A shell is a program that helps you operate your computer by starting other programs. fish offers a command-line interface focused on usability and interactive use.
If you are already familiar with other shells like bash and want to see the scripting differences, see :ref:`Fish For Bash Users <fish_for_bash_users>`.
If fish is your default shell and you want to copy commands from the internet that are written in bash (the default shell on most systems), you can proceed in one of the following two ways:
Because shell scripts are written in many different languages, they need to carry information about which interpreter should be used to execute them. For this, they are expected to have a first line, the shebang line, which names the interpreter executable.
For a script written in another language, just replace ``/bin/bash`` with the interpreter for that language (for example: ``/usr/bin/python`` for a python script, or ``/usr/local/bin/fish`` for a fish script).
This line is only needed when scripts are executed without specifying the interpreter. For functions inside fish or when executing a script with ``fish /path/to/script``, a shebang is not required (but it doesn't hurt!).
Shells like fish are used by giving them commands. Every fish command follows the same basic syntax. A command is executed by writing the name of the command followed by any arguments. For example::
This calls the :ref:`echo <cmd-echo>` command. ``echo`` writes its arguments to the screen. In the example above, the output is ``hello world``. Everything in fish is done with commands. There are commands for repeating other commands, commands for assigning variables, commands for treating a group of commands as a single command, etc. All of these commands follow the same basic syntax.
To learn more about the ``echo`` command, read its manual page by typing ``man echo``. ``man`` is a command for displaying a manual page on a given topic. It takes the name of the manual page to display as an argument. There are manual pages for almost every command. There are also manual pages for many other things, such as system libraries and important files.
Every program on your computer can be used as a command in fish. If the program file is located in one of the PATH_ directories, you can just type the name of the program to use it. Otherwise the whole filename, including the directory (like ``/home/me/code/checkers/checkers`` or ``../checkers``) is required.
Commands and arguments are separated by the space character ``' '``. Every command ends with either a newline (by pressing the return key) or a semicolon ``;``. Multiple commands can be written on the same line by separating them with semicolons.
A switch is a very common special type of argument. Switches almost always start with one or more hyphens ``-`` and alter the way a command operates. For example, the ``ls`` command usually lists the names of all files and directories in the current working directory. By using the ``-l`` switch, the behavior of ``ls`` is changed to not only display the filename, but also the size, permissions, owner, and modification time of each file.
Switches differ between commands and are usually documented on a command's manual page. There are some switches, however, that are common to most commands. For example, ``--help`` will usually display a help text, ``--version`` will usually display the command version, and ``-i`` will often turn on interactive prompting before taking action.
-**Builtin**: A command that is implemented by the shell. Builtins are so closely tied to the operation of the shell that it is impossible to implement them as external commands.
-**Function**: A block of commands that can be called as if they were a single command. By using functions, it is possible to string together multiple simple commands into one more advanced command.
Sometimes features like `parameter expansion <#expand>`_ and `character escapes <#escapes>`_ get in the way. When that happens, you can use quotes, either single (``'``) or double (``"``). Between single quotes, fish performs no expansions. Between double quotes, fish only performs :ref:`variable expansion <expand-variable>`. No other kind of expansion (including :ref:`brace expansion <expand-brace>` or parameter expansion) is performed, and escape sequences (for example, ``\n``) are ignored. Within quotes, whitespace is not used to separate arguments, allowing quoted arguments to contain spaces.
The only meaningful escape sequences in single quotes are ``\'``, which escapes a single quote and ``\\``, which escapes the backslash symbol. The only meaningful escapes in double quotes are ``\"``, which escapes a double quote, ``\$``, which escapes a dollar character, ``\`` followed by a newline, which deletes the backslash and the newline, and ``\\``, which escapes the backslash symbol.
Some characters cannot be written directly on the command line. For these characters, so-called escape sequences are provided. These are:
-``\a`` represents the alert character.
-``\e`` represents the escape character.
-``\f`` represents the form feed character.
-``\n`` represents a newline character.
-``\r`` represents the carriage return character.
-``\t`` represents the tab character.
-``\v`` represents the vertical tab character.
-``\xHH``, where ``HH`` is a hexadecimal number, represents the ASCII character with the specified value. For example, ``\x9`` is the tab character.
-``\XHH``, where ``HH`` is a hexadecimal number, represents a byte of data with the specified value. If you are using a multibyte encoding, this can be used to enter invalid strings. Only use this if you know what you are doing.
-``\ooo``, where ``ooo`` is an octal number, represents the ASCII character with the specified value. For example, ``\011`` is the tab character.
-``\uXXXX``, where ``XXXX`` is a hexadecimal number, represents the 16-bit Unicode character with the specified value. For example, ``\u9`` is the tab character.
-``\UXXXXXXXX``, where ``XXXXXXXX`` is a hexadecimal number, represents the 32-bit Unicode character with the specified value. For example, ``\U9`` is the tab character.
-``\cX``, where ``X`` is a letter of the alphabet, represents the control sequence generated by pressing the control key and the specified letter. For example, ``\ci`` is the tab character
Some characters have special meaning to the shell. For example, an apostrophe ``'`` disables expansion (see :ref:`Quotes<quotes>`). To tell the shell to treat these characters literally, escape them with a backslash. For example, the command::
echo \'hello world\'
outputs ``'hello world'`` (including the apostrophes), while the command::
echo 'hello world'
outputs ``hello world`` (without the apostrophes). In the former case the shell treats the apostrophes as literal ``'`` characters, while in the latter case it treats them as special expansion modifiers.
The special characters and their escape sequences are:
-:code:`\ ` (backslash space) escapes the space character. This keeps the shell from splitting arguments on the escaped space.
-``\$`` escapes the dollar character.
-``\\`` escapes the backslash character.
-``\*`` escapes the star character.
-``\?`` escapes the question mark character (this is not necessary if the ``qmark-noglob``:ref:`feature flag<featureflags>` is enabled).
-``\~`` escapes the tilde character.
-``\#`` escapes the hash character.
-``\(`` escapes the left parenthesis character.
-``\)`` escapes the right parenthesis character.
-``\{`` escapes the left curly bracket character.
-``\}`` escapes the right curly bracket character.
The destination of a stream can be changed using something called *redirection*. For example, ``echo hello > output.txt``, redirects the standard output of the ``echo`` command to a text file.
- A filename. The output will be written to the specified file. Often ``>/dev/null`` to silence output by writing it to the special "sinkhole" file.
- An ampersand (``&``) followed by the number of another file descriptor like ``&2`` for standard error. The output will be written to the destination descriptor.
