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.
Every program on your computer can be used as a command in fish. If the program file is located in one of the :envvar:`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. Try ``man your-command-here`` to get information on your command's switches.
So the basic idea of fish is the same as with other unix shells: It gets a commandline, runs :ref:`expansions <expand>`, and the result is then run as a command.
-**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. In ``echo foo``, the "echo" is a builtin.
-**Command**: A program that the shell can run, or more specifically an external program that the shell runs in another process. External commands are provided on your system, as executable files. In ``echo foo`` the "echo" is a builtin command, in ``command echo foo`` the "echo" is an external command, provided by a file like /bin/echo.
-**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.
-**Pipeline**: A set of commands strung together so that the output of one command is the input of the next command. ``echo foo | grep foo`` is a pipeline.
-**Switch** or **Option**: A special kind of argument that alters the behavior of a command. A switch almost always begins with one or two hyphens. In ``echo -n foo`` the "-n" is an option.
Sometimes features like :ref:`parameter expansion <expand>` and :ref:`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.
Single quotes have no special meaning within double quotes and vice versa.
Example::
rm "cumbersome filename.txt"
removes the file ``cumbersome filename.txt``, while
::
rm cumbersome filename.txt
removes two files, ``cumbersome`` and ``filename.txt``.
Another example::
grep 'enabled)$' foo.txt
searches for lines ending in ``enabled)`` in ``foo.txt`` (the ``$`` is special to ``grep``: it matches the end of the line).
.._escapes:
Escaping Characters
-------------------
Some characters cannot be written directly on the command line. For these characters, so-called escape sequences are provided. These are:
-``\xHH`` or ``\XHH``, where ``HH`` is a hexadecimal number, represents a byte of data with the specified value. For example, ``\x9`` is the tab character. If you are using a multibyte encoding, this can be used to enter invalid strings. Typically fish is run with the ASCII or UTF-8 encoding, so anything up to ``\X7f`` is an ASCII character.
-``\ooo``, where ``ooo`` is an octal number, represents the ASCII character with the specified value. For example, ``\011`` is the tab character. The highest allowed value is ``\177``.
-``\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. The highest allowed value is \U10FFFF.
-``\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:
As a special case, ``\`` immediately followed by a literal new line is a "continuation" and tells fish to ignore the line break and resume input at the start of the next line (without introducing any whitespace or terminating a token).
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.
- An ampersand (``&``) followed by the number of another file descriptor like ``&2`` for standard error. The output will be written to the destination descriptor.
- An ampersand followed by a minus sign (``&-``). The file descriptor will be closed. Note: This may cause the program to fail because its writes will be unsuccessful.
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 removed. 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::
cat foo.txt | head
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.
It is possible to pipe a different output file descriptor by prepending its FD number and the output redirect symbol to the pipe. For example::
make fish 2>| less
will attempt to build ``fish``, and any errors will be shown using the ``less`` pager. [#]_
..[#] A "pager" here is a program that takes output and "paginates" it. ``less`` doesn't just do pages, it allows arbitrary scrolling (even back!).
.._syntax-job-control:
Job control
-----------
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 :doc:`fg <cmds/fg>` (foreground) command.
At the moment, functions cannot be started in the background. Functions that are stopped and then restarted in the background using the :doc:`bg <cmds/bg>` command will not execute correctly.
If the ``&`` character is followed by a non-separating character, it is not interpreted as background operator. Separating characters are whitespace and the characters ``;<>&|``.
Functions are programs written in the fish syntax. They group together various commands and their arguments using a single name.
For example, here's a simple function to list directories::
function ll
ls -l $argv
end
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.
Fish's prompt is also defined in a function, called :doc:`fish_prompt <cmds/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 :doc:`funced <cmds/funced>`, and to save a function :doc:`funcsave <cmds/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 around ``ls`` might look like this::
There are a few important things that need to be noted about aliases:
- Always take care to add the :ref:`$argv <variables-argv>` variable to the list of parameters to the wrapped command. This makes sure that if the user specifies any additional parameters to the function, they are passed on to the underlying command.
- If the alias has the same name as the aliased command, you need to prefix the call to the program with ``command`` to tell fish that the function should not call itself, but rather a command with the same name. If you forget to do so, the function would call itself until the end of time. Usually fish is smart enough to figure this out and will refrain from doing so (which is hopefully in your interest).
To easily create a function of this form, you can use the :doc:`alias <cmds/alias>` command. Unlike other shells, this just makes functions - fish has no separate concept of an "alias", we just use the word for a simple wrapping function like this. :doc:`alias <cmds/alias>` immediately creates a function. Consider using ``alias --save`` or :doc:`funcsave <cmds/funcsave>` to save the created function into an autoload file instead of recreating the alias each time.
