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.
In addition, it only splits them on newlines instead of $IFS. If you want to split on something else, use :ref:`string split <cmd-string-split>`, :ref:`string split0 <cmd-string-split>` or :ref:`string collect <cmd-string-collect>`. If those are used as the last command in a command substitution the splits they create are carried over. So::
Fish sets and erases variables with :ref:`set <cmd-set>` instead of ``VAR=VAL`` and a variety of separate builtins like ``declare`` and ``unset`` and ``export``. ``set`` takes options to determine the scope and exportedness of a variable::
Fish does not perform word splitting. Once a variable has been set to a value, that value stays as it is, so double-quoting variable expansions isn't the necessity it is in bash. [#]_
All variables are "arrays" (we use the term "lists"), and expanding a variable expands to all its elements, with each element as its own argument (like bash's ``"${var[@]}"``::
Fish only supports the ``*`` and ``**`` glob (and the deprecated ``?`` glob) as syntax. If a glob doesn't match it fails the command (like with bash's ``failglob``) unless the command is ``for``, ``set`` or ``count`` or the glob is used with an environment override (``VAR=* command``), in which case it expands to nothing (like with bash's ``nullglob`` option).
Fish does not have ``${foo%bar}``, ``${foo#bar}`` and ``${foo/bar/baz}``. Instead string manipulation is done by the :ref:`string <cmd-string>` builtin.
1. Read/interpret the string, with special rules, up to the terminator. [#]_
2. Write the resulting string to a temporary file.
3. Start the command the heredoc is attached to with that file as stdin.
This means it is essentially the same as just reading from a pipe, so::
echo "foo" | cat
is mostly the same as
..code-block:: sh
cat <<EOF
foo
EOF
Just like with heredocs, the command has to be prepared to read from stdin. Sometimes this requires special options to be used, often giving a filename of ``-`` turns it on.
For example::
echo "xterm
rxvt-unicode" | pacman --remove -
# is the same as (the `-` makes pacman read arguments from stdin)
So heredocs really are just minor syntactical sugar that introduces a lot of special rules, which is why fish doesn't have them. Pipes are a core concept, and are simpler and compose nicer.
..[#] For example, the "EOF" is just a convention, the terminator can be an arbitrary string, something like "THISISTHEEND" also works. And using ``<<-`` trims leading *tab* characters (but not other whitespace), so you can indent the lines, but only with tabs. Substitutions (variables, commands) are done on the heredoc by default, but not if the terminator is quoted: ``cat << "EOF"``.
Fish has a POSIX-compatible ``test`` or ``[`` builtin. There is no ``[[`` and ``test`` does not accept ``==`` as a synonym for ``=``. It can compare floating point numbers, however.
``set -q`` can be used to determine if a variable exists or has a certain number of elements (``set -q foo[2]``).
You can pass arguments to ``math`` separately like above or in quotes. Because fish uses ``()`` parentheses for :ref:`command substitutions <bash-command-substitutions>`, quoting is needed if you want to use them in your expression::
> math '(5 + 2) * 4'
Both ``*`` and ``x`` are valid ways to spell multiplication, but ``*`` needs to be quoted because it looks like a :ref:`glob <bash-globs>`.
Fish does not use the ``$PS1``, ``$PS2`` and so on variables. Instead the prompt is the output of the :ref:`fish_prompt <cmd-fish_prompt>` function, plus the :ref:`fish_mode_prompt <cmd-fish_mode_prompt>` function if vi-mode is enabled and the :ref:`fish_right_prompt <cmd-fish_right_prompt>` function for the right prompt.
As an example, here's a relatively simple bash prompt:
..code-block:: sh
# <$HOSTNAME> <$PWD in blue> <Prompt Sign in Yellow> <Rest in default light white>
- Fish provides :ref:`set_color <cmd-set_color>` to color text. It can use the 16 named colors and also RGB sequences (so you could also use ``set_color 5555FF``)
- Instead of introducing specific escapes like ``\h`` for the hostname, the prompt is simply a function. To achieve the effect of ``\h``, fish provides helper functions like :ref:`prompt_hostname <cmd-prompt_hostname>`, which prints a shortened version of the hostname.
- Fish offers other helper functions for adding things to the prompt, like :ref:`fish_vcs_prompt <cmd-fish_vcs_prompt>` for adding a display for common version control systems (git, mercurial, svn), and :ref:`prompt_pwd <cmd-prompt_pwd>` for showing a shortened ``$PWD`` (the user's home directory becomes ``~`` and any path component is shortened).
Bash has a feature called "subshells", where it will start another shell process for certain things. That shell will then be independent and e.g. any changes it makes to variables won't be visible in the main shell.
``()`` 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::
Subshells are also frequently confused with :ref:`command substitutions <bash-command-substitutions>`, which bash writes as ```command``` or ``$(command)`` and fish writes as ``$(command)`` or ``(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.
By now it has become apparent that fish puts much more of a focus on its builtins and external commands rather than its syntax. So here are some helpful builtins and their rough equivalent in bash:
-:ref:`string <cmd-string>` - this replaces most of the string transformation (``${i%foo}`` et al) and can also be used instead of ``grep`` and ``sed`` and such.
-:ref:`math <cmd-math>` - this replaces ``$((i + 1))`` arithmetic and can also do floats and some simple functions (sine and friends).
-:ref:`argparse <cmd-argparse>` - this can handle a script's option parsing, for which bash would probably use ``getopt`` (zsh provides ``zparseopts``).
-:ref:`count <cmd-count>` can be used to count things and therefore replaces ``$#`` and can be used instead of ``wc``.
-:ref:`status <cmd-status>` provides information about the shell status, e.g. if it's interactive or what the current linenumber is. This replaces ``$-`` and ``$BASH_LINENO`` and other variables.