As a convenience, the redirection ``&>`` can be used to direct both stdout and stderr to the same destination. For example, ``echo hello &> all_output.txt`` redirects both stdout and stderr to the file ``all_output.txt``. This is equivalent to ``echo hello > all_output.txt 2>&1``.
..[#] Previous versions of fish also allowed specifying this as ``^DESTINATION``, but that made another character special so it was deprecated and will be removed in the future. See :ref:`feature flags<featureflags>`.
Another way to redirect streams is a *pipe*. A pipe connects streams with each other. Usually the standard output of one command is connected with the standard input of another. This is done by separating commands with the pipe character ``|``. For example::
The command ``cat foo.txt`` sends the contents of ``foo.txt`` to stdout. This output is provided as input for the ``head`` program, which prints the first 10 lines of its input.
When you start a job in fish, fish itself will pause, and give control of the terminal to the program just started. Sometimes, you want to continue using the commandline, and have the job run in the background. To create a background job, append an \& (ampersand) to your command. This will tell fish to run the job in the background. Background jobs are very useful when running programs that have a graphical user interface.
Most programs allow you to suspend the program's execution and return control to fish by pressing :kbd:`Control`\ +\ :kbd:`Z` (also referred to as ``^Z``). Once back at the fish commandline, you can start other programs and do anything you want. If you then want you can go back to the suspended command by using the :ref:`fg <cmd-fg>` (foreground) command.
The first line tells fish to define a function by the name of ``ll``, so it can be used by simply writing ``ll`` on the commandline. The second line tells fish that the command ``ls -l $argv`` should be called when ``ll`` is invoked. :ref:`$argv <variables-argv>` is a :ref:`list variable <variables-lists>`, which always contains all arguments sent to the function. In the example above, these are simply passed on to the ``ls`` command. The ``end`` on the third line ends the definition.
Calling this as ``ll /tmp/`` will end up running ``ls -l /tmp/``, which will list the contents of /tmp.
This is a kind of function known as a :ref:`wrapper <syntax-function-wrappers>` or "alias".
Fish's prompt is also defined in a function, called :ref:`fish_prompt <cmd-fish_prompt>`. It is run when the prompt is about to be displayed and its output forms the prompt::
To edit a function, you can use :ref:`funced <cmd-funced>`, and to save a function :ref:`funcsave <cmd-funcsave>`. This will store it in a function file that fish will :ref:`autoload <syntax-function-autoloading>` when needed.
One of the most common uses for functions is to slightly alter the behavior of an already existing command. For example, one might want to redefine the ``ls`` command to display colors. The switch for turning on colors on GNU systems is ``--color=auto``. An alias, or wrapper, around ``ls`` might look like this::
- Always take care to add the :ref:`$argv <variables-argv>` variable to the list of parameters to the wrapped command. This makes sure that if the user specifies any additional parameters to the function, they are passed on to the underlying command.
- If the alias has the same name as the aliased command, you need to prefix the call to the program with ``command`` to tell fish that the function should not call itself, but rather a command with the same name. If you forget to do so, the function would call itself until the end of time. Usually fish is smart enough to figure this out and will refrain from doing so (which is hopefully in your interest).
- Autoloading isn't applicable to aliases. Since, by definition, the function is created at the time the alias command is executed. You cannot autoload aliases.
When fish needs to load a function, it searches through any directories in the :ref:`list variable <variables-lists>```$fish_function_path`` for a file with a name consisting of the name of the function plus the suffix ``.fish`` and loads the first it finds.
- A directory for end-users to keep their own functions, usually ``~/.config/fish/functions`` (controlled by the ``XDG_CONFIG_HOME`` environment variable).
- A directory for systems administrators to install functions for all users on the system, usually ``/etc/fish/functions`` (really ``$__fish_sysconfdir/functions``).
- Directories for other software to put their own functions. These are in the directories in the ``XDG_DATA_DIRS`` environment variable, in a subdirectory called ``fish/vendor_functions.d``. The default is usually ``/usr/share/fish/vendor_functions.d`` and ``/usr/local/share/fish/vendor_functions.d``.
Autoloading also won't work for `event handlers <#event>`_, since fish cannot know that a function is supposed to be executed when an event occurs when it hasn't yet loaded the function. See the `event handlers <#event>`_ section for more information.
If you are developing another program and want to install fish functions for it, install them to the "vendor" functions directory. As this path varies from system to system, you can use ``pkgconfig`` to discover it with the output of ``pkg-config --variable functionsdir fish``. Your installation system should support a custom path to override the pkgconfig path, as other distributors may need to alter it easily.
Fish has some builtins that let you execute commands only if a specific criterion is met: :ref:`if <cmd-if>`, :ref:`switch <cmd-switch>`, :ref:`and <cmd-and>` and :ref:`or <cmd-or>`, and also the familiar :ref:`&&/|| <tut-combiners>` syntax.
The :ref:`switch <cmd-switch>` command is used to execute one of possibly many blocks of commands depending on the value of a string. See the documentation for :ref:`switch <cmd-switch>` for more information.
Unlike programming languages you might know, :ref:`if <cmd-if>` doesn't take a *condition*, it takes a *command*. If that command returned a successful :ref:`exit status <variables-status>` (that's 0), the ``if`` branch is taken, otherwise the :ref:`else <cmd-else>` branch.
Parameter expansion is limited to 524288 items. There is a limit to how many arguments the operating system allows for any command, and 524288 is far above it. This is a measure to stop the shell from hanging doing useless computation.
-``**`` matches any number of characters (including zero), and also descends into subdirectories. If ``**`` is a segment by itself, that segment may match zero times, for compatibility with other shells.
-``?`` can match any single character except ``/``. This is deprecated and can be disabled via the ``qmark-noglob``:ref:`feature flag<featureflags>`, so ``?`` will just be an ordinary character.
Other shells, such as zsh, have a much richer glob syntax, like ``**(.)`` to only match regular files. Fish does not. Instead of reinventing the whell, use programs like ``find`` to look for files. For example::
Wildcard matches are sorted case insensitively. When sorting matches containing numbers, they are naturally sorted, so that the strings '1' '5' and '12' would be sorted like 1, 5, 12.
-``~/.*`` matches all hidden files (also known as "dotfiles") and directories in your home directory.