For an alternative, try :ref:`abbreviations <abbreviations>`. These are words that are expanded while you type, instead of being actual functions inside the shell.
.._syntax-function-autoloading:
Autoloading functions
^^^^^^^^^^^^^^^^^^^^^
Functions can be defined on the commandline or in a configuration file, but they can also be automatically loaded. This has some advantages:
- An autoloaded function becomes available automatically to all running shells.
- If the function definition is changed, all running shells will automatically reload the altered version, after a while.
- Startup time and memory usage is improved, etc.
When fish needs to load a function, it searches through any directories in the :ref:`list variable <variables-lists>```$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.
For example if you try to execute something called ``banana``, fish will go through all directories in $fish_function_path looking for a file called ``banana.fish`` and load the first one it finds.
- A directory for users to keep their own functions, usually ``~/.config/fish/functions`` (controlled by the ``XDG_CONFIG_HOME`` environment variable).
- A directory for functions for all users on the system, usually ``/etc/fish/functions`` (really ``$__fish_sysconfdir/functions``).
- Directories for other software to put their own functions. These are in the directories under ``$__fish_user_data_dir`` (usually ``~/.local/share/fish``, controlled by the ``XDG_DATA_HOME`` environment variable) and in the ``XDG_DATA_DIRS`` environment variable, in a subdirectory called ``fish/vendor_functions.d``. The default value for ``XDG_DATA_DIRS`` is usually ``/usr/share/fish/vendor_functions.d`` and ``/usr/local/share/fish/vendor_functions.d``.
As we've explained, autoload files are loaded *by name*, so, while you can put multiple functions into one file, the file will only be loaded automatically once you try to execute the one that shares the name.
Autoloading also won't work for :ref:`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 :ref:`event handlers <event>` section for more information.
If a file of the right name doesn't define the function, fish will not read other autoload files, instead it will go on to try builtins and finally commands. This allows masking a function defined later in $fish_function_path, e.g. if your administrator has put something into /etc/fish/functions that you want to skip.
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.
Comments
--------
Anything after a ``#`` until the end of the line is a comment. That means it's purely for the reader's benefit, fish ignores it.
This is useful to explain what and why you are doing something::
function ls
# The function is called ls,
# so we have to explicitly call `command ls` to avoid calling ourselves.
command ls --color=auto $argv
end
There are no multiline comments. If you want to make a comment span multiple lines, simply start each line with a ``#``.
Fish has some builtins that let you execute commands only if a specific criterion is met: :doc:`if <cmds/if>`, :doc:`switch <cmds/switch>`, :doc:`and <cmds/and>` and :doc:`or <cmds/or>`, and also the familiar :ref:`&&/|| <tut-combiners>` syntax.
The :doc:`switch <cmds/switch>` command is used to execute one of possibly many blocks of commands depending on the value of a string. See the documentation for :doc:`switch <cmds/switch>` for more information.
Unlike programming languages you might know, :doc:`if <cmds/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 :doc:`else <cmds/else>` branch.
Note that combiners are *lazy* - only the part that is necessary to determine the final status is run.
Compare::
if sleep 2; and false
echo 'How did I get here? This should be impossible'
end
and::
if false; and sleep 2
echo 'How did I get here? This should be impossible'
end
These do essentially the same thing, but the former takes 2 seconds longer because the ``sleep`` always needs to run. So, in cases like these, the ordering is quite important for performance.
Or you can have a case where it is necessary to stop early::
if command -sq foo; and foo
If this went on after seeing that the command "foo" doesn't exist, it would try to run ``foo`` and error because it wasn't found!
will print "Still running" once a second. You can abort it with ctrl-c.
``for`` loops work like in other shells, which is more like python's for-loops than e.g. C's::
for file in *
echo file: $file
end
will print each file in the current directory. The part after the ``in`` is just a list of arguments, so you can use any :ref:`expansions <expand>` there::
set moreanimals bird fox
for animal in {cat,}fish dog $moreanimals
echo I like the $animal
end
If you need a list of numbers, you can use the ``seq`` command to create one::
In addition there's a :doc:`begin <cmds/begin>` block that just groups commands together so you can redirect to a block or use a new :ref:`variable scope <variables-scope>` without any repetition::
Parameter expansion is limited to 524288 items. There is a limit to how many arguments the operating system allows for any command, and 524288 is far above it. This is a measure to stop the shell from hanging doing useless computation.
.._expand-wildcard:
Wildcards ("Globbing")
^^^^^^^^^^^^^^^^^^^^^^
When a parameter includes an :ref:`unquoted <quotes>```*`` star (or "asterisk") or a ``?`` question mark, fish uses it as a wildcard to match files.
-``*`` matches any number of characters (including zero) in a file name, not including ``/``.
