Fish (Unix shell)

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Fish

Version 3.3.1 of fish
Original author(s)	Axel Liljencrantz
Developer(s)	Fish-shell developers[1]
Initial release	13 February 2005; 19 years ago (2005-02-13)
Stable release	3.7.1[2]  / 19 March 2024; 17 December 2024
Repository	github.com/fish-shell/fish-shell
Written in	Rust[3]
Operating system	Unix-like
Type	Unix shell
License	GPL-2.0-only[4]
Website	fishshell.com

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.^[5] Fish is considered an exotic shell since it does not adhere to POSIX shell standards, at the discretion of its maintainers.^[6]

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.^[7]

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.^[8]

The syntax resembles a POSIX compatible shell (such as Bash), but deviates in many ways^[9]

# 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

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

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

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

• 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).

$var

${var[@]}

${var[*]}

"$var"

"${var[@]}"

$var

"${var[*]}"

"$var"

(expression)

fish -c expression

"$(expression)" 

<(expression)

(expression | psub)

!cmd && echo FAIL || echo OK

not command
and echo FAIL
or echo OK

var=value 

set var value

"${HOME/alice/bob}"

string replace alice bob $HOME

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 var 

set --export var 

local var

set --local var

unset var 

set --erase var 

test -v var

set --query var

var=( a b c ) 

set var a b c

for i in "${var[@]}"; do
  echo "$i"
done

for i in $var
  echo $i
end

"$@" 

$argv 

"$1" 

$argv[1] 

$#

(count $argv)

shift

set --erase argv[1]

PATH="$PATH:$HOME/.local/bin"

set PATH $PATH $HOME/.local/bin

LANG=C.UTF-8 python3 

env LANG=C.UTF-8 python3

$((10/3))

math '10/3'

$'\e'

\e 

'mom'\''s final backslash: \'

'mom\'s final backslash: \\'

• Comparison of command shells
• Unix
• Unix shell

• 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