For most commands, if any wildcard fails to expand, the command is not executed, :ref:`$status <variables-status>` is set to nonzero, and a warning is printed. This behavior is like what bash does with ``shopt -s failglob``. There are exactly 4 exceptions, namely :ref:`set <cmd-set>`, overriding variables in :ref:`overrides <variables-override>`, :ref:`count <cmd-count>` and :ref:`for <cmd-for>`. Their globs will instead expand to zero arguments (so the command won't see them at all), like with ``shopt -s nullglob`` in bash.
..[#] Technically, unix allows filenames with newlines, and this splits the ``find`` output on newlines. If you want to avoid that, use find's ``-print0`` option and :ref:`string split0<cmd-string-split0>`.
When you write a command in parenthesis like ``outercommand (innercommand)``, the ``innercommand`` will be executed first. Its output will be taken and each line given as a separate argument to ``outercommand``, which will then be executed. [#]_
If the output is piped to :ref:`string split or string split0 <cmd-string-split>` as the last step, those splits are used as they appear instead of splitting lines.
The exit status of the last run command substitution is available in the `status <#variables-status>`_ variable if the substitution happens in the context of a :ref:`set <cmd-set>` command (so ``if set -l (something)`` checks if ``something`` returned true).
Only part of the output can be used, see :ref:`index range expansion <expand-index-range>` for details.
Fish has a default limit of 100 MiB on the data it will read in a command sustitution. If that limit is reached the command (all of it, not just the command substitution - the outer command won't be executed at all) fails and ``$status`` is set to 122. This is so command substitutions can't cause the system to go out of memory, because typically your operating system has a much lower limit, so reading more than that would be useless and harmful. This limit can be adjusted with the ``fish_read_limit`` variable (`0` meaning no limit). This limit also affects the :ref:`read <cmd-read>` command.
Sometimes you want to pass the output of a command to another command that only accepts files. If it's just one file, you can usually just pass it via a pipe, like::
grep fish myanimallist1 | wc -l
but if you need multiple or the command doesn't read from standard input, "process substitution" is useful. Other shells [#]_ allow this via ``foo <(bar) <(baz)``, and fish uses the :ref:`psub <cmd-psub>` command::
# Compare just the lines containing "fish" in two files:
diff -u (grep fish myanimallist1 | psub) (grep fish myanimallist2 | psub)
This creates a temporary file, stores the output of the command in that file and prints the filename, so it is given to the outer command.
Curly braces can be used to write comma-separated lists. They will be expanded with each element becoming a new parameter, with the surrounding string attached. This is useful to save on typing, and to separate a variable name from surrounding text.
One of the most important expansions in fish is the "variable expansion". This is the replacing of a dollar sign (``$``) followed by a variable name with the _value_ of that variable. For more on shell variables, read the :ref:`Shell variables <variables>` section.
Unlike all the other expansions, variable expansion also happens in double quoted strings. Inside double quotes (``"these"``), variables will always expand to exactly one argument. If they are empty or undefined, it will result in an empty string. If they have one element, they'll expand to that element. If they have more than that, the elements will be joined with spaces, unless the variable is a :ref:`path variable <variables-path>` - in that case it will use a colon (`:`) instead [#]_.
Outside of double quotes, variables will expand to as many arguments as they have elements. That means an empty list will expand to nothing, a variable with one element will expand to that element, and a variable with multiple elements will expand to each of those elements separately.
If a variable expands to nothing, it will cancel out any other strings attached to it. See the :ref:`cartesian product <cartesian-product>` section for more information.
When using this feature together with list brackets, the brackets will be used from the inside out. ``$$foo[5]`` will use the fifth element of ``$foo`` as a variable name, instead of giving the fifth element of all the variables $foo refers to. That would instead be expressed as ``$$foo[1][5]`` (take the first element of ``$foo``, use it as a variable name, then give the fifth element of that).
When lists are expanded with other parts attached, they are expanded with these parts still attached. Even if two lists are attached to each other, they are expanded in all combinations. This is referred to as the `cartesian product` (like in mathematics), and works basically like :ref:`brace expansion <expand-brace>`.
Sometimes this may be unwanted, especially that tokens can disappear after expansion. In those cases, you should double-quote variables - ``echo "$c"word``.
This also happens after :ref:`command substitution <expand-command-substitution>`. To avoid tokens disappearing there, make the inner command return a trailing newline, or store the output in a variable and double-quote it.
Because :ref:`$PATH <path>` is a list, this expands to all the files in all the directories in it. And if there are no directories in $PATH, the right answer here is to expand to no files.
Sometimes it's necessary to access only some of the elements of a list, or some of the lines a command substitution outputs. Both are possible in fish by writing a set of indices in brackets, like::
In index brackets, fish understands ranges written like ``a..b`` ('a' and 'b' being indices). They are expanded into a sequence of indices from a to b (so ``a a+1 a+2 ... b``), going up if b is larger and going down if a is larger. Negative indices can also be used - they are taken from the end of the list, so ``-1`` is the last element, and ``-2`` the one before it. If an index doesn't exist the range is clamped to the next possible index.
If a list has 5 elements the indices go from 1 to 5, so a range of ``2..16`` will only go from element 2 to element 5.
If the end is negative the range always goes up, so ``2..-2`` will go from element 2 to 4, and ``2..-16`` won't go anywhere because there is no way to go from the second element to one that doesn't exist, while going up.
If the start is negative the range always goes down, so ``-2..1`` will go from element 4 to 1, and ``-16..2`` won't go anywhere because there is no way to go from the second element to one that doesn't exist, while going down.
A missing starting index in a range defaults to 1. This is allowed if the range is the first index expression of the sequence. Similarly, a missing ending index, defaulting to -1 is allowed for the last index range in the sequence.
The ``~`` (tilde) character at the beginning of a parameter, followed by a username, is expanded into the home directory of the specified user. A lone ``~``, or a ``~`` followed by a slash, is expanded into the home directory of the process owner.
Variables are a way to save data and pass it around. They can be used just by the shell, or they can be ":ref:`exported <variables-export>`", so that a copy of the variable is available to any external command the shell starts. An exported variable is referred to as an "environment variable".
To set a variable value, use the :ref:`set <cmd-set>` command. A variable name can not be empty and can contain only letters, digits, and underscores. It may begin and end with any of those characters.
- Local variables are specific to the current fish session, and associated with a specific block of commands, and automatically erased when a specific block goes out of scope. A block of commands is a series of commands that begins with one of the commands ``for``, ``while`` , ``if``, ``function``, ``begin`` or ``switch``, and ends with the command ``end``.
