.. _tutorial:

*******************
Tutorial
*******************

xonsh is a shell language and command prompt. Unlike other shells, xonsh is
based on Python, with additional syntax added that makes calling subprocess
commands, manipulating the environment, and dealing with the file system
easy.  The xonsh command prompt gives users interactive access to the xonsh
language.

While all Python code is also xonsh, not all Bash code can be used in xonsh.
That would defeat the purpose, and Python is better anyway! Still, xonsh is
Bash-wards compatible in the ways that matter, such as for running commands,
reading in the Bash environment, and utilizing Bash tab completion.

The purpose of this tutorial is to teach you xonsh. There are many excellent
guides out there for learning Python, and this will not join their ranks.
Similarly, you'd probably get the most out of this tutorial if you have already
used a command prompt or interactive interpreter.

Let's dive in!

Starting xonsh
========================
Assuming you have successfully installed xonsh (see http://xon.sh),
you can start up the xonsh interpreter via the ``xonsh`` command. Suppose
you are in a lesser terminal:

.. code-block:: console

    $ xonsh
    snail@home ~ $

Now we are in a xonsh shell. Our username happens to be ``snail``, our
hostname happens to be ``home``, and we are in our home directory (``~``).
Alternatively, you can setup your terminal emulator (xterm, gnome-terminal,
etc) to run xonsh automatically when it starts up. This is recommended.

Basics
=======================
The xonsh language is based on Python, and the xonsh shell uses Python to
interpret any input it receives. This makes simple things, like arithmetic,
simple:

.. code-block:: xonshcon

    >>> 1 + 1
    2

.. note:: From here on we'll be using ``>>>`` to prefix (or prompt) any
          xonsh input. This follows the Python convention and helps trick
          syntax highlighting, though ``$`` is more traditional for shells.

Since this is just Python, we are able to import modules, print values,
and use other built-in Python functionality:

.. code-block:: xonshcon

    >>> import sys
    >>> print(sys.version)
    3.4.2 |Continuum Analytics, Inc.| (default, Oct 21 2014, 17:16:37)
    [GCC 4.4.7 20120313 (Red Hat 4.4.7-1)]


We can also create and use literal data types, such as ints, floats, lists,
sets, and dictionaries. Everything that you are used to if you already know
Python is there:

.. code-block:: xonshcon

    >>> d = {'xonsh': True}
    >>> d.get('bash', False)
    False

The xonsh shell also supports multi-line input for more advanced flow control.
The multi-line mode is automatically entered whenever the first line of input
is not syntactically valid on its own.  Multi-line mode is then exited when
enter (or return) is pressed when the cursor is in the first column.

.. code-block:: xonshcon

    >>> if True:
    ...     print(1)
    ... else:
    ...     print(2)
    ...
    1

Flow control, of course, includes loops.

.. code-block:: xonshcon

    >>> for i, x in enumerate('xonsh'):
    ...     print(i, x)
    ...
    0 x
    1 o
    2 n
    3 s
    4 h

We can also define and call functions and classes. I'll mostly spare you the
details, but this *is* pretty cool:

.. code-block:: xonshcon

    >>> def f():
    ...     return "xonsh"
    ...
    >>> f()
    'xonsh'

For easier indentation, Shift+Tab will enter 4 spaces.
And that about wraps it up for the basics section.  It is just like Python.

Environment Variables
=======================
Environment variables are written as ``$`` followed by a name.  For example,
``$HOME``, ``$PWD``, and ``$PATH``.

.. code-block:: xonshcon

    >>> $HOME
    '/home/snail'

You can set (and export) environment variables like you would set any other
variable in Python.  The same is true for deleting them too.

.. code-block:: xonshcon

    >>> $GOAL = 'Become the Lord of the Files'
    >>> print($GOAL)
    Become the Lord of the Files
    >>> del $GOAL

Very nice.

.. note::

   To update ``os.environ`` when the xonsh environment changes set
   :ref:`$UPDATE_OS_ENVIRON <update_os_environ>` to ``True``.

The Environment Itself ``${...}``
---------------------------------

All environment variables live in the built-in ``${...}`` (aka ``__xonsh__.env``) mapping.
You can access this mapping directly, but in most situations, you shouldn’t need to.

If you want for example to check if an environment variable is present in your current
session (say, in your awesome new ``xonsh`` script) you can use the membership operator:

.. code-block:: xonshcon

   >>> 'HOME' in ${...}
   True

To get information about a specific environment variable you can use the
:func:`~xonsh.environ.Env.help` method.

.. code-block:: xonshcon

   >>> ${...}.help('XONSH_DEBUG')

One helpful method on the ``${...}`` is :func:`~xonsh.environ.Env.swap`.
It can be used to temporarily set an environment variable:

.. code-block:: xonshcon

    >>> with ${...}.swap(SOMEVAR='foo'):
    ...     echo $SOMEVAR
    ...
    ...
    foo
    >>> echo $SOMEVAR

    >>>

Environment Lookup with ``${<expr>}``
-------------------------------------

The ``$NAME`` is great as long as you know the name of the environment
variable you want to look up.  But what if you want to construct the name
programmatically, or read it from another variable?  Enter the ``${}``
operator.

.. warning:: In Bash, ``$NAME`` and ``${NAME}`` are syntactically equivalent.
             In xonsh, they have separate meanings.

We can place any valid Python expression inside of the curly braces in
``${<expr>}``. This result of this expression will then be used to look up a
value in the environment. Here are a couple of examples in action:

.. code-block:: xonshcon

    >>> x = 'USER'
    >>> ${x}
    'snail'
    >>> ${'HO' + 'ME'}
    '/home/snail'

Not bad, xonsh, not bad.

Environment Types
-----------------

Like other variables in Python, environment variables have a type. Sometimes
this type is imposed based on the variable name. The current rules are pretty
simple:

* ``\w*PATH``: any variable whose name ends in PATH is a list of strings.
* ``\w*DIRS``: any variable whose name ends in DIRS is a list of strings.

Futhermore, a number of predefined environment variables listed `here <envvars.html>`_ have a static type.
For example,
* ``XONSH_HISTORY_SIZE``: is an int, and
* ``CASE_SENSITIVE_COMPLETIONS``: is a boolean.

xonsh will automatically convert back and forth to untyped (string-only)
representations of the environment as needed (mostly by subprocess commands).
When in xonsh, you'll always have the typed version.

Variables that do not match the rules above are converted to strings using ``str``,
except they are ``None``. In this case the empty string is used.

Here are a couple of
PATH examples:

.. code-block:: xonshcon

    >>> $PATH
    ['/home/snail/.local/bin', '/home/snail/sandbox/bin',
    '/home/snail/miniconda3/bin', '/usr/local/bin', '/usr/local/sbin',
    '/usr/bin', '/usr/sbin', '/bin', '/sbin', '.']
    >>> $LD_LIBRARY_PATH
    ['/home/snail/.local/lib', '']

Also note that *any* Python object can go into the environment. It is sometimes
useful to have more sophisticated types, like functions, in the environment.
There are handful of environment variables that xonsh considers special.
They can be seen on the `Environment Variables page <envvars.html>`_.

.. note:: In subprocess mode, referencing an undefined environment variable
          will produce an empty string.  In Python mode, however, a
          ``KeyError`` will be raised if the variable does not exist in the
          environment.


Running Commands
==============================
As a shell, xonsh is meant to make running commands easy and fun.
Running subprocess commands should work like in any other shell.

