Fish

Fish is an interactive shell with autosuggestions, syntax highlighting and sensible defaults out of the box, but its scripting syntax is not POSIX-compatible. Use it as a friendly login shell and keep portable scripts in Bash. Pick between shells with Shell Comparison.

Documentation: Fish Documentation Project: Fish shell

Checked against fish 4.2.1. Examples were run unless marked (unverified).

Not Bash #

BASH	FISH
`NAME=value`	`set NAME value`
`export NAME=value`	`set -x NAME value`
`unset NAME`	`set -e NAME`
`$(cmd)`	`(cmd)` or `$(cmd)`
`cmd1 && cmd2`	`cmd1; and cmd2` (`&&` also works in current fish)
`cmd1 \|\| cmd2`	`cmd1; or cmd2` (`\|\|` also works in current fish)
`if [[ -f f ]]; then ... fi`	`if test -f f; ...; end`
`for i in 1 2; do ... done`	`for i in 1 2; ...; end`
`$?`	`$status`
`${PIPESTATUS[@]}`	`$pipestatus`
`"$@"`	`$argv`
`{ cmd; }`	`begin; cmd; end`
`alias ll='ls -l'`	`alias ll 'ls -l'` (creates a function)
Unquoted `$var` is split on whitespace	Variables are never word-split; every variable is a list
`~/.bashrc`	`~/.config/fish/config.fish`

Fish has no $((...)) arithmetic and no <<EOF here-docs; use math and string (below) for arithmetic and string handling.

Variables #

Every fish variable is a list, indexed from 1.

COMMAND	DESCRIPTION
`set a x y z`	Set a list
`set -l NAME value`	Local to the current block or function
`set -g NAME value`	Global to this session
`set -U NAME value`	Universal: shared by all sessions and kept across restarts
`set -x NAME value`	Exported to child processes (`set -gx` for global and exported)
`set -e NAME`	Erase
`set -q NAME`	Test whether set (exit status only)
`set -S NAME`	Show scope, export state and elements
`$a[1]`, `$a[-1]`	First and last element
`$a[2..3]`	Range
`count $a`	Number of elements
`set a $a w`	Append
`contains y $a`	Exit status 0 if `y` is in the list; add `-i` to print its index
`echo {a,b}c`	Brace expansion: `ac bc`

PATH #

COMMAND	DESCRIPTION
`fish_add_path /usr/local/bin`	Add to `PATH` persistently (stored in `fish_user_paths`, a universal variable); ignores directories that do not exist
`echo $PATH`	Prints one element per entry, not colon-separated

Functions #

fish
function hi --description "say hi"
    echo hi $argv
end

hi there          # hi there
functions hi      # show the definition
funcsave hi       # save it to ~/.config/fish/functions/hi.fish

COMMAND	DESCRIPTION
`functions`	List functions
`functions -e NAME`	Erase a function
`funcsave NAME`	Write a function to its own autoloaded file
`funced NAME`	Edit a function in `$EDITOR`
`type NAME`	What `NAME` is (also shows alias definitions)
`command -q NAME`	Exit status 0 if the command exists (use with `and`)

Functions in ~/.config/fish/functions/NAME.fish are autoloaded on first use, so there is no need to source them from config.fish.

Abbreviations #

An abbreviation expands in place as you type, so history shows the full command.

COMMAND	DESCRIPTION
`abbr -a gco "git checkout"`	Add an abbreviation
`abbr --show`	List them as `abbr` commands
`abbr -e gco`	Erase one

Control Flow #

fish
if test -f /etc/passwd
    echo file
else if test -d /etc/passwd
    echo directory
end

for i in (seq 3)
    echo $i
end

while true
    break
end

switch $argv[1]
    case "f*"
        echo starts with f
    case "*"
        echo other
end

test supports the same operators as POSIX test (-f, -d, -z, -n, -eq, =). not, and and or are commands that combine exit statuses.

string and math #

string replaces most uses of sed, cut and tr on single values.

COMMAND	DESCRIPTION
`string upper hello`, `string lower ABC`	Change case
`string replace -a l L hello`	Replace all matches (`heLLo`)
`string match "*.txt" a.txt`	Glob match (prints the match)
`string match -r "h(.)l" hello`	Regex match, prints the match then each capture group
`string split , a,b,c`	Split into a list, one item per line
`string join - a b c`	Join arguments with a separator (`a-b-c`)
`string length hello`	Length in characters
`string sub -s 2 -l 2 hello`	Substring from position 2, length 2 (`el`)
`string trim " x "`	Strip surrounding whitespace
`string pad -w 6 ab`	Pad to width 6 (right-aligned by default)
`string repeat -n 2 ab`	Repeat (`abab`)
`math 2 + 3 \* 4`	Arithmetic (escape `*` or quote the expression)
`math -s0 7 / 2`	Integer result (`3`); `-s` sets the number of decimal places
`math "sqrt 16"`	Functions available, e.g. `sqrt`
`read -l x`	Read a line into a local variable (`echo a \| read -l x`)

Redirection and Pipes #

COMMAND	DESCRIPTION
`cmd > file`, `cmd >> file`	Stdout to file, append
`cmd 2> file`	Stderr to file
`cmd &> file`	Stdout and stderr to a file
`cmd 2>&1 \| other`	Pipe both streams
`cmd \| tee file`	Show and save output
`diff (sort a \| psub) (sort b \| psub)`	Process substitution equivalent (`psub`)

Interactive Features #

KEY / COMMAND	DESCRIPTION
`→` or `Ctrl+F`	Accept the autosuggestion
`Alt+→`	Accept one word of the suggestion `(unverified)`
`Ctrl+R`	History search
`Alt+S`	Prepend `sudo` to the current command (or the previous one if the line is empty) `(unverified)`
`fish_config`	Browser-based theme and prompt picker
`history`	Show history
`history search TEXT`	Search history
`history delete --contains TEXT`	Delete matching entries
`fish_key_reader`	Show the key sequence a key press sends
`set -U fish_greeting`	Empty the welcome message (set it to nothing)
`status is-interactive`	Exit status 0 in an interactive session (guard `config.fish` with it)
`fish_vi_key_bindings`, `fish_default_key_bindings`	Switch to vi or back to the default bindings

Configuration #

fish
# ~/.config/fish/config.fish
if status is-interactive
    abbr -a gco "git checkout"
    fish_add_path ~/.local/bin
end

Cross-shell tools such as Starship Prompt support Fish: starship init fish | source. Other initialisers follow the same pattern, e.g. zoxide init fish | source (see Modern CLI Tools). Both init fish forms are from the tools’ documentation and were not run here (unverified).