Variables can be explicitly set to be universal with the ``-U`` or ``--universal`` switch, global with the ``-g`` or ``--global`` switch, or local with the ``-l`` or ``--local`` switch. The scoping rules when creating or updating a variable are:
- As a special case, when no scope is given and no variable has been defined the variable will belong to the scope of the currently executing *function*. Note that this is different from the ``--local`` flag, which would make the variable local to the current *block*.
There can be many variables with the same name, but different scopes. When you :ref:`use a variable <expand-variable>`, the smallest scoped variable of that name will be used. If a local variable exists, it will be used instead of the global or universal variable of the same name.
There are a few possible uses for different scopes.
Typically inside funcions you should use local scope::
function something
set -l file /path/to/my/file
if not test -e "$file"
set file /path/to/my/otherfile
end
end
If you want to set something in config.fish, or set something in a function and have it available for the rest of the session, global scope is a good choice::
# Don't shorten the working directory in the prompt
set -g fish_prompt_pwd_dir_length 0
# Set my preferred cursor style:
function setcursors
set -g fish_cursor_default block
set -g fish_cursor_insert line
set -g fish_cursor_visual underscore
end
# Set my language (also :ref:`exported <variables-export>`):
set -gx LANG de_DE.UTF-8
If you want to set some personal customization, universal variables are nice::
# Typically you'd run this interactively, fish takes care of keeping it.
Unlike other shells, this does *not* inhibit any lookup (aliases or similar). Calling a command after setting a variable override will result in the exact same command being run.
Universal variables are variables that are shared between all the user's fish sessions on the computer. Fish stores many of its configuration options as universal variables. This means that in order to change fish settings, all you have to do is change the variable value once, and it will be automatically updated for all sessions, and preserved across computer reboots and login/logout.
To see universal variables in action, start two fish sessions side by side, and issue the following command in one of them ``set fish_color_cwd blue``. Since ``fish_color_cwd`` is a universal variable, the color of the current working directory listing in the prompt will instantly change to blue on both terminals.
`Universal variables <#variables-universal>`_ are stored in the file ``.config/fish/fish_variables``. Do not edit this file directly, as your edits may be overwritten. Edit the variables through fish scripts or by using fish interactively instead.
Do not append to universal variables in :ref:`config.fish <initialization>`, because these variables will then get longer with each new shell instance. Instead, simply set them once at the command line.
When calling a function, all current local variables temporarily disappear. This shadowing of the local scope is needed since the variable namespace would become cluttered, making it very easy to accidentally overwrite variables from another function.
Variables in fish can be "exported", so they will be inherited by any commands started by fish. In particular, this is necessary for variables used to configure external commands like $LESS or $GOPATH, but also for variables that contain general system settings like $PATH or $LANGUAGE. If an external command needs to know a variable, it needs to be exported.
This also applies to fish - when it starts up, it receives environment variables from its parent (usually the terminal). These typically include system configuration like :ref:`$PATH <PATH>` and :ref:`locale variables <variables-locale>`.
Variables can be explicitly set to be exported with the ``-x`` or ``--export`` switch, or not exported with the ``-u`` or ``--unexport`` switch. The exporting rules when setting a variable are identical to the scoping rules for variables:
- If a variable is not explicitly set to be exported or not exported, but has been previously defined, the previous exporting rule for the variable is kept.
- If a variable has local scope and is exported, any function called receives a *copy* of it, so any changes it makes to the variable disappear once the function returns.
set -gx ANDROID_HOME ~/.android # /opt/android-sdk
set -gx CDPATH . ~ (test -e ~/Videos; and echo ~/Videos)
set -gx EDITOR emacs -nw
set -gx GOPATH ~/dev/go
set -gx GTK2_RC_FILES "$XDG_CONFIG_HOME/gtk-2.0/gtkrc"
set -gx LESSHISTFILE "-"
It typically makes sense to make exported variables global as well, but local-exported variables can be useful if you need something more specific than :ref:`Overrides <variables-override>`. They are *copied* to functions so the function can't alter them outside, and still available to commands.
Note that list indices start at 1 in fish, not 0 like in other languages. This is because it requires less subtracting of 1 and many common Unix tools like ``seq`` work better with it (``seq 5`` prints 1 to 5, not 0 to 5). An invalid index is silently ignored resulting in no value (not even an empty string, just no argument at all).
If you don't use any brackets, all the elements of the list will be passed to the command as separate items. This means you can iterate over a list with ``for``::
for i in $PATH
echo $i is in the path
end
This goes over every directory in $PATH separately and prints a line saying it is in the path.
If you specify a negative index when expanding or assigning to a list variable, the index will be taken from the *end* of the list. For example, the index -1 is the last element of the list::
All lists are one-dimensional and can't contain other lists, although it is possible to fake nested lists using dereferencing - see :ref:`variable expansion <expand-variable>`.
When a list is exported as an environment variable, it is either space or colon delimited, depending on whether it is a :ref:`path variable <variables-path>`::
Fish automatically creates lists from all environment variables whose name ends in PATH (like $PATH, $CDPATH or $MANPATH), by splitting them on colons. Other variables are not automatically split.
A nice thing about lists is that they are passed to commands one element as one argument, so once you've set your list, you can just pass it::
set -l grep_args -r "my string"
grep $grep_args . # will run the same as `grep -r "my string"` .
Unlike other shells, fish does not do "word splitting" - elements in a list stay as they are, even if they contain spaces or tabs.
.._variables-argv:
Argument Handling
-----------------
An important list is ``$argv``, which contains the arguments to a function or script. For example::
function myfunction
echo $argv[1]
echo $argv[3]
end
This function takes whatever arguments it gets and prints the first and third::
> myfunction first second third
first
third
> myfunction apple cucumber banana
apple
banana
Commandline tools often get various options and flags and positional arguments, and $argv would contain all of these.
A more robust approach to argument handling is :ref:`argparse <cmd-argparse>`, which checks the defined options and puts them into various variables, leaving only the positional arguments in $argv. Here's a simple example::
function mybetterfunction
argparse h/help s/second -- $argv
or return # exit if argparse failed because it found an option it didn't recognize - it will print an error
# If -h or --help is given, we print a little help text and return
Path variables are a special kind of variable used to support colon-delimited path lists including PATH, CDPATH, MANPATH, PYTHONPATH, etc. All variables that end in "PATH" (case-sensitive) become PATH variables.
- The locale variables ``LANG``, ``LC_ALL``, ``LC_COLLATE``, ``LC_CTYPE``, ``LC_MESSAGES``, ``LC_MONETARY``, ``LC_NUMERIC`` and ``LC_TIME`` set the language option for the shell and subprograms. See the section `Locale variables <#variables-locale>`_ for more information.