.. code-block:: xonshcon

    >>> echo "Yoo hoo"
    Yoo hoo
    >>> cd xonsh
    >>> ls
    build  docs     README.rst  setup.py  xonsh           __pycache__
    dist   license  scripts     tests     xonsh.egg-info
    >>> dir scripts
    xonsh  xonsh.bat
    >>> git status
    On branch main
    Your branch is up-to-date with 'origin/main'.
    Changes not staged for commit:
      (use "git add <file>..." to update what will be committed)
      (use "git checkout -- <file>..." to discard changes in working directory)

        modified:   docs/tutorial.rst

    no changes added to commit (use "git add" and/or "git commit -a")
    >>> exit

This should feel very natural.


Python-mode vs Subprocess-mode
================================
It is sometimes helpful to make the distinction between lines that operate
in pure Python mode and lines that use shell-specific syntax, edit the
execution environment, and run commands. Unfortunately, it is not always
clear from the syntax alone what mode is desired. This ambiguity stems from
most command line utilities looking a lot like Python operators.

Take the case of ``ls -l``.  This is valid Python code, though it could
have also been written as ``ls - l`` or ``ls-l``.  So how does xonsh know
that ``ls -l`` is meant to be run in subprocess-mode?

For any given line that only contains an expression statement (expr-stmt,
see the Python AST docs for more information), if all the names cannot
be found as current variables xonsh will try to parse the line as a
subprocess command instead.  In the above, if ``ls`` and ``l`` are not
variables, then subprocess mode will be attempted. If parsing in subprocess
mode fails, then the line is left in Python-mode.

In the following example, we will list the contents of the directory
with ``ls -l``. Then we'll make new variable names ``ls`` and ``l`` and then
subtract them. Finally, we will delete ``ls`` and ``l`` and be able to list
the directories again.

.. code-block:: xonshcon

    >>> # this will be in subproc-mode, because ls doesn't exist
    >>> ls -l
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh
    >>> # set ls and l variables to force python-mode
    >>> ls = 44
    >>> l = 2
    >>> ls -l
    42
    >>> # deleting ls will return us to subproc-mode
    >>> del ls
    >>> ls -l
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh

The determination between Python- and subprocess-modes is always done in the
safest possible way. If anything goes wrong, it will favor Python-mode.
The determination between the two modes is done well ahead of any execution.
You do not need to worry about partially executed commands - that is
impossible.

.. note:: If you would like to explicitly run a subprocess command, you can always
          use the formal xonsh subprocess syntax that we will see in the following
          sections. For example: ``![ls -l]``.

Quoting
=======

Single or double quotes can be used to remove the special meaning
of certain characters or words to xonsh. If a subprocess command
contains characters that collide with xonsh syntax then quotes
must be used to force xonsh to not interpret them.

.. code-block:: xonshcon

    >>> echo ${
    ...
    SyntaxError: <xonsh-code>:1:5: ('code: {',)
    echo ${
         ^
    >>> echo '${'
    ${

.. warning:: There is no notion of an escaping character in xonsh like the
             backslash (\\) in bash.


Captured Subprocess with ``$()`` and ``!()``
============================================
The ``$(<expr>)`` operator in xonsh executes a subprocess command and
*captures* some information about that command.

The ``$()`` syntax captures and returns the standard output stream of the
command as a Python string.  This is similar to how ``$()`` performs in Bash.
For example,

.. code-block:: xonshcon

    >>> $(ls -l)
    'total 0\n-rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh\n'

The ``!()`` syntax captured more information about the command, as an instance
of a class called ``CommandPipeline``.  This object contains more information
about the result of the given command, including the return code, the process
id, the standard output and standard error streams, and information about how
input and output were redirected.  For example:

.. code-block:: xonshcon

    >>> !(ls nonexistent_directory)
    CommandPipeline(
        stdin=<_io.BytesIO object at 0x7f1948182bf8>,
        stdout=<_io.BytesIO object at 0x7f1948182af0>,
        stderr=<_io.BytesIO object at 0x7f19483a6200>,
        pid=26968,
        returncode=2,
        args=['ls', 'nonexistent_directory'],
        alias=['ls', '--color=auto', '-v'],
        stdin_redirect=['<stdin>', 'r'],
        stdout_redirect=[9, 'wb'],
        stderr_redirect=[11, 'w'],
        timestamps=[1485235484.5016758, None],
        executed_cmd=['ls', '--color=auto', '-v', 'nonexistent_directory'],
        input=None,
        output=,
        errors=None
    )

This object will be "truthy" if its return code was 0, and it is equal (via
``==``) to its return code. It also hashes to its return code. Converting the object
to the string will return the output. This allows for some interesting new
kinds of interactions with subprocess commands, for example:

.. code-block:: xonshcon

    def check_file(file):
        if !(test -e @(file)):
            if !(test -f @(file)) or !(test -d @(file)):
                print("File is a regular file or directory")
            else:
                print("File is not a regular file or directory")
        else:
            print("File does not exist")

    def wait_until_google_responds():
        while not !(ping -c 1 google.com):
            sleep 1


If you iterate over the ``CommandPipeline`` object, it will yield lines of its
output.  Using this, you can quickly and cleanly process output from commands.
Additionally, these objects expose a method ``itercheck``, which behaves the same
as the built-in iterator but raises ``XonshCalledProcessError`` if the process
had a nonzero return code.

.. code-block:: xonshcon

    def get_wireless_interface():
        """Returns devicename of first connected wifi, None otherwise"""
        for line in !(nmcli device):
            dev, typ, state, conn_name = line.split(None, 3)
            if typ == 'wifi' and state == 'connected':
                return dev

    def grep_path(path, regexp):
        """Recursively greps `path` for perl `regexp`

        Returns a dict of 'matches' and 'failures'.
        Matches are files that contain the given regexp.
        Failures are files that couldn't be scanned.
        """
        matches = []
        failures = []

        try:
            for match in !(grep -RPl @(regexp) @(str(path))).itercheck():
                matches.append(match)
        except XonshCalledProcessError as error:
            for line in error.stderr.split('\n'):
                if not line.strip():
                    continue
                filename = line.split('grep: ', 1)[1].rsplit(':', 1)[0]
                failures.append(filename)
        return {'matches': matches, 'failures': failures}


The ``$()`` and ``!()`` operators are expressions themselves. This means that
we can assign the results to a variable or perform any other manipulations we
want.

.. code-block:: xonshcon

    >>> x = $(ls -l)
    >>> print(x.upper())
    TOTAL 0
    -RW-RW-R-- 1 SNAIL SNAIL 0 MAR  8 15:46 XONSH
    >>> y = !(ls -l)
    >>> print(y.returncode)
    0
    >>> print(y.rtn)  # alias to returncode
    0


.. warning:: Job control is not implemented for captured subprocesses.

While in subprocess-mode or inside of a captured subprocess, we can always
still query the environment with ``$NAME`` variables or the ``${}`` syntax,
or inject Python values with the ``@()`` operator:

.. code-block:: xonshcon

    >>> $(echo $HOME)
    '/home/snail\n'

Uncaptured Subprocess with ``$[]`` and ``![]``
===============================================
Uncaptured subprocesses are denoted with the ``$[]`` and ``![]`` operators. They are
the same as ``$()`` captured subprocesses in almost every way. The only
difference is that the subprocess's stdout passes directly through xonsh and
to the screen.  The return value of ``$[]`` is always ``None``.

In the following, we can see that the results of ``$[]`` are automatically
printed, and that the return value is not a string.