-``**`` matches any number of characters (including zero), and also descends into subdirectories. If ``**`` is a segment by itself, that segment may match zero times, for compatibility with other shells.
-``?`` can match any single character except ``/``. This is deprecated and can be disabled via the ``qmark-noglob``:ref:`feature flag<featureflags>`, so ``?`` will just be an ordinary character.
Wildcard matches are sorted case insensitively. When sorting matches containing numbers, they are naturally sorted, so that the strings '1' '5' and '12' would be sorted like 1, 5, 12.
Hidden files (where the name begins with a dot) are not considered when wildcarding unless the wildcard string has a dot in that place.
Examples:
-``a*`` matches any files beginning with an 'a' in the current directory.
-``**`` matches any files and directories in the current directory and all of its subdirectories.
-``~/.*`` matches all hidden files (also known as "dotfiles") and directories in your home directory.
For most commands, if any wildcard fails to expand, the command is not executed, :ref:`$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 exceptions, namely :doc:`set <cmds/set>` and :doc:`path <cmds/path>`, overriding variables in :ref:`overrides <variables-override>`, :doc:`count <cmds/count>` and :doc:`for <cmds/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.
Unlike bash (by default), fish will not pass on the literal glob character if no match was found, so for a command like ``apt install`` that does the matching itself, you need to add quotes::
One of the most important expansions in fish is the "variable expansion". This is the replacing of a dollar sign (``$``) followed by a variable name with the _value_ of that variable.
Some variables like ``$HOME`` are already set because fish sets them by default or because fish's parent process passed them to fish when it started it. You can define your own variables by setting them with :doc:`set <cmds/set>`::
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.
Unlike other shells, fish doesn't do what is known as "Word Splitting". Once a variable is set to a particular set of elements, those elements expand as themselves. They aren't split on spaces or newlines or anything::
That means quoting isn't the absolute necessity it is in other shells. Most of the time, not quoting a variable is correct. The exception is when you need to ensure that the variable is passed as one element, even if it might be unset or have multiple elements. This happens often with :doc:`test <cmds/test>`::
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..-1][5]`` (take all elements of ``$foo``, use them as variable names, then give the fifth element of those).
When you write a command in parentheses like ``outercommand (innercommand)``, fish first runs ``innercommand``, and then uses each line of its output as a separate argument to ``outercommand``, which will then be executed. Unlike other shells, the value of ``$IFS`` is not used [#]_, fish splits on newlines.
A command substitution can have a dollar sign before the opening parenthesis like ``outercommand $(innercommand)``. This variant is also allowed inside double quotes. When using double quotes, the command output is not split up by lines, but trailing empty lines are still removed.
If the output is piped to :doc:`string split or string split0 <cmds/string-split>` as the last step, those splits are used as they appear instead of splitting lines.
The exit status of the last run command substitution is available in the :ref:`status <variables-status>` variable if the substitution happens in the context of a :doc:`set <cmds/set>` command (so ``if set -l (something)`` checks if ``something`` returned true).
# Set ``$data`` to the contents of data, splitting on NUL-bytes.
set data (cat data | string split0)
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::
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 :doc:`psub <cmds/psub>` command::
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 ``$status`` is set to 122. This is so command substitutions can't cause the system to go out of memory, because typically your operating system has a much lower limit, so reading more than that would be useless and harmful. This limit can be adjusted with the ``fish_read_limit`` variable (`0` meaning no limit). This limit also affects the :doc:`read <cmds/read>` command.
..[#] One exception: Setting ``$IFS`` to empty will disable line splitting. This is deprecated, use :doc:`string split <cmds/string-split>` instead.
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.
Examples::
> echo input.{c,h,txt}
input.c input.h input.txt
# Move all files with the suffix '.c' or '.h' to the subdirectory src.
> mv *.{c,h} src/
# Make a copy of `file` at `file.bak`.
> cp file{,.bak}
> set -l dogs hot cool cute "good "
> echo {$dogs}dog
hotdog cooldog cutedog good dog
If there is no "," or variable expansion between the curly braces, they will not be expanded::
# This {} isn't special
> echo foo-{}
foo-{}
# This passes "HEAD@{2}" to git
> git reset --hard HEAD@{2}
> echo {{a,b}}
{a} {b} # because the inner brace pair is expanded, but the outer isn't.
If after expansion there is nothing between the braces, the argument will be removed (see :ref:`the cartesian product section <cartesian-product>`)::
> echo foo-{$undefinedvar}
# Output is an empty line, just like a bare `echo`.
If there is nothing between a brace and a comma or two commas, it's interpreted as an empty element::
> echo {,,/usr}/bin
/bin /bin /usr/bin
To use a "," as an element, :ref:`quote <quotes>` or :ref:`escape <escapes>` it.
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>`.