- A number of variable starting with the prefixes ``fish_color`` and ``fish_pager_color``. See `Variables for changing highlighting colors <#variables-color>`__ for more information.
-``fish_ambiguous_width`` controls the computed width of ambiguous-width characters. This should be set to 1 if your terminal renders these characters as single-width (typical), or 2 if double-width.
-``fish_emoji_width`` controls whether fish assumes emoji render as 2 cells or 1 cell wide. This is necessary because the correct value changed from 1 to 2 in Unicode 9, and some terminals may not be aware. Set this if you see graphical glitching related to emoji (or other "special" characters). It should usually be auto-detected.
-``FISH_DEBUG`` and ``FISH_DEBUG_OUTPUT`` control what debug output fish generates and where it puts it, analogous to the ``--debug`` and ``--debug-output`` options. These have to be set on startup, via e.g. ``FISH_DEBUG='reader*' FISH_DEBUG_OUTPUT=/tmp/fishlog fish``.
-``fish_escape_delay_ms`` sets how long fish waits for another key after seeing an escape, to distinguish pressing the escape key from the start of an escape sequence. The default is 30ms. Increasing it increases the latency but allows pressing escape instead of alt for alt+character bindings. For more information, see :ref:`the chapter in the bind documentation <cmd-bind-escape>`.
-``fish_greeting``, the greeting message printed on startup. This is printed by a function of the same name that can be overridden for more complicated changes (see :ref:`funced <cmd-funced>`
-``fish_handle_reflow``, determines whether fish should try to repaint the commandline when the terminal resizes. In terminals that reflow text this should be disabled. Set it to 1 to enable, anything else to disable.
-``fish_trace``, if set and not empty, will cause fish to print commands before they execute, similar to ``set -x`` in bash. The trace is printed to the path given by the :ref:`--debug-output <cmd-fish>` option to fish (stderr by default).
-``umask``, the current file creation mask. The preferred way to change the umask variable is through the :ref:`umask <cmd-umask>` function. An attempt to set umask to an invalid value will always fail.
-``BROWSER``, your preferred web browser. If this variable is set, fish will use the specified browser instead of the system default browser to display the fish documentation.
Fish also provides additional information through the values of certain environment variables. Most of these variables are read-only and their value can't be changed with ``set``.
-``argv``, a list of arguments to the shell or function. ``argv`` is only defined when inside a function call, or if fish was invoked with a list of arguments, like ``fish myscript.fish foo bar``. This variable can be changed.
-``COLUMNS`` and ``LINES``, the current size of the terminal in height and width. These values are only used by fish if the operating system does not report the size of the terminal. Both variables must be set in that case otherwise a default of 80x24 will be used. They are updated when the window size changes.
-``IFS``, the internal field separator that is used for word splitting with the :ref:`read <cmd-read>` builtin. Setting this to the empty string will also disable line splitting in `command substitution <#expand-command-substitution>`_. This variable can be changed.
-``status``, the `exit status <#variables-status>`_ of the last foreground job to exit. If the job was terminated through a signal, the exit status will be 128 plus the signal number.
-``status_generation``, the "generation" count of ``$status``. This will be incremented only when the previous command produced an explicit status. (For example, background jobs will not increment this).
As a convention, an uppercase name is usually used for exported variables, while lowercase variables are not exported. (``CMD_DURATION`` is an exception for historical reasons). This rule is not enforced by fish, but it is good coding practice to use casing to distinguish between exported and unexported variables.
Fish also uses some variables internally, their name usually starting with ``__fish``. These are internal and should not typically be modified directly.
Whenever a process exits, an exit status is returned to the program that started it (usually the shell). This exit status is an integer number, which tells the calling application how the execution of the command went. In general, a zero exit status means that the command executed without problem, but a non-zero exit status means there was some form of problem.
The colors used by fish for syntax highlighting can be configured by changing the values of a various variables. The value of these variables can be one of the colors accepted by the :ref:`set_color <cmd-set_color>` command. The ``--bold`` or ``-b`` switches accepted by ``set_color`` are also accepted.
The "locale" of a program is its set of language and regional settings. In UNIX, there are a few separate variables to control separate things - ``LC_CTYPE`` defines the text encoding while ``LC_TIME`` defines the time format.
The locale variables are: ``LANG``, ``LC_ALL``, ``LC_COLLATE``, ``LC_CTYPE``, ``LC_MESSAGES``, ``LC_MONETARY``, ``LC_NUMERIC`` and ``LC_TIME``. These variables work as follows: ``LC_ALL`` forces all the aspects of the locale to the specified value. If ``LC_ALL`` is set, all other locale variables will be ignored (this is typically not recommended!). The other ``LC_`` variables set the specified aspect of the locale information. ``LANG`` is a fallback value, it will be used if none of the ``LC_`` variables are specified.
The most common way to set the locale to use a command like ``set -gx LANG en_GB.utf8``, which sets the current locale to be the English language, as used in Great Britain, using the UTF-8 character set. That way any program that requires one setting differently can easily override just that and doesn't have to resort to LC_ALL. For a list of available locales on your system, try ``locale -a``.
- Builtins that manipulate the shell state - :ref:`cd <cmd-cd>` changes directory, :ref:`set <cmd-set>` sets variables
- Builtins for dealing with data, like :ref:`string <cmd-string>` for strings and :ref:`math <cmd-math>` for numbers, :ref:`count <cmd-count>` for counting lines or arguments
-:ref:`status <cmd-status>` for asking about the shell's status
-:ref:`printf <cmd-printf>` and :ref:`echo <cmd-echo>` for creating output
-:ref:`test <cmd-test>` for checking conditions
-:ref:`argparse <cmd-argparse>` for parsing function arguments
-:ref:`source <cmd-source>` to read a script in the current shell (so changes to variables stay) and :ref:`eval <cmd-eval>` to execute a string as script
-:ref:`random <cmd-random>` to get random numbers or pick a random element from a list
For a list of all builtins, use ``builtin -n``.
For a list of all builtins, functions and commands shipped with fish, see the :ref:`list of commands <Commands>`. The documentation is also available by using the ``--help`` switch.
The names given to variables and functions (so called "identifiers") have to follow certain rules:
- A variable name cannot be empty. It can contain only letters, digits, and underscores. It may begin and end with any of those characters.
- A function name cannot be empty. It may not begin with a hyphen ("-") and may not contain a slash ("/"). All other characters, including a space, are valid.