.. code-block:: xonshcon

    >>> x = $[ls -l]
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh
    >>> x is None
    True

The ``![]`` operator is similar to the ``!()`` in that it returns an object
containing information about the result of executing the given command.
However, its standard output and standard error streams are directed to the
terminal, and the resulting object is not displayed.  For example

.. code-block:: xonshcon

    >>> x = ![ls -l] and ![echo "hi"]
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh
    hi


Python Evaluation with ``@()``
===============================

The ``@(<expr>)`` operator form works in subprocess mode, and will evaluate
arbitrary Python code. The result is appended to the subprocess command list.
If the result is a string or bytes, it is appended to the argument list. If the result
is a list or other non-string sequence, the contents are converted to strings
and appended to the argument list in order. If the result in the first position
is a function, it is treated as an alias (see the section on `Aliases`_ below),
even if it was not explicitly added to the ``aliases`` mapping.  Otherwise, the
result is automatically converted to a string. For example,

.. code-block:: xonshcon

    >>> x = 'xonsh'
    >>> y = 'party'
    >>> echo @(x + ' ' + y)
    xonsh party
    >>> echo @(2+2)
    4
    >>> echo @([42, 'yo'])
    42 yo
    >>> echo "hello" | @(lambda a, s=None: s.read().strip() + " world\n")
    hello world
    >>> @(['echo', 'hello', 'world'])
    hello world
    >>> @('echo hello world')  # note that strings are not split automatically
    xonsh: subprocess mode: command not found: echo hello world

This syntax can be used inside of a captured or uncaptured subprocess, and can
be used to generate any of the tokens in the subprocess command list.

.. code-block:: xonshcon

    >>> out = $(echo @(x + ' ' + y))
    >>> out
    'xonsh party\n'
    >>> @("ech" + "o") "hey"
    hey

Thus, ``@()`` allows us to create complex commands in Python-mode and then
feed them to a subprocess as needed.  For example:

.. code-block:: xonshcon

    for i in range(20):
        $[touch @('file%02d' % i)]

The ``@()`` syntax may also be used inside of subprocess
arguments, not just as a stand-alone argument. For example:

  .. code-block:: xonshcon

    >>> x = 'hello'
    >>> echo /path/to/@(x)
    /path/to/hello

When used inside of a subprocess argument and ``<expr>`` evaluates to a
non-string iterable, ``@()`` will expand to the outer product of all
given values:

  .. code-block:: sh

    >>> echo /path/to/@(['hello', 'world'])
    /path/to/hello /path/to/world

    >>> echo @(['a', 'b']):@('x', 'y')
    a:x a:y b:x b:y


Command Substitution with ``@$()``
==================================

A common use of the ``@()`` and ``$()`` operators is allowing the output of a
command to replace the command itself (command substitution):
``@([i.strip() for i in $(cmd).split()])``.  Xonsh offers a
short-hand syntax for this operation: ``@$(cmd)``.

Consider the following example:

.. code-block:: xonshcon

    >>> # this returns a string representing stdout
    >>> $(which ls)
    'ls --color=auto\n'

    >>> # this attempts to run the command, but as one argument
    >>> # (looks for 'ls --color=auto\n' with spaces and newline)
    >>> @($(which ls).strip())
    xonsh: subprocess mode: command not found: ls --color=auto

    >>> # this actually executes the intended command
    >>> @([i.strip() for i in $(which ls).split()])
    some_file  some_other_file

    >>> # this does the same thing, but is much more concise
    >>> @$(which ls)
    some_file  some_other_file


Nesting Subprocesses
=====================================
Though I am begging you not to abuse this, it is possible to nest the
subprocess operators that we have seen so far (``$()``, ``$[]``, ``${}``,
``@()``, ``@$()``).  An instance of ``ls -l`` that is on the wrong side of the
border of the absurd is shown below:

.. code-block:: console

    >>> $[@$(which @($(echo ls).strip())) @('-' + $(printf 'l'))]
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh

With great power, and so forth...

.. note:: Nesting these subprocess operators inside of ``$()`` and/or ``$[]``
          works because the contents of those operators are executed in
          subprocess mode.  Since ``@()`` and ``${}`` run their contents in
          Python mode, it is not possible to nest other subprocess operators
          inside of them.

To understand how xonsh executes the subprocess commands try
to set :ref:`$XONSH_TRACE_SUBPROC <xonsh_trace_subproc>` to ``True``:

.. code-block:: console

    >>> $XONSH_TRACE_SUBPROC = True
    >>> $[@$(which @($(echo ls).strip())) @('-' + $(printf 'l'))]
    TRACE SUBPROC: (['echo', 'ls'],)
    TRACE SUBPROC: (['which', 'ls'],)
    TRACE SUBPROC: (['printf', 'l'],)
    TRACE SUBPROC: (['ls', '--color=auto', '-v', '-l'],)
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh


Pipes
====================

In subprocess-mode, xonsh allows you to use the ``|`` character to pipe
together commands as you would in other shells.

.. code-block:: xonshcon

    >>> env | uniq | sort | grep PATH
    DATAPATH=/usr/share/MCNPX/v260/Data/
    DEFAULTS_PATH=/usr/share/gconf/awesome-gnome.default.path
    LD_LIBRARY_PATH=/home/snail/.local/lib:
    MANDATORY_PATH=/usr/share/gconf/awesome-gnome.mandatory.path
    PATH=/home/snail/.local/bin:/home/snail/sandbox/bin:/usr/local/bin
    XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
    XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0

This is only available in subprocess-mode because ``|`` is otherwise a
Python operator.
If you are unsure of what pipes are, there are many great references out there.
You should be able to find information on StackOverflow or Google.

Logical Subprocess And
=======================

Subprocess-mode also allows you to use the ``and`` operator to chain together
subprocess commands. The truth value of a command is evaluated as whether
its return code is zero (i.e. ``proc.returncode == 0``).  Like in Python,
if the command evaluates to ``False``, subsequent commands will not be executed.
For example, suppose we want to lists files that may or may not exist:

.. code-block:: xonshcon

    >>> touch exists
    >>> ls exists and ls doesnt
    exists
    /bin/ls: cannot access doesnt: No such file or directory

However, if you list the file that doesn't exist first,
you would have only seen the error:

.. code-block:: xonshcon

    >>> ls doesnt and ls exists
    /bin/ls: cannot access doesnt: No such file or directory

Also, don't worry. Xonsh directly translates the ``&&`` operator into ``and``
for you. It is less Pythonic, of course, but it is your shell!

Logical Subprocess Or
=======================

Much like with ``and``, you can use the ``or`` operator to chain together
subprocess commands. The difference, to be certain, is that
subsequent commands will be executed only if the
if the return code is non-zero (i.e. a failure). Using the file example
from above:

.. code-block:: xonshcon

    >>> ls exists or ls doesnt
    exists

This doesn't even try to list a non-existent file!
However, if you list the file that doesn't exist first,
you will see the error and then the file that does exist:

.. code-block:: xonshcon

    >>> ls doesnt or ls exists
    /bin/ls: cannot access doesnt: No such file or directory
    exists

Never fear! Xonsh also directly translates the ``||`` operator into ``or``,
too. Your muscle memory is safe now, here with us.

Input/Output Redirection
====================================

xonsh also allows you to redirect ``stdin``, ``stdout``, and/or ``stderr``.
This allows you to control where the output of a command is sent, and where
it receives its input from.  xonsh has its own syntax for these operations,
but, for compatibility purposes, xonsh also support Bash-like syntax.

The basic operations are "write to" (``>``), "append to" (``>>``), and "read
from" (``<``).  The details of these are perhaps best explained through
examples.

.. note:: The target of the redirection should be separated by a space,
          otherwise xonsh will raise a SyntaxError.

Redirecting ``stdout``
----------------------

All of the following examples will execute ``COMMAND`` and write its regular
output (stdout) to a file called ``output.txt``, creating it if it does not
exist:

.. code-block:: xonshcon

    >>> COMMAND > output.txt
    >>> COMMAND out> output.txt
    >>> COMMAND o> output.txt
    >>> COMMAND 1> output.txt # included for Bash compatibility

These can be made to append to ``output.txt`` instead of overwriting its contents
by replacing ``>`` with ``>>`` (note that ``>>`` will still create the file if it
does not exist).