# All elements in the brace combine with the parts outside of the braces
>_ echo {good,bad}" apples"
good apples bad apples
# The same thing happens with variable expansion.
>_ set -l a x y z
>_ set -l b 1 2 3
# $a is {x,y,z}, $b is {1,2,3},
# so this is `echo {x,y,z}{1,2,3}`
>_ echo $a$b
x1 y1 z1 x2 y2 z2 x3 y3 z3
# Same thing if something is between the lists
>_ echo $a"-"$b
x-1 y-1 z-1 x-2 y-2 z-2 x-3 y-3 z-3
# Or a brace expansion and a variable
>_ echo {x,y,z}$b
x1 y1 z1 x2 y2 z2 x3 y3 z3
# A combined brace-variable expansion
>_ echo {$b}word
1word 2word 3word
# Special case: If $c has no elements, this expands to nothing
>_ echo {$c}word
# Output is an empty line
Sometimes this may be unwanted, especially that tokens can disappear after expansion. In those cases, you should double-quote variables - ``echo "$c"word``.
This also happens after :ref:`command substitution <expand-command-substitution>`. To avoid tokens disappearing there, make the inner command return a trailing newline, or store the output in a variable and double-quote it.
E.g.
::
>_ set b 1 2 3
>_ echo (echo x)$b
x1 x2 x3
>_ echo (printf '%s' '')banana
# the printf prints nothing, so this is nothing times "banana",
# which is nothing.
>_ echo (printf '%s\n' '')banana
# the printf prints a newline,
# so the command substitution expands to an empty string,
Because :envvar:`PATH` is a list, this expands to all the files in all the directories in it. And if there are no directories in :envvar:`PATH`, the right answer here is to expand to no files.
Sometimes it's necessary to access only some of the elements of a :ref:`list <variables-lists>` (all fish variables are lists), or some of the lines a :ref:`command substitution <expand-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 an element that doesn't exist to the second element, 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.
Multiple ranges are also possible, separated with a space.
Some examples::
echo (seq 10)[1 2 3]
# Prints: 1 2 3
# Limit the command substitution output
echo (seq 10)[2..5]
# Uses elements from 2 to 5
# Output is: 2 3 4 5
echo (seq 10)[7..]
# Prints: 7 8 9 10
# Use overlapping ranges:
echo (seq 10)[2..5 1..3]
# Takes elements from 2 to 5 and then elements from 1 to 3
# Output is: 2 3 4 5 1 2 3
# Reverse output
echo (seq 10)[-1..1]
# Uses elements from the last output line to
# the first one in reverse direction
# Output is: 10 9 8 7 6 5 4 3 2 1
# The command substitution has only one line,
# so these will result in empty output:
echo (echo one)[2..-1]
echo (echo one)[-3..1]
The same works when setting or expanding variables::
# Reverse path variable
set PATH $PATH[-1..1]
# or
set PATH[-1..1] $PATH
# Use only n last items of the PATH
set n -3
echo $PATH[$n..-1]
Variables can be used as indices for expansion of variables, like so::
set index 2
set letters a b c d
echo $letters[$index] # returns 'b'
However using variables as indices for command substitution is currently not supported, so::
echo (seq 5)[$index] # This won't work
set sequence (seq 5) # It needs to be written on two lines like this.
echo $sequence[$index] # returns '2'
When using indirect variable expansion with multiple ``$`` (``$$name``), you have to give all indices up to the variable you want to slice::
> set -l list 1 2 3 4 5
> set -l name list
> echo $$name[1]
1 2 3 4 5
> echo $$name[1..-1][1..3] # or $$name[1][1..3], since $name only has one element.
1 2 3
.._expand-home:
Home directory expansion
^^^^^^^^^^^^^^^^^^^^^^^^
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::
ls ~/Music # lists my music directory
echo ~root # prints root's home directory, probably "/root"
.._combine:
Combining different expansions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
All of the above expansions can be combined. If several expansions result in more than one parameter, all possible combinations are created.
When combining multiple parameter expansions, expansions are performed in the following order:
- Command substitutions
- Variable expansions
- Bracket expansion
- Wildcard expansion
Expansions are performed from right to left, nested bracket expansions are performed from the inside and out.
Example:
If the current directory contains the files 'foo' and 'bar', the command ``echo a(ls){1,2,3}`` will output ``abar1 abar2 abar3 afoo1 afoo2 afoo3``.
.._variables:
Shell variables
---------------
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 :doc:`set <cmds/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.
- Function variables are specific to the currently executing function. They are erased ("go out of scope") when the current function ends. Outside of a function, they don't go out of scope.
- Local variables are specific to the current 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``. Outside of a block, this is the same as the function scope.