Other things have other restrictions. For instance what is allowed for file names depends on your system, but at the very least they cannot contain a "/" (because that is the path separator) or NULL byte (because that is how UNIX ends strings).
Fish has an extensive help system. Use the :ref:`help <cmd-help>` command to obtain help on a specific subject or command. For instance, writing ``help syntax`` displays the `syntax section <#syntax>`_ of this documentation.
Help on a specific builtin can also be obtained with the ``-h`` parameter. For instance, to obtain help on the :ref:`fg <cmd-fg>` builtin, either type ``fg -h`` or ``help fg``.
fish suggests commands as you type, based on `command history <#history-search>`_, completions, and valid file paths. As you type commands, you will see a suggestion offered after the cursor, in a muted gray color (which can be changed with the ``fish_color_autosuggestion`` variable).
To accept the autosuggestion (replacing the command line contents), press :kbd:`→` or :kbd:`Control`\ +\ :kbd:`F`. To accept the first suggested word, press :kbd:`Alt`\ +\ :kbd:`→` or :kbd:`Alt`\ +\ :kbd:`F`. If the autosuggestion is not what you want, just ignore it: it won't execute unless you accept it.
Autosuggestions are a powerful way to quickly summon frequently entered commands, by typing the first few characters. They are also an efficient technique for navigating through directory hierarchies.
Tab completion is a time saving feature of any modern shell. When you type :kbd:`Tab`, fish tries to guess the rest of the word under the cursor. If it finds just one possibility, it inserts it. If it finds more, it inserts the longest unambiguous part and then opens a menu (the "pager") that you can navigate to find what you're looking for.
The pager can be navigated with the arrow keys, :kbd:`Page Up` / :kbd:`Page Down`, :kbd:`Tab` or :kbd:`Shift`\ +\ :kbd:`Tab`. Pressing :kbd:`Control`\ +\ :kbd:`S` (the ``pager-toggle-search`` binding - :kbd:`/` in vi-mode) opens up a search menu that you can use to filter the list.
- Filenames, even on strings with wildcards such as ``*`` and ``**``.
It also provides a large number of program specific scripted completions. Most of these completions are simple options like the ``-l`` option for ``ls``, but some are more advanced. For example:
Fish interprets the command line as it is typed and uses syntax highlighting to provide feedback. The most important feedback is the detection of potential errors. By default, errors are marked red.
Detected errors include:
- Non existing commands.
- Reading from or appending to a non existing file.
- Incorrect use of output redirects
- Mismatched parenthesis
When the cursor is over a parenthesis or a quote, fish also highlights its matching quote or parenthesis.
To customize the syntax highlighting, you can set the environment variables listed in the `Variables for changing highlighting colors <#variables-color>`__ section.
.._title:
Programmable title
------------------
When using most virtual terminals, it is possible to set the message displayed in the titlebar of the terminal window. This can be done automatically in fish by defining the :ref:`fish_title <cmd-fish_title>` function. The :ref:`fish_title <cmd-fish_title>` function is executed before and after a new command is executed or put into the foreground and the output is used as a titlebar message. The :ref:`status current-command <cmd-status>` builtin will always return the name of the job to be put into the foreground (or ``fish`` if control is returning to the shell) when the `fish_prompt <cmd-fish_prompt>` function is called. The first argument to fish_title will contain the most recently executed foreground command as a string, starting with fish 2.2.
Examples:
The default fish title is::
function fish_title
echo (status current-command) ' '
pwd
end
To show the last command in the title::
function fish_title
echo $argv[1]
end
.._prompt:
Programmable prompt
-------------------
When fish waits for input, it will display a prompt by evaluating the :ref:`fish_prompt <cmd-fish_prompt>` and :ref:`fish_right_prompt <cmd-fish_right_prompt>` functions. The output of the former is displayed on the left and the latter's output on the right side of the terminal. The output of :ref:`fish_mode_prompt <cmd-fish_mode_prompt>` will be prepended on the left, though the default function only does this when in `vi-mode <#vi-mode>`__.
.._greeting:
Configurable greeting
---------------------
If a function named :ref:`fish_greeting <cmd-fish_greeting>` exists, it will be run when entering interactive mode. Otherwise, if an environment variable named :ref:`fish_greeting <cmd-fish_greeting>` exists, it will be printed.
.._private-mode:
Private mode
-------------
If ``$fish_private_mode`` is set to a non-empty value, commands will not be written to the history file on disk.
You can also launch with ``fish --private`` (or ``fish -P`` for short). This both hides old history and prevents writing history to disk. This is useful to avoid leaking personal information (e.g. for screencasts) or when dealing with sensitive information.
You can query the variable ``fish_private_mode`` (``if set -q fish_private_mode ...``) if you would like to respect the user's wish for privacy and alter the behavior of your own fish scripts.
The fish editor features copy and paste, a `searchable history <#history-search>`_ and many editor functions that can be bound to special keyboard shortcuts.
Similar to bash, fish has Emacs and Vi editing modes. The default editing mode is Emacs. You can switch to Vi mode with ``fish_vi_key_bindings`` and switch back with ``fish_default_key_bindings``. You can also make your own key bindings by creating a function and setting $fish_key_bindings to its name. For example::
Some bindings are shared between emacs- and vi-mode because they aren't text editing bindings or because what Vi/Vim does for a particular key doesn't make sense for a shell.
-:kbd:`Tab``completes <#tab-completion>`_ the current token. :kbd:`Shift`\ +\ :kbd:`Tab` completes the current token and starts the pager's search mode.
-:kbd:`←` (Left) and :kbd:`→` (Right) move the cursor left or right by one character. If the cursor is already at the end of the line, and an autosuggestion is available, :kbd:`→` accepts the autosuggestion.
-:kbd:`Alt`\ +\ :kbd:`←` and :kbd:`Alt`\ +\ :kbd:`→` move the cursor one word left or right (to the next space or punctuation mark), or moves forward/backward in the directory history if the command line is empty. If the cursor is already at the end of the line, and an autosuggestion is available, :kbd:`Alt`\ +\ :kbd:`→` (or :kbd:`Alt`\ +\ :kbd:`F`) accepts the first word in the suggestion.
-:kbd:`Control`\ +\ :kbd:`←` and :kbd:`Control`\ +\ :kbd:`→` move the cursor one word left or right. These accept one word of the autosuggestion - the part they'd move over.
-:kbd:`Shift`\ +\ :kbd:`←` and :kbd:`Shift`\ +\ :kbd:`→` move the cursor one word left or right, without stopping on punctuation. These accept one big word of the autosuggestion.