Redirecting ``stderr``
----------------------

All of the following examples will execute ``COMMAND`` and write its error
output (stderr) to a file called ``errors.txt``, creating it if it does not
exist:

.. code-block:: xonshcon

    >>> COMMAND err> errors.txt
    >>> COMMAND e> errors.txt
    >>> COMMAND 2> errors.txt # included for Bash compatibility

As above, replacing ``>`` with ``>>`` will cause the error output to be
appended to ``errors.txt``, rather than replacing its contents.

Combining Streams
----------------------

It is possible to send all of ``COMMAND``'s output (both regular output and
error output) to the same location.  All of the following examples accomplish
that task:

.. code-block:: xonshcon

    >>> COMMAND all> combined.txt
    >>> COMMAND a> combined.txt
    >>> COMMAND &> combined.txt # included for Bash compatibility

It is also possible to explicitly merge stderr into stdout so that error
messages are reported to the same location as regular output.  You can do this
with the following syntax:

.. code-block:: xonshcon

    >>> COMMAND err>out
    >>> COMMAND err>o
    >>> COMMAND e>out
    >>> COMMAND e>o
    >>> COMMAND 2>&1  # included for Bash compatibility

This merge can be combined with other redirections, including pipes (see the
section on `Pipes`_ above):

.. code-block:: xonshcon

    >>> COMMAND err>out | COMMAND2
    >>> COMMAND e>o > combined.txt

It is worth noting that this last example is equivalent to: ``COMMAND a> combined.txt``

Similarly, you can also send stdout to stderr with the following syntax:

.. code-block:: xonshcon

    >>> COMMAND out>err
    >>> COMMAND out>e
    >>> COMMAND o>err
    >>> COMMAND o>e
    >>> COMMAND 1>&2  # included for Bash compatibility

Redirecting ``stdin``
---------------------

It is also possible to have a command read its input from a file, rather
than from ``stdin``.  The following examples demonstrate two ways to accomplish this:

.. code-block:: xonshcon

    >>> COMMAND < input.txt
    >>> < input.txt COMMAND

Combining I/O Redirects
------------------------

It is worth noting that all of these redirections can be combined.  Below is
one example of a complicated redirect.

.. code-block:: xonshcon

    >>> COMMAND1 e>o < input.txt | COMMAND2 > output.txt e>> errors.txt

This line will run ``COMMAND1`` with the contents of ``input.txt`` fed in on
stdin, and will pipe all output (stdout and stderr) to ``COMMAND2``; the
regular output of this command will be redirected to ``output.txt``, and the
error output will be appended to ``errors.txt``.


Background Jobs
===============

Typically, when you start a program running in xonsh, xonsh itself will pause
and wait for that program to terminate.  Sometimes, though, you may want to
continue giving commands to xonsh while that program is running.  In subprocess
mode, you can start a process "in the background" (i.e., in a way that allows
continued use of the shell) by adding an ampersand (``&``) to the end of your
command.  Background jobs are very useful when running programs with graphical
user interfaces.

The following shows an example with ``emacs``.

.. code-block:: xonshcon

    >>> emacs &
    >>>

Note that the prompt is returned to you after emacs is started.

Normally background commands end upon the shell closing. To allow a background
command to continue running after the shell has exited, use the ``disown``
command which accepts either no arguments (to disown the most recent job)
or an arbitrary number of job identifiers.

Job Control
===========

If you start a program in the foreground (with no ampersand), you can suspend
that program's execution and return to the xonsh prompt by pressing Control-Z.
This will give control of the terminal back to xonsh, and will keep the program
paused in the background.

.. note:: Suspending processes via Control-Z is not yet supported when
	  running on Windows.

To unpause the program and bring it back to the foreground, you can use the
``fg`` command.  To unpause the program have it continue in the background
(giving you continued access to the xonsh prompt), you can use the ``bg``
command.

You can get a listing of all currently running jobs with the ``jobs`` command.

Each job has a unique identifier (starting with 1 and counting upward).  By
default, the ``fg`` and ``bg`` commands operate on the job that was started
most recently.  You can bring older jobs to the foreground or background by
specifying the appropriate ID; for example, ``fg 1`` brings the job with ID 1
to the foreground. Additionally, specify "+" for the most recent job and "-"
for the second most recent job.

String Literals in Subprocess-mode
====================================
Strings can be used to escape special characters in subprocess-mode. The
contents of the string are passed directly to the subprocess command as a
single argument.  So whenever you are in doubt, or if there is a xonsh syntax
error because of a filename, just wrap the offending portion in a string.

A common use case for this is files with spaces in their names. This
detestable practice refuses to die. "No problem!" says xonsh, "I have
strings."  Let's see it go!

.. code-block:: xonshcon

    >>> touch "sp ace"
    >>> ls -l
    total 0
    -rw-rw-r-- 1 snail snail 0 Mar  8 17:50 sp ace
    -rw-rw-r-- 1 snail snail 0 Mar  8 15:46 xonsh

By default, the name of an environment variable inside a string will be
replaced by the contents of that variable (in subprocess mode only).  For
example:

.. code-block:: xonshcon

    >>> print("my home is $HOME")
    my home is $HOME
    >>> echo "my home is $HOME"
    my home is /home/snail

You can avoid this expansion within a particular command by forcing the strings
to be evaluated in Python mode using the ``@()`` syntax:

.. code-block:: xonshcon

    >>> echo "my home is $HOME"
    my home is /home/snail
    >>> echo @("my home is $HOME")
    my home is $HOME


.. note::

    You can also disable environment variable expansion completely by setting
    ``$EXPAND_ENV_VARS`` to ``False``.

Advanced String Literals
========================

For the fine control of environment variables (envvar) substitutions, brace substitutions and backslash escapes
there are extended list of literals:

- ``""`` - regular string: backslash escapes. Envvar substitutions in subprocess-mode.
- ``r""`` - raw string: unmodified.
- ``f""`` - formatted string: brace substitutions, backslash escapes. Envvar substitutions in subprocess-mode.
- ``fr""`` - raw formatted string: brace substitutions.
- ``p""`` - path string: backslash escapes, envvar substitutions, returns Path.
- ``pr""`` - raw Path string: envvar substitutions, returns Path.
- ``pf""`` - formatted Path string: backslash escapes, brace and envvar substitutions, returns Path.

To complete understanding let's set environment variable ``$EVAR`` to ``1`` and local variable ``var`` to ``2``
and make a table that shows how literal changes the string in Python- and subprocess-mode:

.. table::

    ========================  ==========================  =======================  =====================
         String literal            As python object       print(<String literal>)  echo <String literal>
    ========================  ==========================  =======================  =====================
    ``"/$EVAR/\'{var}\'"``    ``"/$EVAR/'{var}'"``        ``/$EVAR/'{var}'``       ``/1/'{var}'``
    ``r"/$EVAR/\'{var}\'"``   ``"/$EVAR/\\'{var}\\'"``    ``/$EVAR/\'{var}\'``     ``/$EVAR/\'{var}\'``
    ``f"/$EVAR/\'{var}\'"``   ``"/$EVAR/'2'"``            ``/$EVAR/'2'``           ``/1/'2'``
    ``fr"/$EVAR/\'{var}\'"``  ``"/$EVAR/\\'2\\'"``        ``/$EVAR/\'2\'``         ``/$EVAR/\'2\'``
    ``p"/$EVAR/\'{var}\'"``   ``Path("/1/'{var}'")``      ``/1/'{var}'``           ``/1/'{var}'``
    ``pr"/$EVAR/\'{var}\'"``  ``Path("/1/\\'{var}\\'")``  ``/1/\'{var}\'``         ``/1/\'{var}\'``
    ``pf"/$EVAR/\'{var}\'"``  ``Path("/1/'2'")``          ``/1/'2'``               ``/1/'2'``
    ========================  ==========================  =======================  =====================

Filename Globbing with ``*``
===============================
Filename globbing with the ``*`` character is also allowed in subprocess-mode.
This simply uses Python's glob module under-the-covers.  See there for more
details.  As an example, start with a lovely bunch of xonshs:

.. code-block:: xonshcon

    >>> touch xonsh conch konk quanxh
    >>> ls
    conch  konk  quanxh  xonsh
    >>> ls *h
    conch  quanxh  xonsh
    >>> ls *o*
    conch  konk  xonsh

This is not available in Python-mode because multiplication is pretty
important.