Variables can be explicitly set to be universal with the ``-U`` or ``--universal`` switch, global with ``-g`` or ``--global``, function-scoped with ``-f`` or ``--function`` and local to the current block with ``-l`` or ``--local``. The scoping rules when creating or updating a variable are:
- When no scope is given and no variable of that name exists, the variable is created in function scope if inside a function, or global scope if no function is executing.
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.
Example:
There are a few possible uses for different scopes.
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
When in doubt, use function-scoped variables. When you need to make a variable accessible everywhere, make it global. When you need to persistently store configuration, make it universal. When you want to use a variable only in a short block, make it local.
foo=gagaga echo $foo # prints gagaga, while in other shells it might print "banana"
Multiple elements can be given in a :ref:`brace expansion<expand-brace>`::
# Call bash with a reasonable default path.
PATH={/usr,}/{s,}bin bash
Or with a :ref:`glob <expand-wildcard>`::
# Run vlc on all mp3 files in the current directory
# If no file exists it will still be run with no arguments
mp3s=*.mp3 vlc $mp3s
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.
This syntax is supported since fish 3.1.
.._variables-universal:
More on universal variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.
:ref:`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 <configuration>`, 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 ``PAGER`` 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. Exported variables are also often called "environment variables".
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 :envvar:`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 similar to the scoping rules for variables - when an option is passed it is respected, otherwise the variable's existing state is used. If no option is passed and the variable didn't exist yet it is not exported.
Note: Exporting is not a :ref:`scope <variables-scope>`, but an additional state. 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. Global variables are accessible to functions whether they are exported or not.
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``::
To create a variable ``smurf``, containing the items ``blue`` and ``small``, simply write::
set smurf blue small
It is also possible to set or erase individual elements of a list::
# Set smurf to be a list with the elements 'blue' and 'small'
set smurf blue small
# Change the second element of smurf to 'evil'
set smurf[2] evil
# Erase the first element
set -e smurf[1]
# Output 'evil'
echo $smurf
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::
> set fruit apple orange banana
> echo $fruit[-1]
banana
> echo $fruit[-2..-1]
orange
banana
> echo $fruit[-1..1] # reverses the list
banana
orange
apple
As you see, you can use a range of indices, see :ref:`index range expansion <expand-index-range>` for details.
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 :envvar:`PATH`, :envvar:`CDPATH` or :envvar:`MANPATH`), by splitting them on colons. Other variables are not automatically split.
That covers the positional arguments, but commandline tools often get various options and flags, and $argv would contain them intermingled with the positional arguments. Typical unix argument handling allows short options (``-h``, also grouped like in ``ls -lah``), long options (``--help``) and allows those options to take arguments (``--color=auto`` or ``--position anywhere`` or ``complete -C"git "``) as well as a ``--`` separator to signal the end of options. Handling all of these manually is tricky and error-prone.
A more robust approach to option handling is :doc:`argparse <cmds/argparse>`, which checks the defined options and puts them into various variables, leaving only the positional arguments in $argv. Here's a simple example::
Path variables are a special kind of variable used to support colon-delimited path lists including :envvar:`PATH`, :envvar:`CDPATH`, :envvar:`MANPATH`, :envvar:`PYTHONPATH`, etc. All variables that end in "PATH" (case-sensitive) become PATH variables by default.
Path variables will also be exported in the colon form, so ``set -x MYPATH 1 2 3`` will have external commands see it as ``1:2:3``.
::
> set -gx MYPATH /bin /usr/bin /sbin
> env | grep MYPATH
MYPATH=/bin:/usr/bin:/sbin
This is for compatibility with other tools. Unix doesn't have variables with multiple elements, the closest thing it has are colon-lists like :envvar:`PATH`. For obvious reasons this means no element can contain a ``:``.
The locale variables :envvar:`LANG`, :envvar:`LC_ALL`, :envvar:`LC_COLLATE`, :envvar:`LC_CTYPE`, :envvar:`LC_MESSAGES`, :envvar:`LC_MONETARY`, :envvar:`LC_NUMERIC`, and :envvar:`LANG` set the language option for the shell and subprograms. See the section :ref:`Locale variables <variables-locale>` for more information.
A number of variable starting with the prefixes ``fish_color`` and ``fish_pager_color``. See :ref:`Variables for changing highlighting colors <variables-color>` for more information.
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.
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.
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.
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>`.
determines where fish looks for completion. When trying to complete for a command, fish looks for files in the directories in this variable.
..envvar:: fish_function_path
determines where fish looks for functions. When fish :ref:`autoloads <syntax-function-autoloading>` a function, it will look for files in these directories.
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 :doc:`funced <cmds/funced>`)
the current history session name. If set, all subsequent commands within an
interactive fish session will be logged to a separate file identified by the value of the
variable. If unset, the default session name "fish" is used. If set to an
empty string, history is not saved to disk (but is still available within the interactive
session).