-:kbd:`↑` (Up) and :kbd:`↓` (Down) (or :kbd:`Control`\ +\ :kbd:`P` and :kbd:`Control`\ +\ :kbd:`N` for emacs aficionados) search the command history for the previous/next command containing the string that was specified on the commandline before the search was started. If the commandline was empty when the search started, all commands match. See the `history <#history-search>`_ section for more information on history searching.
-:kbd:`Alt`\ +\ :kbd:`↑` and :kbd:`Alt`\ +\ :kbd:`↓` search the command history for the previous/next token containing the token under the cursor before the search was started. If the commandline was not on a token when the search started, all tokens match. See the `history <#history-search>`_ section for more information on history searching.
-:kbd:`Control`\ +\ :kbd:`D` delete one character to the right of the cursor. If the command line is empty, :kbd:`Control`\ +\ :kbd:`D` will exit fish.
-:kbd:`Control`\ +\ :kbd:`R` searches the history if there is something in the commandline. This is mainly to ease the transition from other shells, where ctrl+r initiates the history search.
-:kbd:`Alt`\ +\ :kbd:`L` lists the contents of the current directory, unless the cursor is over a directory argument, in which case the contents of that directory will be listed.
-:kbd:`Alt`\ +\ :kbd:`P` adds the string ``&| less;`` to the end of the job under the cursor. The result is that the output of the command will be paged.
-:kbd:`Alt`\ +\ :kbd:`E` edit the current command line in an external editor. The editor is chosen from the first available of the ``$VISUAL`` or ``$EDITOR`` variables.
-:kbd:`Control`\ +\ :kbd:`Space` Inserts a space without expanding an :ref:`abbreviation <abbreviations>`. For vi-mode this only applies to insert-mode.
-:kbd:`End` or :kbd:`Control`\ +\ :kbd:`E` moves to the end of line. If the cursor is already at the end of the line, and an autosuggestion is available, :kbd:`End` or :kbd:`Control`\ +\ :kbd:`E` accepts the autosuggestion.
-:kbd:`Control`\ +\ :kbd:`B`, :kbd:`Control`\ +\ :kbd:`F` move the cursor one character left or right or accept the autosuggestion just like the :kbd:`←` (Left) and :kbd:`→` (Right) shared bindings (which are available as well).
Vi mode allows for the use of Vi-like commands at the prompt. Initially, `insert mode <#vi-mode-insert>`_ is active. :kbd:`Escape` enters `command mode <#vi-mode-command>`_. The commands available in command, insert and visual mode are described below. Vi mode shares `some bindings <#shared-binds>`_ with `Emacs mode <#emacs-mode>`_.
When in vi-mode, the :ref:`fish_mode_prompt <cmd-fish_mode_prompt>` function will display a mode indicator to the left of the prompt. To disable this feature, override it with an empty function. To display the mode elsewhere (like in your right prompt), use the output of the ``fish_default_mode_prompt`` function.
When a binding switches the mode, it will repaint the mode-prompt if it exists, and the rest of the prompt only if it doesn't. So if you want a mode-indicator in your ``fish_prompt``, you need to erase ``fish_mode_prompt`` e.g. by adding an empty file at ``~/.config/fish/functions/fish_mode_prompt.fish``. (Bindings that change the mode are supposed to call the `repaint-mode` bind function, see :ref:`bind <cmd-bind>`)
The ``fish_vi_cursor`` function will be used to change the cursor's shape depending on the mode in supported terminals. The following snippet can be used to manually configure cursors after enabling vi-mode::
# Emulates vim's cursor shape behavior
# Set the normal and visual mode cursors to a block
If the cursor shape does not appear to be changing after setting the above variables, it's likely your terminal emulator does not support the capabilities necessary to do this. It may also be the case, however, that ``fish_vi_cursor`` has not detected your terminal's features correctly (for example, if you are using ``tmux``). If this is the case, you can force ``fish_vi_cursor`` to set the cursor shape by setting ``$fish_vi_force_cursor`` in ``config.fish``. You'll have to restart fish for any changes to take effect. If cursor shape setting remains broken after this, it's almost certainly an issue with your terminal emulator, and not fish.
-:kbd:`[` and :kbd:`]` search the command history for the previous/next token containing the token under the cursor before the search was started. See the `history <#history-search>`_ section for more information on history searching.
In addition to the standard bindings listed here, you can also define your own with :ref:`bind <cmd-bind>`::
# Just clear the commandline on control-c
bind \cc 'commandline -r ""'
Put ``bind`` statements into :ref:`config.fish <initialization>` or a function called ``fish_user_key_bindings``.
The key sequence (the ``\cc``) here depends on your setup, in particular the terminal. To find out what the terminal sends use :ref:`fish_key_reader <cmd-fish_key_reader>`::
Note that some key combinations are indistinguishable or unbindable. For instance control-i *is the same* as the tab key. This is a terminal limitation that fish can't do anything about.
Also, :kbd:`Escape` is the same thing as :kbd:`Alt` in a terminal. To distinguish between pressing :kbd:`Escape` and then another key, and pressing :kbd:`Alt` and that key (or an escape sequence the key sends), fish waits for a certain time after seeing an escape character. This is configurable via the ``fish_escape_delay_ms`` variable.
If you want to be able to press :kbd:`Escape` and then a character and have it count as :kbd:`Alt`\ +\ that character, set it to a higher value, e.g.::
Fish uses an Emacs-style kill ring for copy and paste functionality. For example, use :kbd:`Control`\ +\ :kbd:`K` (`kill-line`) to cut from the current cursor position to the end of the line. The string that is cut (a.k.a. killed in emacs-ese) is inserted into a list of kills, called the kill ring. To paste the latest value from the kill ring (emacs calls this "yanking") use :kbd:`Control`\ +\ :kbd:`Y` (the ``yank`` input function). After pasting, use :kbd:`Alt`\ +\ :kbd:`Y` (``yank-pop``) to rotate to the previous kill.
Copy and paste from outside are also supported, both via the :kbd:`Control`\ +\ :kbd:`X` / :kbd:`Control`\ +\ :kbd:`V` bindings (the ``fish_clipboard_copy`` and ``fish_clipboard_paste`` functions [#]_) and via the terminal's paste function, for which fish enables "Bracketed Paste Mode", so it can tell a paste from manually entered text.
In addition, when pasting inside single quotes, pasted single quotes and backslashes are automatically escaped so that the result can be used as a single token simply by closing the quote after.
..[#] These rely on external tools. Currently xsel, xclip, wl-copy/wl-paste and pbcopy/pbpaste are supported.