Advanced Path Search with Backticks
===================================

xonsh offers additional ways to find path names beyond regular globbing, both
in Python mode and in subprocess mode.

Regular Expression Globbing
---------------------------

If you have ever felt that normal globbing could use some more octane,
then regex globbing is the tool for you! Any string that uses backticks
(`````) instead of quotes (``'``, ``"``) is interpreted as a regular
expression to match filenames against.  Like with regular globbing, a
list of successful matches is returned.  In Python-mode, this is just a
list of strings. In subprocess-mode, each filename becomes its own argument
to the subprocess command.

Let's see a demonstration with some simple filenames:


.. code-block:: xonshcon

    >>> touch a aa aaa aba abba aab aabb abcba
    >>> ls `a(a+|b+)a`
    aaa  aba  abba
    >>> print(`a(a+|b+)a`)
    ['aaa', 'aba', 'abba']
    >>> len(`a(a+|b+)a`)
    3

This same kind of search is performed if the backticks are prefaced with ``r``.
So the following expressions are equivalent: ```test``` and ``r`test```.

Other than the regex matching, this functions in the same way as normal
globbing.  For more information, please see the documentation for the ``re``
module in the Python standard library.

.. warning:: In Xonsh, the meaning of backticks is very different from their
             meaning in Bash.
             In Bash, backticks mean to run a captured subprocess
	     (``$()`` in Xonsh).


Normal Globbing
---------------

In subprocess mode, normal globbing happens without any special syntax.
However, the backtick syntax has an additional feature: it is available inside
of Python mode as well as subprocess mode.

Similarly to regex globbing, normal globbing can be performed (either in Python
mode or subprocess mode) by using the ``g````:

.. code-block:: xonshcon

    >>> touch a aa aaa aba abba aab aabb abcba
    >>> ls a*b*
    aab  aabb  aba  abba  abcba
    >>> ls g`a*b*`
    aab  aabb  aba  abba  abcba
    >>> print(g`a*b*`)
    ['aab', 'aabb', 'abba', 'abcba', 'aba']
    >>> len(g`a*b*`)
    5


Custom Path Searches
--------------------

In addition, if normal globbing and regular expression globbing are not enough,
xonsh allows you to specify your own search functions.

A search function is defined as a function of a single argument (a string) that
returns a list of possible matches to that string.  Search functions can then
be used with backticks with the following syntax: ``@<name>`test```

The following example shows the form of these functions:

.. code-block:: xonshcon

    >>> def foo(s):
    ...     return [i for i in os.listdir('.') if i.startswith(s)]
    >>> @foo`aa`
    ['aa', 'aaa', 'aab', 'aabb']


Path Output
-----------

Using the ``p`` modifier with either regex or glob backticks changes the
return type from a list of strings to a list of :class:`pathlib.Path` objects:

.. code-block:: xonshcon

    >>> p`.*`
    [Path('foo'), Path('bar')]
    >>> [x for x in pg`**` if x.is_symlink()]
    [Path('a_link')]


Path Literals
-------------

Path objects can be instantiated directly using *p-string* syntax. Path objects
can be converted back to plain strings with `str()`, and this conversion is
handled implicitly in subprocess mode.

.. code-block:: xonshcon

    >>> mypath = p'/foo/bar'
    >>> mypath
    Path('/foo/bar')
    >>> mypath.stem
    'bar'
    >>> echo @(mypath)
    /foo/bar

Path object allows do some tricks with paths. Globbing certain path, checking and getting info:

.. code-block:: xonshcon

    >>> mypath = p'/etc'
    >>> sorted(mypath.glob('**/*bashrc*'))
    [Path('/etc/bash.bashrc'), Path('/etc/skel/.bashrc')]
    >>> [mypath.exists(), mypath.is_dir(), mypath.is_file(), mypath.parent, mypath.owner()]
    [True, True, False, Path('/'), 'root']

Help & Superhelp with ``?`` & ``??``
=====================================================
From IPython, xonsh allows you to inspect objects with question marks.
A single question mark (``?``) is used to display the normal level of help.
Double question marks (``??``) are used to display a higher level of help,
called superhelp. Superhelp usually includes source code if the object was
written in pure Python.

Let's start by looking at the help for the int type:

.. code-block:: xonshcon

    >>> int?
    Type:            type
    String form:     <class 'int'>
    Init definition: (self, *args, **kwargs)
    Docstring:
    int(x=0) -> integer
    int(x, base=10) -> integer

    Convert a number or string to an integer, or return 0 if no arguments
    are given.  If x is a number, return x.__int__().  For floating point
    numbers, this truncates towards zero.

    If x is not a number or if base is given, then x must be a string,
    bytes, or bytearray instance representing an integer literal in the
    given base.  The literal can be preceded by '+' or '-' and be surrounded
    by whitespace.  The base defaults to 10.  Valid bases are 0 and 2-36.
    Base 0 means to interpret the base from the string as an integer literal.
    >>> int('0b100', base=0)
    4
    <class 'int'>

Now, let's look at the superhelp for the xonsh built-in that enables
regex globbing:

.. code-block:: xonshcon

    >>> __xonsh__.regexsearch??
    Type:         function
    String form:  <function regexsearch at 0x7efc8b367d90>
    File:         /usr/local/lib/python3.5/dist-packages/xonsh/built_ins.py
    Definition:   (s)
    Source:
    def regexsearch(s):
        s = expand_path(s)
        return reglob(s)


    <function xonsh.built_ins.regexsearch>

Note that both help and superhelp return the object that they are inspecting.
This allows you to chain together help inside of other operations and
ask for help several times in an object hierarchy.  For instance, let's get
help for both the dict type and its key() method simultaneously:

.. code-block:: xonshcon

    >>> dict?.keys??
    Type:            type
    String form:     <class 'dict'>
    Init definition: (self, *args, **kwargs)
    Docstring:
    dict() -> new empty dictionary
    dict(mapping) -> new dictionary initialized from a mapping object's
        (key, value) pairs
    dict(iterable) -> new dictionary initialized as if via:
        d = {}
        for k, v in iterable:
            d[k] = v
    dict(**kwargs) -> new dictionary initialized with the name=value pairs
        in the keyword argument list.  For example:  dict(one=1, two=2)
    Type:        method_descriptor
    String form: <method 'keys' of 'dict' objects>
    Docstring:   D.keys() -> a set-like object providing a view on D's keys
    <method 'keys' of 'dict' objects>

Of course, for subprocess commands, you still want to use the ``man`` command.


Compile, Evaluate, & Execute
================================
Like Python and Bash, xonsh provides built-in hooks to compile, evaluate,
and execute strings of xonsh code.  To prevent this functionality from having
serious name collisions with the Python built-in ``compile()``, ``eval()``,
and ``exec()`` functions, the xonsh equivalents all append an 'x'.  So for
xonsh code you want to use the ``compilex()``, ``evalx()``, and ``execx()``
functions. If you don't know what these do, you probably don't need them.


Aliases
==============================
Another important xonsh built-in is the ``aliases`` mapping.  This is
like a dictionary that affects how subprocess commands are run.  If you are
familiar with the Bash ``alias`` built-in, this is similar.  Alias command
matching only occurs for the first element of a subprocess command.