..envvar:: 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 `--debug-output` option to fish or the :envvar:`FISH_DEBUG_OUTPUT` variable. It goes to stderr by default.
the current file creation mask. The preferred way to change the umask variable is through the :doc:`umask <cmds/umask>` function. An attempt to set umask to an invalid value will always fail.
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``.
the name of the currently running command (though this is deprecated, and the use of ``status current-command`` is preferred).
..envvar:: 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.
..envvar:: CMD_DURATION
the runtime of the last command in milliseconds.
..describe:: 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.
..envvar:: fish_kill_signal
the signal that terminated the last foreground job, or 0 if the job exited normally.
..envvar:: fish_killring
a list of entries in fish's :ref:`kill ring <killring>` of cut text.
the internal field separator that is used for word splitting with the :doc:`read <cmds/read>` builtin. Setting this to the empty string will also disable line splitting in :ref:`command substitution <expand-command-substitution>`. This variable can be changed.
the :ref:`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.
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).
the type of the current terminal. When fish tries to determine how the terminal works - how many colors it supports, what sequences it sends for keys and other things - it looks at this variable and the corresponding information in the terminfo database (see ``man terminfo``).
Note: Typically this should not be changed as the terminal sets it to the correct value.
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.
.._variables-status:
The status variable
^^^^^^^^^^^^^^^^^^^
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.
Fish stores the exit status of the last process in the last job to exit in the ``status`` variable.
If fish encounters a problem while executing a command, the status variable may also be set to a specific value:
The status can be negated with :doc:`not <cmds/not>` (or ``!``), which is useful in a :ref:`condition <syntax-conditional>`. This turns a status of 0 into 1 and any non-zero status into 0.
There is also ``$pipestatus``, which is a list of all ``status`` values of processes in a pipe. One difference is that :doc:`not <cmds/not>` applies to ``$status``, but not ``$pipestatus``, because it loses information.
Here ``$status`` reflects the status of ``grep``, which returns 0 if it found something, negated with ``not`` (so 1 if it found something, 0 otherwise). ``$pipestatus`` reflects the status of ``cat`` (which returns non-zero for example when it couldn't find the file) and ``grep``, without the negation.
So if both ``cat`` and ``grep`` succeeded, ``$status`` would be 1 because of the ``not``, and ``$pipestatus`` would be 0 and 0.
It's possible for the first command to fail while the second succeeds. One common example is when the second program quits early.
For example, if you have a pipeline like::
cat file1 file2 | head -n 50
This will tell ``cat`` to print two files, "file1" and "file2", one after the other, and the ``head`` will then only print the first 50 lines. In this case you might often see this constellation::
> cat file1 file2 | head -n 50
# 50 lines of output
> echo $pipestatus
141 0
Here, the "141" signifies that ``cat`` was killed by signal number 13 (128 + 13 == 141) - a ``SIGPIPE``. You can also use :envvar:`fish_kill_signal` to see the signal number. This happens because it was still working, and then ``head`` closed the pipe, so ``cat`` received a signal that it didn't ignore and so it died.
Whether ``cat`` here will see a SIGPIPE depends on how long the file is and how much it writes at once, so you might see a pipestatus of "0 0", depending on the implementation. This is a general unix issue and not specific to fish. Some shells feature a "pipefail" feature that will call a pipeline failed if one of the processes in it failed, and this is a big problem with it.
The "locale" of a program is its set of language and regional settings that depend on language and cultural convention. In UNIX, these are made up of several categories. The categories are:
This is the typical environment variable for specifying a locale. A user may set this variable to express the language they speak, their region, and a character encoding. The actual values are specific to their platform, except for special values like ``C`` or ``POSIX``.
The value of LANG is used for each category unless the variable for that category was set or LC_ALL is set. So typically you only need to set LANG.
An example value might be ``en_US.UTF-8`` for the american version of english and the UTF-8 encoding, or ``de_AT.UTF-8`` for the austrian version of german and the UTF-8 encoding.
Your operating system might have a ``locale`` command that you can call as ``locale -a`` to see a list of defined locales.
Overrides the :envvar:`LANG` environment variable and the values of the other ``LC_*`` variables. If this is set, none of the other variables are used for anything.
Usually the other variables should be used instead. Use LC_ALL only when you need to override something.
This determines classification rules, like if the type of character is an alpha, digit, and so on.
Most importantly, it defines the text *encoding* - which numbers map to which characters. On modern systems, this should typically be something ending in "UTF-8".
