NAME

This is to give you a quick overview if you come from bash (or to a lesser extent other shells like zsh or ksh) and want to know how fish differs. Fish is intentionally not POSIX-compatible and as such some of the things you are used to work differently.

Many things are similar - they both fundamentally expand commandlines to execute commands, have pipes, redirections, variables, globs, use command output in various ways. This document is there to quickly show you the differences.

COMMAND SUBSTITUTIONS

In addition, it only splits them on newlines instead of $IFS. If you want to split on something else, use string split, string split0 or string collect. If those are used as the last command in a command substitution the splits they create are carried over. So:

for i in (find . -print0 | string split0)

will correctly handle all possible filenames.

VARIABLES

# Define $PAGER global and exported, so this is like ``export PAGER=less``
set -gx PAGER less
# Define $alocalvariable only locally - like ``local alocalvariable=foo``
set -l alocalvariable foo

or to erase variables:

set -e PAGER

VAR=VAL statements are available as environment overrides:

PAGER=cat git log

Fish does not perform word splitting. Once a variable has been set to a value, that value stays as it is, so double-quoting variable expansions isn't the necessity it is in bash. [1]

For instance, here's bash

> foo="bar baz"
> printf '"%s"\n' $foo # will print two lines, because we didn't double-quote, so the variable is split
"bar"
"baz"

And here is fish:

> set foo "bar baz"
> printf '"%s"\n' $foo # foo was set as one element, so it will be passed as one element, so this is one line
"bar baz"

All variables are "arrays" (we use the term "lists"), and expanding a variable expands to all its elements, with each element as its own argument (like bash's "${var[@]}":

> set var "foo bar" banana
> printf %s\n $var
foo bar
banana

Specific elements of a list can be selected:

echo $list[5..7]

[1]

zsh also does not perform word splitting by default (the SH_WORD_SPLIT option controls this)

WILDCARDS (GLOBS)

Globbing doesn't happen on expanded variables, so:

set foo "*"
echo $foo

will not match any files.

There are no options to control globbing so it always behaves like that.

QUOTING

There is no $'', instead the sequences that would transform are transformed when unquoted:

> echo a\nb
a
b

STRING MANIPULATION

SPECIAL VARIABLES

PROCESS SUBSTITUTION

Note that both of these are bashisms, and most things can easily be expressed without. E.g. instead of:

source (command | psub)

just use:

command | source

as fish's source can read from stdin.

HEREDOCS

cat <<EOF
some string
some more string
EOF

use:

printf %s\n "some string" "some more string"

or:

echo "some string
some more string"

Quotes are followed across newlines.

TEST (TEST, [, [[)

set -q can be used to determine if a variable exists or has a certain number of elements (set -q foo[2]).

ARITHMETIC EXPANSION

math $i + 1

It can handle floating point numbers:

> math 5 / 2
2.5

PROMPTS

As an example, here's a relatively simple bash prompt:

# <$HOSTNAME> <$PWD in blue> <Prompt Sign in Yellow> <Rest in default light white>
export PS1='\h\[\e[1;34m\]\w\[\e[m\] \[\e[1;32m\]\$\[\e[m\] '

and a rough fish equivalent:

function fish_prompt
    set -l prompt_symbol '$'
    fish_is_root_user; and set prompt_symbol '#'
    echo -s $hostname (set_color blue) (prompt_pwd) \
    (set_color yellow) $prompt_symbol (set_color normal)
end

This shows a few differences:

The default prompt is reasonably full-featured and its code can be read via type fish_prompt.

Fish does not have $PS2 for continuation lines, instead it leaves the lines indented to show that the commandline isn't complete yet.

BLOCKS AND LOOPS

for i in 1 2 3; do
   echo $i
done
# becomes
for i in 1 2 3
   echo $i
end
while true; do
   echo Weeee
done
# becomes
while true
   echo Weeeeeee
end
{
   echo Hello
}
# becomes
begin
   echo Hello
end
if true; then
   echo Yes I am true
else
   echo "How is true not true?"
fi
# becomes
if true
   echo Yes I am true
else
   echo "How is true not true?"
end
foo() {
   echo foo
}
# becomes
function foo
    echo foo
end
# (note that bash specifically allows the word "function" as an extension, but POSIX only specifies the form without, so it's more compatible to just use the form without)

Fish does not have an until. Use while not or while !.

SUBSHELLS

This includes things like:

# A list of commands in `()` parentheses
(foo; bar) | baz
# Both sides of a pipe
foo | while read -r bar; do
    # This variable will not be visible outside of the while loop.
    VAR=VAL
    # This background process will not be, either
    baz &
done

() subshells are often confused with {} grouping, which does not use a subshell. When you just need to group, you can use begin; end in fish:

(foo; bar) | baz
# when it should really have been:
{ foo; bar } | baz
# becomes
begin; foo; bar; end | baz

The pipe will simply be run in the same process, so while read loops can set variables outside:

foo | while read bar
    set -g VAR VAL
    baz &
end
echo $VAR # will print VAL
jobs # will show "baz"

Subshells are also frequently confused with command substitutions, which bash writes as `command` or $(command) and fish writes as (command). Bash also uses subshells to implement them.

The isolation can usually be achieved by just scoping variables (with set -l), but if you really do need to run your code in a new shell environment you can always use fish -c 'your code here' to do so explicitly.

BUILTINS AND OTHER COMMANDS

AUTHOR

COPYRIGHT