The keys of ``aliases`` are strings that act as commands in subprocess-mode.
The values are lists of strings, where the first element is the command, and
the rest are the arguments.

.. code-block:: xonshcon

    >>> aliases['ls']
    ['ls', '--color=auto', '-v']

You can also set the value to a string. If the string is a xonsh expression,
it will be converted to a list automatically with xonsh's ``Lexer.split()`` method.
For example, the following creates several aliases for the ``git`` version
control software. Both styles (list of strings and single string) are shown:

.. code-block:: xonshcon

    >>> aliases['g'] = 'git status -sb'
    >>> aliases['gco'] = 'git checkout'
    >>> aliases['gp'] = ['git', 'pull']

If you were to run ``gco feature-fabulous`` with the above aliases in effect,
the command would reduce to ``['git', 'checkout', 'feature-fabulous']`` before
being executed.

If the string is representing a block of xonsh code, the alias will be registered
as an ``ExecAlias``, which is a callable alias. This block of code will then be
executed whenever the alias is run. The arguments are available in the list ``$args``
or by the index in ``$arg<n>`` environment variables.

.. code-block:: xonshcon

    >>> aliases['answer'] = 'echo @(21+21)'
    >>> aliases['piu'] = 'pip install -U @($args)'
    >>> aliases['cdls'] = 'cd $arg0 && ls'

.. note::

   To add multiple aliases there is merge operator: ``aliases |= {'e': 'echo', 'g': 'git'}``.


Callable Aliases
----------------
Lastly, if an alias value is a function (or other callable), then this
function is called *instead* of going to a subprocess command. Such functions
may have one of the following signatures:

.. code-block:: python

    def mycmd0():
        """This form takes no arguments but may return output or a return code.
        """
        return "some output."

    def mycmd1(args):
        """This form takes a single argument, args. This is a list of strings
        representing the arguments to this command. Feel free to parse them
        however you wish!
        """
        # perform some action.
        return 0

    def mycmd2(args, stdin=None):
        """This form takes two arguments. The args list like above, as a well
        as standard input. stdin will be a file like object that the command
        can read from, if the user piped input to this command. If no input
        was provided this will be None.
        """
        # do whatever you want! Anything you print to stdout or stderr
        # will be captured for you automatically. This allows callable
        # aliases to support piping.
        print('I go to stdout and will be printed or piped')

        # Note: that you have access to the xonsh
        # built-ins if you 'import builtins'.  For example, if you need the
        # environment, you could do the following:
        import builtins
        env = builtins.__xonsh__.env

        # The return value of the function can either be None,
        return

        # a single string representing stdout
        return  'I am out of here'

        # or you can build up strings for stdout and stderr and then
        # return a (stdout, stderr) tuple. Both of these may be
        # either a str or None. Any results returned like this will be
        # concatenated with the strings printed elsewhere in the function.
        stdout = 'I commanded'
        stderr = None
        return stdout, stderr

        # Lastly, a 3-tuple return value can be used to include an integer
        # return code indicating failure (> 0 return code). In the previous
        # examples the return code would be 0/success.
        return (None, "I failed", 2)

    def mycmd3(args, stdin=None, stdout=None):
        """This form has three parameters.  The first two are the same as above.
        The last argument represents the standard output.  This is a file-like
        object that the command may write too.
        """
        # you can either use stdout
        stdout.write("Hello, ")
        # or print()!
        print("Mom!")
        return

    def mycmd4(args, stdin=None, stdout=None, stderr=None):
        """The next form of subprocess callables takes all of the
        arguments shown above as well as the standard error stream.
        As with stdout, this is a write-only file-like object.
        """
        # This form allows "streaming" data to stdout and stderr
        import time
        for i in range(5):
            time.sleep(i)
            print(i, file=stdout)

        # In this form, the return value should be a single integer
        # representing the "return code" of the alias (zero if successful,
        # non-zero otherwise)
        return 0

    def mycmd5(args, stdin=None, stdout=None, stderr=None, spec=None):
        """This form of subprocess callables takes all of the
        arguments shown above as well as a subprocess specification
        SubprocSpec object. This holds many attributes that dictate how
        the command is being run.  For instance this can be useful for
        knowing if the process is captured by $() or !().
        """
        import xonsh.proc
        if spec.captured in xonsh.proc.STDOUT_CAPTURE_KINDS:
            print("I'm being captured!")
        elif not spec.last_in_pipeline:
            print("Going through a pipe!")
        else:
            print("Hello terminal!")
        return 0

    def mycmd6(args, stdin=None, stdout=None, stderr=None, spec=None, stack=None):
        """Lastly, the final form of subprocess callables takes a stack argument
        in addition to the arguments shown above. The stack is a list of
        FrameInfo namedtuple objects, as described in the standard library
        inspect module. The stack is computed such the the call site is the
        first and innermost entry, while the outer frame is the last entry.

        The stack is only computed if the alias has a "stack" argument.
        However, the stack is also accessible as "spec.stack".
        """
        for frame_info in stack:
            frame = frame_info[0]
            print('In function ' + frame_info[3])
            print('  locals', frame.f_locals)
            print('  globals', frame.f_globals)
            print('\n')
        return 0


Adding, Modifying, and Removing Aliases
---------------------------------------

We can dynamically alter the aliases present simply by modifying the
built-in mapping.  Here is an example using a function value:

.. code-block:: xonshcon

    >>> def _banana(args, stdin=None):
    ...     return ('My spoon is tooo big!', None)
    >>> aliases['banana'] = _banana
    >>> banana
    'My spoon is tooo big!'


To redefine an alias, simply assign a new function, here using a python lambda
with keyword arguments:

.. code-block:: xonshcon

    >>> aliases['banana'] = lambda: "Banana for scale.\n"
    >>> banana
    Banana for scale.


Removing an alias is as easy as deleting the key from the alias dictionary:

.. code-block:: xonshcon

    >>> del aliases['banana']

.. note::

   Alias functions should generally be defined with a leading underscore.
   Otherwise, they may shadow the alias itself, as Python variables take
   precedence over aliases when xonsh executes commands.


Anonymous Aliases
-----------------
As mentioned above, it is also possible to treat functions outside this mapping
as aliases, by wrapping them in ``@()``.  For example:

.. code-block:: xonshcon

    >>> @(_banana)
    'My spoon is tooo big!'
    >>> echo "hello" | @(lambda args, stdin=None: stdin.read().strip() + ' ' + args[0] + '\n') world
    hello world


Unthreadable Aliases
-----------------------
Usually, callable alias commands will be run in a separate thread so that
they may be run in the background.  However, some aliases may need to be
executed on the thread that they were called from. This is mostly useful for
debuggers and profilers. To make an alias run in the foreground, decorate its
function with the ``xonsh.tools.unthreadable`` decorator.

.. code-block:: python

    from xonsh.tools import unthreadable

    @unthreadable
    def _mycmd(args, stdin=None):
        return 'In your face!'

    aliases['mycmd'] = _mycmd

Uncapturable Aliases
-----------------------
Also, callable aliases by default will be executed such that their output is
captured (like most commands in xonsh that don't enter alternate mode).
However, some aliases may want to run alternate-mode commands themselves.
Thus the callable alias can't be captured without dire consequences (tm).
To prevent this, you can declare a callable alias uncapturable. This is mostly
useful for aliases that then open up text editors, pagers, or the like.
To make an alias uncapturable, decorate its
function with the ``xonsh.tools.uncapturable`` decorator. This is probably
best used in conjunction with the ``unthreadable`` decorator.  For example:

.. code-block:: xonshcon

    from xonsh.tools import unthreadable, uncapturable

    @uncapturable
    @unthreadable
    def _binvi(args, stdin=None):
        vi -b @(args)  # Edit binary files

    aliases['bvi'] = _binvi

Note that ``@()`` is required to pass the python list ``args`` to a subprocess
command.