- Builtins for dealing with data, like :doc:`string <cmds/string>` for strings and :doc:`math <cmds/math>` for numbers, :doc:`count <cmds/count>` for counting lines or arguments, :doc:`path <cmds/path>` for dealing with path
-:doc:`status <cmds/status>` for asking about the shell's status
-:doc:`printf <cmds/printf>` and :doc:`echo <cmds/echo>` for creating output
-:doc:`test <cmds/test>` for checking conditions
-:doc:`argparse <cmds/argparse>` for parsing function arguments
-:doc:`source <cmds/source>` to read a script in the current shell (so changes to variables stay) and :doc:`eval <cmds/eval>` to execute a string as script
-:doc:`random <cmds/random>` to get random numbers or pick a random element from a list
-:doc:`read <cmds/read>` for reading from a pipe or the terminal
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.
When fish is told to run something, it goes through multiple steps to find it.
If it contains a ``/``, fish tries to execute the given file, from the current directory on.
If it doesn't contain a ``/``, it could be a function, builtin, or external command, and so fish goes through the full lookup.
In order:
1. It tries to resolve it as a :ref:`function <syntax-function>`.
- If the function is already known, it uses that
- If there is a file of the name with a ".fish" suffix in :envvar:`fish_function_path`, it :ref:`loads that <syntax-function-autoloading>`. (If there is more than one file only the first is used)
- If the function is now defined it uses that
2. It tries to resolve it as a :ref:`builtin <builtin-overview>`.
3. It tries to find an executable file in :envvar:`PATH`.
- If it finds a file, it tells the kernel to run it.
- If the kernel knows how to run the file (e.g. via a ``#!`` line - ``#!/bin/sh`` or ``#!/usr/bin/python``), it does it.
- If the kernel reports that it couldn't run it because of a missing interpreter, and the file passes a rudimentary check, fish tells ``/bin/sh`` to run it.
If none of these work, fish runs the function :doc:`fish_command_not_found <cmds/fish_command_not_found>` and sets :envvar:`status` to 127.
You can use :doc:`type <cmds/type>` to see how fish resolved something::
Let's make up an example. This function will :ref:`glob <expand-wildcard>` the files in all the directories it gets as :ref:`arguments <variables-argv>`, and :ref:`if <syntax-conditional>` there are :doc:`more than five <cmds/test>` it will ask the user if it is supposed to show them, but only if it is connected to a terminal::
# This will glob on all arguments. Any non-directories will be ignored.
set -l files $argv/*
# If there are more than 5 files
if test (count $files) -gt 5
# and both stdin (for reading input) and stdout (for writing the prompt)
# are terminals
and isatty stdin
and isatty stdout
# Keep asking until we get a valid response
while read --nchars 1 -l response --prompt-str="Are you sure? (y/n)"
or return 1 # if the read was aborted with ctrl-c/ctrl-d
switch $response
case y Y
echo Okay
# We break out of the while and go on with the function
break
case n N
# We return from the function without printing
echo Not showing
return 1
case '*'
# We go through the while loop and ask again
echo Not valid input
continue
end
end
end
# And now we print the files
printf '%s\n' $files
end
If you run this as ``show_files /``, it will most likely ask you until you press Y/y or N/n. If you run this as ``show_files / | cat``, it will print the files without asking. If you run this as ``show_files .``, it might just print something without asking because there are fewer than five files.
- A function name cannot be empty. It may not begin with a hyphen ("-") and may not contain a slash ("/"). All other characters, including a space, are valid. A function name also can't be the same as a reserved keyword or essential builtin like ``if`` or ``set``.
- A bind mode name (e.g., ``bind -m abc ...``) must be a valid variable name.
Other things have other restrictions. For instance what is allowed for file names depends on your system, but at the very least they cannot contain a "/" (because that is the path separator) or NULL byte (because that is how UNIX ends strings).
- Directories for others to ship configuration snippets for their software. Fish searches the directories under ``$__fish_user_data_dir`` (usually ``~/.local/share/fish``, controlled by the ``XDG_DATA_HOME`` environment variable) and in the ``XDG_DATA_DIRS`` environment variable for a ``fish/vendor_conf.d`` directory; if not defined, the default value of ``XDG_DATA_DIRS`` is ``/usr/share/fish/vendor_conf.d`` and ``/usr/local/share/fish/vendor_conf.d``, unless your distribution customized this.
If there are multiple files with the same name in these directories, only the first will be executed.
They are executed in order of their filename, sorted (like globs) in a natural order (i.e. "01" sorts before "2").
- System-wide configuration files, where administrators can include initialization for all users on the system - similar to ``/etc/profile`` for POSIX-style shells - in ``$__fish_sysconf_dir`` (usually ``/etc/fish/config.fish``).
- User configuration, usually in ``~/.config/fish/config.fish`` (controlled by the ``XDG_CONFIG_HOME`` environment variable, and accessible as ``$__fish_config_dir``).
``~/.config/fish/config.fish`` is sourced *after* the snippets. This is so you can copy snippets and override some of their behavior.
