix shell
License	GPL-2.0-only
Website	fishshell.com

Features

Fish displays incremental suggestions as the user types, based on command history and the current directory. This functions similarly to Bash's Ctrl+R history search, but is always on, giving the user continuous feedback while typing commands. Fish also includes feature-rich tab completion, with support for expanding file paths (with wildcards and brace expansion), environment variables, and command-specific completions. Command-specific completions, including options with descriptions, can be to some extent generated from the commands' man pages, but custom completions can also be included with software or written by users of the shell.

The creator of Fish preferred to add new features as commands rather than syntax. This made features more discoverable, as the built-in features allow searching commands with options and help texts. Functions can also include human readable descriptions. A special help command gives access to all the fish documentation in the user's web browser.

Syntax

The syntax resembles a POSIX compatible shell (such as Bash), but deviates in many ways

# Variable assignment
#
# Set the variable 'foo' to the value 'bar'. 
# Fish doesn't use the = operator, which is inherently whitespace sensitive. 
# The 'set' command 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 backticks (``), which can't be nested and may be confused with single quotes (' '). 

> set wd (pwd)
> set wd $(pwd) # since version 3.4
> 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 $A[(seq 3)]
3 5 7
# Erase the third and fifth elements of 'A'
> set --erase A[$B]
> echo $A
3 5 9

# for-loop, convert jpegs to pngs
> for i in *.jpg
      convert $i (basename $i .jpg).png
  end

# fish supports multi-line history and editing.
# Semicolons work like newlines:
> for i in *.jpg; convert $i (basename $i .jpg).png; end



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

# String replacement (replacing all i by I)
> string replace -a "i" "I" "Wikipedia"
WIkIpedIa

No implicit subshell

Some language constructs, like pipelines, functions and loops, have been implemented using so called subshells in other shell languages. Subshells are child programs that run a few commands in order to perform a task, then exit back to the parent shell. This implementation detail typically has the side effect that any state changes made in the subshell, such as variable assignments, do not propagate to the main shell. Fish never creates subshells for language features; all builtins happen within the parent shell.

# 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

Variable assignment example

This Bash example doesn't do what it seems: because the loop body is a subshell, the update to $found is not persistent.

found=''
cat /etc/fstab | while read dev mnt rest; do
  if test "$mnt" = "/"; then
    found="$dev"
  fi
done

Workaround:

found=''
while read dev mnt rest; do
  if test "$mnt" = "/"; then
    found="$dev"
  fi
done < /etc/fstab

Fish example:

set found ''
cat /etc/fstab | while read dev mnt rest
  if test "$mnt" = "/"
    set found $dev
  end
end

Universal variables

Fish has a feature known as universal variables, which allows 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 '--universal' (or '-U') tells fish to
# make this a universal variable.
> set --universal EDITOR emacs

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

Other features

• Advanced tab completion (with support for writing custom completions).
• Syntax highlighting with extensive error checking.
• Support for the X clipboard.
• Smart terminal handling based on terminfo.
• Searchable command history.
• Web-based configuration (fish_config).

Bash/fish translation table

Feature	Bash syntax	fish syntax	Comment
variable expansion: with word splitting and glob interpretation	$var or ${var[@]} or ${var[*]}	deliberately omitted	Identified as a primary cause of bugs in posix compatible shell languages
variable expansion: scalar	"$var"	deliberately omitted	Every variable is an array
variable expansion: array	"${var[@]}"	$var	Quoting not necessary to suppress word splitting and glob interpretation. Instead, quoting signifies serialization.
variable expansion: as a space separated string	"${var[*]}"	"$var"
edit line in text editor	Ctrl+X,Ctrl+E	Alt+E	Upon invocation, moves line input to a text editor
evaluate line input	Ctrl+Alt+E	—	Evaluates expressions in-place on the line editor
history completion	Ctrl+R	implicit
history substitution	!!	deliberately omitted	Not discoverable
explicit subshell	(expression)	fish -c expression
command substitution	"$(expression)"	"$(expression)" or (expression | string collect)
process substitution	<(expression)	(expression | psub)	Command, not syntax
logical operators	!cmd && echo FAIL || echo OK	not command and echo FAIL or echo OK
variable assignment	var=value	set var value
string processing: replace	"${HOME/alice/bob}"	string replace alice bob $HOME
string processing: remove prefix or suffix pattern, non-greedily or greedily	var=a.b.c "${var#*.}" #b.c "${var##*.}" #c "${var%.*}" #a.b "${var%%.*}" #a	string replace --regex '.*?\.(.*)' '$1' a.b.c #b.c string replace --regex '.*\.(.*)' '$1' a.b.c #c string replace --regex '(.*)\..*' '$1' a.b.c #a.b string replace --regex '(.*?)\..*' '$1' a.b.c #a
export variable	export var	set --export var	Options discoverable via tab completion
function-local variable	local var	by default
scope-local variable	no equivalent	set --local var
remove variable	unset var	set --erase var
check if a variable exists	test -v var	set --query var
array initialization	var=( a b c )	set var a b c	Every variable is an array
array iteration	for i in "${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: length	$#	(count $argv)
argument vector: shift	shift	set --erase argv[1]
array representation in environment variables	PATH="$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 array-like environment variables, like $PATH and $LS_COLORS.
export and run	LANG=C.UTF-8 python3	env LANG=C.UTF-8 python3	env LANG=C.UTF-8 python3 works in any shell, as env is a standalone program.
arithmetic	$((10/3))	math '10/3'	expr 10 / 3 works in any shell, as expr is a standalone program.
escape sequence	$'\e'	\e	printf '\e' works in both shells; their printf builtins are both compatible with the GNU printf standalone program.
single quoted string: escape sequences	'mom'\''s final backslash: \'	'mom\'s final backslash: \\'	Bash only requires replacement of the single quote itself in single quoted strings, but the replacement is 4 characters long. The same replacement works in fish, but fish supports a regular escape sequence for this, thus requires escaping backslashes too (except permits single backslashes that don't precede another backslash or single quote).

See also

• Comparison of command shells
• Unix
• Unix shell

External links

• Official website – containing documentation and downloads
• fish on GitHub (active)
• fish on Gitorious (obsolete)
• fish on SourceForge (obsolete)
• Fish-users – general discussion list for fish users
• Shell Translation Dictionary - another Bash/Fish translation table