-------------

Aliasing is a powerful way that xonsh allows you to seamlessly interact to
with Python and subprocess.

.. warning:: If ``FOREIGN_ALIASES_OVERRIDE`` environment variable is False
             (the default), then foreign shell aliases that try to override
             xonsh aliases will be ignored. The setting of this environment variable
             must happen outside if xonsh, i.e. in the process that starts xonsh.


Up, Down, Tab
==============
The up and down keys search history matching from the start of the line,
much like they do in the IPython shell.

Tab completion is present as well. By default, in Python-mode you are able to
complete based on the variable names in the current builtins, globals, and
locals, as well as xonsh languages keywords & operator, files & directories,
and environment variable names. In subprocess-mode, you additionally complete
on the names of executable files on your ``$PATH``, alias keys, and full Bash
completion for the commands themselves.

xonsh also provides a means of modifying the behavior of the tab completer.  More
detail is available on the `Tab Completion page <tutorial_completers.html>`_.

.. _customprompt:

Customizing the Prompt
======================
Customizing the prompt by modifying ``$PROMPT``, ``$RIGHT_PROMPT`` or ``$BOTTOM_TOOLBAR`` 
is probably the most common reason for altering an environment variable.

.. note:: Note that the ``$PROMPT`` variable will never be inherited from a
          parent process (regardless of whether that parent is a foreign shell
          or an instance of xonsh).

The ``$PROMPT`` variable can be a string, or it can be a function (of no
arguments) that returns a string.  The result can contain keyword arguments,
which will be replaced automatically:

.. code-block:: xonshcon

    >>> $PROMPT = '{user}@{hostname}:{cwd} > '
    snail@home:~ > # it works!
    snail@home:~ > $PROMPT = lambda: '{user}@{hostname}:{cwd} >> '
    snail@home:~ >> # so does that!

-- todo: convert this to jinja template and generate these contents dynamically and mention about $PROMPT_FIELDS

By default, the following variables are available for use:

  -- remove these extra variables and set the attribute on the field itself

  * ``user``: The username of the current user
  * ``hostname``: The name of the host computer
  * ``cwd``: The current working directory, you may use ``$DYNAMIC_CWD_WIDTH`` to
    set a maximum width for this variable and ``$DYNAMIC_CWD_ELISION_CHAR`` to
    set the character used in shortened path.
  * ``short_cwd``: A shortened form of the current working directory; e.g.,
    ``/path/to/xonsh`` becomes ``/p/t/xonsh``
  * ``cwd_dir``: The dirname of the current working directory, e.g. ``/path/to/`` in
    ``/path/to/xonsh``.
  * ``cwd_base``: The basename of the current working directory, e.g. ``xonsh`` in
    ``/path/to/xonsh``.
  * ``env_name``: The name of active virtual environment, if any. The rendering
    of this variable is affected by the ``$VIRTUAL_ENV_PROMPT`` and
    ``$VIRTUAL_ENV_DISABLE_PROMPT`` environment variables; see below.
  * ``env_prefix``: The prefix characters if there is an active virtual environment,
    defaults to ``"("``.
  * ``env_postfix``: The postfix characters if there is an active virtual environment,
    defaults to ``") "``.
  * ``curr_branch``: The name of the current git branch, if any.
  * ``branch_color``: ``{BOLD_GREEN}`` if the current git branch is clean,
    otherwise ``{BOLD_RED}``. This is yellow if the branch color could not be
    determined.
  * ``branch_bg_color``: Like, ``{branch_color}``, but sets a background color
    instead.
  * ``prompt_end``: ``#`` if the user has root/admin permissions ``$`` otherwise
  * ``current_job``: The name of the command currently running in the
    foreground, if any.
  * ``vte_new_tab_cwd``: Issues VTE escape sequence for opening new tabs in the
    current working directory on some linux terminals. This is not usually needed.
  * ``gitstatus``: Informative git status, like ``[main|MERGING|+1…2]``, you
    may refer :py:mod:`xonsh.prompt.gitstatus` for customization options.
  * ``localtime``: The current, local time as given by ``time.localtime()``.
    This is formatted with the time format string found in ``time_format``.
  * ``time_format``: A time format string, defaulting to ``"%H:%M:%S"``.

.. note:: See the section below on ``PROMPT_FIELDS`` for more information on changing.

xonsh obeys the ``$VIRTUAL_ENV_DISABLE_PROMPT`` environment variable
`as defined by virtualenv <https://virtualenv.pypa.io/en/latest/reference/
#envvar-VIRTUAL_ENV_DISABLE_PROMPT>`__. If this variable is truthy, xonsh
will *always* substitute an empty string for ``{env_name}``. Note that unlike 
other shells, ``$VIRTUAL_ENV_DISABLE_PROMPT`` takes effect *immediately*
after being set---it is not necessary to re-activate the environment.

xonsh also allows for an explicit override of the rendering of ``{env_name}``,
via the ``$VIRTUAL_ENV_PROMPT`` environment variable. If this variable is
defined and has any value other than ``None``, ``{env_name}`` will *always*
render as ``str($VIRTUAL_ENV_PROMPT)`` when an environment is activated.
It will still render as an empty string when no environment is active.
``$VIRTUAL_ENV_PROMPT`` is overridden by ``$VIRTUAL_ENV_DISABLE_PROMPT``.

For example:

.. code-block:: xonshcon

    >>> $PROMPT = '{env_name}>>> '
    >>> source env/bin/activate.xsh
    (env) >>> $VIRTUAL_ENV_PROMPT = '~~ACTIVE~~ '
    ~~ACTIVE~~ >>> $VIRTUAL_ENV_DISABLE_PROMPT = 1
    >>> del $VIRTUAL_ENV_PROMPT
    >>> del $VIRTUAL_ENV_DISABLE_PROMPT
    (env) >>>


You can also color your prompt (or print colored messages using ``print_color`` function) easily by inserting
keywords such as ``{GREEN}`` or ``{BOLD_BLUE}``.  Colors have the form shown below:

* ``RESET``: Resets any previously used styling.
* ``COLORNAME``: Inserts a color code for the following basic colors,
  which come in regular (dark) and intense (light) forms:

    - ``BLACK`` or ``INTENSE_BLACK``
    - ``RED`` or ``INTENSE_RED``
    - ``GREEN`` or ``INTENSE_GREEN``
    - ``YELLOW`` or ``INTENSE_YELLOW``
    - ``BLUE`` or ``INTENSE_BLUE``
    - ``PURPLE`` or ``INTENSE_PURPLE``
    - ``CYAN`` or ``INTENSE_CYAN``
    - ``WHITE`` or ``INTENSE_WHITE``