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 want to add configuration for all users of fish 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 won't benefit from the fish-specific configuration. However, if they are required, you can install them to the "vendor" configuration directory. As this path may vary from system to system, ``pkg-config`` should be used to discover it: ``pkg-config --variable confdir fish``.
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.
You can see the current list of features via ``status features``::
-``stderr-nocaret`` was introduced in fish 3.0 (and made the default in 3.3). It makes ``^`` an ordinary character instead of denoting an stderr redirection, to make dealing with quoting and such easier. Use ``2>`` instead. This can no longer be turned off since fish 3.5. The flag can still be tested for compatibility, but a ``no-stderr-nocaret`` value will simply be ignored.
-``qmark-noglob`` was also introduced in fish 3.0. It makes ``?`` an ordinary character instead of a single-character glob. Use a ``*`` instead (which will match multiple characters) or find other ways to match files like ``find``.
-``regex-easyesc`` was introduced in 3.1. It makes it so the replacement expression in ``string replace -r`` does one fewer round of escaping. Before, to escape a backslash you would have to use ``string replace -ra '([ab])' '\\\\\\\\$1'``. After, just ``'\\\\$1'`` is enough. Check your ``string replace`` calls if you use this anywhere.
-``ampersand-nobg-in-token`` was introduced in fish 3.4. It makes it so a ``&`` i no longer interpreted as the backgrounding operator in the middle of a token, so dealing with URLs becomes easier. Either put spaces or a semicolon after the ``&``. This is recommended formatting anyway, and ``fish_indent`` will have done it for you already.
You can also use the version as a group, so ``3.0`` is equivalent to "stderr-nocaret" and "qmark-noglob". Instead of a version, the special group ``all`` enables all features.
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:
- When a signal is delivered
- When a job exits
- When the value of a variable is updated
- When the prompt is about to be shown
Example:
To specify a signal handler for the WINCH signal, write::
Fish already the following named events for the ``--on-event`` switch:
-``fish_prompt`` is emitted whenever a new fish prompt is about to be displayed.
-``fish_preexec`` is emitted right before executing an interactive command. The commandline is passed as the first parameter. Not emitted if command is empty.
-``fish_posterror`` is emitted right after executing a command with syntax errors. The commandline is passed as the first parameter.
-``fish_postexec`` is emitted right after executing an interactive command. The commandline is passed as the first parameter. Not emitted if command is empty.
-``fish_exit`` is emitted right before fish exits.
-``fish_cancel`` is emitted when a commandline is cleared.
Events can be fired with the :doc:`emit <cmds/emit>` command, and do not have to be defined before. The names just need to match. For example::
function handler --on-event imdone
echo generator is done $argv
end
function generator
sleep 1
# The "imdone" is the name of the event
# the rest is the arguments to pass to the handler
emit imdone with $argv
end
If there are multiple handlers for an event, they will all be run, but the order might change between fish releases, so you should not rely on it.
Please note that event handlers only become active when a function is loaded, which means you need to otherwise :doc:`source <cmds/source>` or execute a function instead of relying on :ref:`autoloading <syntax-function-autoloading>`. One approach is to put it into your :ref:`configuration file <configuration>`.
Fish includes basic built-in debugging facilities that allow you to stop execution of a script at an arbitrary point. When this happens you are presented with an interactive prompt where you can execute any fish command to inspect or change state (there are no debug commands as such). For example, you can check or change the value of any variables using :doc:`printf <cmds/printf>` and :doc:`set <cmds/set>`. As another example, you can run :doc:`status print-stack-trace <cmds/status>` to see how the current breakpoint was reached. To resume normal execution of the script, simply type :doc:`exit <cmds/exit>` or :kbd:`Control`\ +\ :kbd:`D`.
To start a debug session simply insert the :doc:`builtin command <cmds/breakpoint>```breakpoint`` at the point in a function or script where you wish to gain control, then run the function or script. Also, the default action of the ``TRAP`` signal is to call this builtin, meaning a running script can be actively debugged by sending it the ``TRAP`` signal (``kill -s TRAP <PID>``). There is limited support for interactively setting or modifying breakpoints from this debug prompt: it is possible to insert new breakpoints in (or remove old ones from) other functions by using the ``funced`` function to edit the definition of a function, but it is not possible to add or remove a breakpoint from the function/script currently loaded and being executed.
Another way to debug script issues is to set the :envvar:`fish_trace` variable, e.g. ``fish_trace=1 fish_prompt`` to see which commands fish executes when running the :doc:`fish_prompt <cmds/fish_prompt>` function.
If you specifically want to debug performance issues, :program:`fish` can be run with the ``--profile /path/to/profile.log`` option to save a profile to the specified path. This profile log includes a breakdown of how long each step in the execution took. See :doc:`fish <cmds/fish>` for more information.