The fish commandline editor can be used to work on commands that are several lines long. There are three ways to make a command span more than a single line:
- Pressing the :kbd:`Enter` key while a block of commands is unclosed, such as when one or more block commands such as ``for``, ``begin`` or ``if`` do not have a corresponding :ref:`end <cmd-end>` command.
- Pressing :kbd:`Alt`\ +\ :kbd:`Enter` instead of pressing the :kbd:`Enter` key.
- By inserting a backslash (``\``) character before pressing the :kbd:`Enter` key, escaping the newline.
The fish commandline editor works exactly the same in single line mode and in multiline mode. To move between lines use the left and right arrow keys and other such keyboard shortcuts.
After a command has been executed, it is remembered in the history list. Any duplicate history items are automatically removed. By pressing the up and down keys, you can search forwards and backwards in the history. If the current command line is not empty when starting a history search, only the commands containing the string entered into the command line are shown.
By pressing :kbd:`Alt`\ +\ :kbd:`↑` and :kbd:`Alt`\ +\ :kbd:`↓`, a history search is also performed, but instead of searching for a complete commandline, each commandline is broken into separate elements just like it would be before execution, and the history is searched for an element matching that under the cursor.
If the commandline reads ``cd m``, place the cursor over the ``m`` character and press :kbd:`Alt`\ +\ :kbd:`↑` to search for previously typed words containing 'm'.
Fish automatically keeps a trail of the recent visited directories with :ref:`cd <cmd-cd>` by storing this history in the ``dirprev`` and ``dirnext`` variables.
Another set of commands, usually also available in other shells like bash, deal with the directory stack. Stack handling is not automatic and needs explicit calls of the following commands:
-:ref:`dirs <cmd-dirs>` prints the stack
-:ref:`pushd <cmd-pushd>` adds a directory on top of the stack and makes it the current working directory
-:ref:`popd <cmd-popd>` removes the directory on top of the stack and changes the current working directory
Normally when fish starts a program, this program will be put in the foreground, meaning it will take control of the terminal and fish will be stopped until the program finishes. Sometimes this is not desirable. For example, you may wish to start an application with a graphical user interface from the terminal, and then be able to continue using the shell. There are several ways to do this:
- Fish puts commands ending with the ``&`` (ampersand) symbol into the background. A background process will be run simultaneous with fish. Fish will keep control of the terminal, so the program will not be able to read from the keyboard.
- By pressing :kbd:`Control`\ +\ :kbd:`Z`, you can stop a currently running foreground program and returns control to fish. Some programs do not support this feature, or remap it to another key. GNU Emacs uses :kbd:`Control`\ +\ :kbd:`X`:kbd:`Z` to stop running.
Note that functions cannot be started in the background. Functions that are stopped and then restarted in the background using the :ref:`bg <cmd-bg>` command will not execute correctly.
On startup, Fish evaluates a number of configuration files, which can be used to control the behavior of the shell. The location of these is controlled by a number of environment variables, and their default or usual location is given below.
- Directories for third-party software vendors to ship their own configuration snippets for their software. Fish searches the directories in the ``XDG_DATA_DIRS`` environment variable for a ``fish/vendor_conf.d`` directory; if this variable is not defined, the default is usually to search ``/usr/share/fish/vendor_conf.d`` and ``/usr/local/share/fish/vendor_conf.d``
- System-wide configuration files, where administrators can include initialization that should be run for all users on the system - similar to ``/etc/profile`` for POSIX-style shells - in ``$__fish_sysconf_dir`` (usually ``/etc/fish/config.fish``).
- User initialization, usually in ``~/.config/fish/config.fish`` (controlled by the ``XDG_CONFIG_HOME`` environment variable, and accessible as ``$__fish_config_dir``).
These files are all executed on the startup of every shell. If you want to run a command only on starting an interactive shell, use the exit status of the command ``status --is-interactive`` to determine if the shell is interactive. If you want to run a command only when using a login shell, use ``status --is-login`` instead. This will speed up the starting of non-interactive or non-login shells.
If you are developing another program, you may wish to install configuration which is run for all users of the fish shell on a system. This is discouraged; if not carefully written, they may have side-effects or slow the startup of the shell. Additionally, users of other shells will not benefit from the Fish-specific configuration. However, if they are absolutely required, you may install them to the "vendor" configuration directory. As this path may vary from system to system, the ``pkgconfig`` framework should be used to discover this path with the output of ``pkg-config --variable confdir fish``.
If you want to add the directory ``~/linux/bin`` to your PATH variable when using a login shell, add the following to your ``~/.config/fish/config.fish`` file::
Feature flags are how fish stages changes that might break scripts. Breaking changes are introduced as opt-in, in a few releases they become opt-out, and eventually the old behavior is removed.
There are two breaking changes in fish 3.0: caret ``^`` no longer redirects stderr, and question mark ``?`` is no longer a glob.
There is one breaking change in fish 3.1: ``string replace -r`` does a superfluous round of escaping for the replacement, so escaping backslashes would look like ``string replace -ra '([ab])' '\\\\\\\$1' a``. This flag removes that if turned on, so ``'\\\\$1'`` is enough.
These changes are off by default. They can be enabled on a per session basis::
When defining a new function in fish, it is possible to make it into an event handler, i.e. a function that is automatically run when a specific event takes place. Events that can trigger a handler currently are:
Please note that event handlers only become active when a function is loaded, which means you might need to otherwise :ref:`source <cmd-source>` or execute a function instead of relying on :ref:`autoloading <syntax-function-autoloading>`. One approach is to put it into your :ref:`initialization file <initialization>`.
Fish includes a built in debugging facility. The debugger allows you to stop execution of a script at an arbitrary point. When this happens you are presented with an interactive prompt. At this prompt you can execute any fish command (there are no debug commands as such). For example, you can check or change the value of any variables using :ref:`printf <cmd-printf>` and :ref:`set <cmd-set>`. As another example, you can run :ref:`status print-stack-trace <cmd-status>` to see how this breakpoint was reached. To resume normal execution of the script, simply type :ref:`exit <cmd-exit>` or :kbd:`Control`\ +\ :kbd:`D`.
To start a debug session simply run the builtin command :ref:`breakpoint <cmd-breakpoint>` at the point in a function or script where you wish to gain control. Also, the default action of the TRAP signal is to call this builtin. So a running script can be debugged by sending it the TRAP signal with the ``kill`` command. Once in the debugger, it is easy to insert new breakpoints by using the funced function to edit the definition of a function.