* ``DEFAULT``: The color code for the terminal's default foreground color.
* ``#HEX``: A ``#`` before a len-3 or len-6 hex code will use that
  hex color, or the nearest approximation that that is supported by
  the shell and terminal.  For example, ``#fff`` and ``#fafad2`` are
  both valid.
* ``BACKGROUND_`` may be added to the beginning of a color name or hex
  color to set a background color.  For example, ``BACKGROUND_INTENSE_RED``
  and ``BACKGROUND_#123456`` can both be used.
* ``bg#HEX`` or ``BG#HEX`` are shortcuts for setting a background hex color.
  Thus you can set ``bg#0012ab`` or the uppercase version.
* ``BOLD_`` is a prefix modifier that increases the intensity of the font.
  It may be used with any foreground color.
  For example, ``BOLD_RED`` and ``BOLD_#112233`` are OK!
* ``FAINT_`` is a prefix modifier that decreases the intensity of the font.
  For example, ``FAINT_YELLOW``.
* ``ITALIC_`` is a prefix modifier that switches to an italic font.
  For example, ``ITALIC_BLUE``.
* ``UNDERLINE_`` is a prefix qualifier that also may be used with any
  foreground color. For example, ``UNDERLINE_GREEN``.
* ``SLOWBLINK_`` is a prefix modifier makes the text blink, slowly.
  For example, ``SLOWBLINK_PURPLE``.
* ``FASTBLINK_`` is a prefix modifier makes the text blink, quickly.
  For example, ``FASTBLINK_CYAN``.
* ``INVERT_`` is a prefix modifier swaps the foreground and background colors.
  For example, ``INVERT_WHITE``.
* ``CONCEAL_`` is a prefix modifier which hides the text. This may not be
  widely supported. For example, ``CONCEAL_BLACK``.
* ``STRIKETHROUGH_`` is a prefix modifier which draws a line through the text.
  For example, ``STRIKETHROUGH_RED``.
* ``BOLDOFF_`` is a prefix modifier for removing the intensity of the font.
  It may be used with any foreground color.
  For example, ``BOLDOFF_RED`` and ``BOLDOFF_#112233`` are OK!
* ``FAINTOFF_`` is a prefix modifier for removing the faintness of the font.
  For example, ``FAINTOFF_YELLOW``.
* ``ITALICOFF_`` is a prefix modifier that removes an italic font.
  For example, ``ITALICOFF_BLUE``.
* ``UNDERLINEOFF_`` is a prefix qualifier for removing the underline of a
  foreground color. For example, ``UNDERLINEOFF_GREEN``.
* ``BLINKOFF_`` is a prefix modifier removing the text blinking,
  whether that is slow or fast. For example, ``BLINKOFF_PURPLE``.
* ``INVERTOFF_`` is a prefix modifier restoring the foreground and background colors.
  For example, ``INVERTOFF_WHITE``.
* ``CONCEALOFF_`` is a prefix modifier which shows the text. This may not be
  widely supported. For example, ``CONCEALOFF_BLACK``.
* ``STRIKETHROUGHOFF_`` is a prefix modifier removing lines through the text.
  For example, ``STRIKETHROUGHOFF_RED``.
* Or any other combination of modifiers, such as
  ``BOLD_UNDERLINE_INTENSE_BLACK``,   which is the most metal color you
  can use!

You can make use of additional variables beyond these by adding them to the
``PROMPT_FIELDS`` environment variable. The values in this dictionary should
be strings (which will be inserted into the prompt verbatim), or functions of
arguments (which will be called each time the prompt is generated, and the results
of those calls will be inserted into the prompt). For example:

.. code-block:: console

    snail@home ~ $ $PROMPT_FIELDS['test'] = "hey"
    snail@home ~ $ $PROMPT = "{test} {cwd} $ "
    hey ~ $
    hey ~ $ import random
    hey ~ $ $PROMPT_FIELDS['test'] = lambda: random.randint(1,9)
    3 ~ $
    5 ~ $
    2 ~ $
    8 ~ $

Environment variables and functions are also available with the ``$``
prefix.  For example:

.. code-block:: console

    snail@home ~ $ $PROMPT = "{$LANG} >"
    en_US.utf8 >

Note that some entries of the ``$PROMPT_FIELDS`` are not always applicable, for
example, ``curr_branch`` returns ``None`` if the current directory is not in a
repository. The ``None`` will be interpreted as an empty string.

But let's consider a problem:

.. code-block:: console

    snail@home ~/xonsh $ $PROMPT = "{cwd_base} [{curr_branch}] $ "
    xonsh [main] $ cd ..
    ~ [] $

We want the branch to be displayed in square brackets, but we also don't want
the brackets (and the extra space) to be displayed when there is no branch. The
solution is to add a nested format string (separated with a colon) that will be
invoked only if the value is not ``None``:

.. code-block:: console

    snail@home ~/xonsh $ $PROMPT = "{cwd_base}{curr_branch: [{}]} $ "
    xonsh [main] $ cd ..
    ~ $

The curly brackets act as a placeholder, because the additional part is an
ordinary format string. What we're doing here is equivalent to this expression:

.. code-block:: python

    " [{}]".format(curr_branch()) if curr_branch() is not None else ""


Executing Commands and Scripts
==============================
When started with the ``-c`` flag and a command, xonsh will execute that command
and exit, instead of entering the command loop.

.. code-block:: console

    $ xonsh -c "echo @(7+3)"
    10

Longer scripts can be run either by specifying a filename containing the script,
or by feeding them to xonsh via stdin.  For example, consider the following
script, stored in ``test.xsh``:

.. code-block:: xonshcon

    #!/usr/bin/env xonsh

    ls

    print('removing files')
    rm `file\d+.txt`

    ls

    print('adding files')
    # This is a comment
    for i, x in enumerate("xonsh"):
        echo @(x) > @("file{0}.txt".format(i))

    print($(ls).replace('\n', ' '))


This script could be run by piping its contents to xonsh:

.. code-block:: console

    $ cat test.xsh | xonsh
    file0.txt  file1.txt  file2.txt  file3.txt  file4.txt  test_script.sh
    removing files
    test_script.sh
    adding files
    file0.txt file1.txt file2.txt file3.txt file4.txt test_script.sh

or by invoking xonsh with its filename as an argument:

.. code-block:: console

    $ xonsh test.xsh
    file0.txt  file1.txt  file2.txt  file3.txt  file4.txt  test_script.sh
    removing files
    test_script.sh
    adding files
    file0.txt file1.txt file2.txt file3.txt file4.txt test_script.sh

xonsh scripts can also accept command line arguments and parameters.
These arguments are made available to the script in two different ways:

#. In either mode, as individual variables ``$ARG<n>`` (e.g., ``$ARG1``)
#. In Python mode only, as a list ``$ARGS``

For example, consider a slight variation of the example script from above that
operates on a given argument, rather than on the string ``'xonsh'`` (notice how
``$ARGS`` and ``$ARG1`` are used):


.. code-block:: xonshcon

    #!/usr/bin/env xonsh

    print($ARGS)

    ls

    print('removing files')
    rm `file\d+.txt`

    ls

    print('adding files')
    # This is a comment
    for i, x in enumerate($ARG1):
        echo @(x) > @("file{0}.txt".format(i))

    print($(ls).replace('\n', ' '))
    print()


.. code-block:: console

    $ xonsh test2.xsh snails
    ['test_script.sh', 'snails']
    file0.txt  file1.txt  file2.txt  file3.txt  file4.txt  file5.txt  test_script.sh
    removing files
    test_script.sh
    adding files
    file0.txt file1.txt file2.txt file3.txt file4.txt file5.txt test_script.sh

    $ echo @(' '.join($(cat @('file%d.txt' % i)).strip() for i in range(6)))
    s n a i l s

Additionally, if the script should exit if a command fails, set the
environment variable ``$RAISE_SUBPROC_ERROR = True`` at the top of the
file. Errors in Python mode will already raise exceptions and so this
is roughly equivalent to Bash's ``set -e``.

Furthermore, you can also toggle the ability to print source code lines with the
``trace on`` and ``trace off`` commands.  This is roughly equivalent to
Bash's ``set -x`` or Python's ``python -m trace``, but you know, better.

Importing Xonsh (``*.xsh``)
==============================
You can import xonsh source files with the ``*.xsh`` file extension using
the normal Python syntax.  Say you had a file called ``mine.xsh``, you could,
therefore, perform a Bash-like source into your current shell with the
following:

.. code-block:: xonshcon

    from mine import *


That's All, Folks
======================
To leave xonsh, hit ``Ctrl-D``, type ``EOF``, type ``quit``, or type ``exit``.
On Windows, you can also type ``Ctrl-Z``.

.. code-block:: xonshcon

    >>> exit

To exit from the xonsh script just call the ``exit(code)`` function.

Now it is your turn.
