
``os`` --- Miscellaneous operating system interfaces
****************************************************

This module provides a portable way of using operating system
dependent functionality.  If you just want to read or write a file see
``open()``, if you want to manipulate paths, see the ``os.path``
module, and if you want to read all the lines in all the files on the
command line see the ``fileinput`` module.  For creating temporary
files and directories see the ``tempfile`` module, and for high-level
file and directory handling see the ``shutil`` module.

Notes on the availability of these functions:

* The design of all built-in operating system dependent modules of
  Python is such that as long as the same functionality is available,
  it uses the same interface; for example, the function
  ``os.stat(path)`` returns stat information about *path* in the same
  format (which happens to have originated with the POSIX interface).

* Extensions peculiar to a particular operating system are also
  available through the ``os`` module, but using them is of course a
  threat to portability.

* All functions accepting path or file names accept both bytes and
  string objects, and result in an object of the same type, if a path
  or file name is returned.

* An "Availability: Unix" note means that this function is commonly
  found on Unix systems.  It does not make any claims about its
  existence on a specific operating system.

* If not separately noted, all functions that claim "Availability:
  Unix" are supported on Mac OS X, which builds on a Unix core.

Note: All functions in this module raise ``OSError`` in the case of
  invalid or inaccessible file names and paths, or other arguments
  that have the correct type, but are not accepted by the operating
  system.

exception exception os.error

   An alias for the built-in ``OSError`` exception.

os.name

   The name of the operating system dependent module imported.  The
   following names have currently been registered: ``'posix'``,
   ``'nt'``, ``'mac'``, ``'os2'``, ``'ce'``, ``'java'``.

   See also:

      ``sys.platform`` has a finer granularity.  ``os.uname()`` gives
      system-dependent version information.

      The ``platform`` module provides detailed checks for the
      system's identity.


File Names, Command Line Arguments, and Environment Variables
=============================================================

In Python, file names, command line arguments, and environment
variables are represented using the string type. On some systems,
decoding these strings to and from bytes is necessary before passing
them to the operating system. Python uses the file system encoding to
perform this conversion (see ``sys.getfilesystemencoding()``).

Changed in version 3.1: On some systems, conversion using the file
system encoding may fail. In this case, Python uses the
``surrogateescape`` encoding error handler, which means that
undecodable bytes are replaced by a Unicode character U+DCxx on
decoding, and these are again translated to the original byte on
encoding.

The file system encoding must guarantee to successfully decode all
bytes below 128. If the file system encoding fails to provide this
guarantee, API functions may raise UnicodeErrors.


Process Parameters
==================

These functions and data items provide information and operate on the
current process and user.

os.environ

   A mapping object representing the string environment. For example,
   ``environ['HOME']`` is the pathname of your home directory (on some
   platforms), and is equivalent to ``getenv("HOME")`` in C.

   This mapping is captured the first time the ``os`` module is
   imported, typically during Python startup as part of processing
   ``site.py``.  Changes to the environment made after this time are
   not reflected in ``os.environ``, except for changes made by
   modifying ``os.environ`` directly.

   If the platform supports the ``putenv()`` function, this mapping
   may be used to modify the environment as well as query the
   environment.  ``putenv()`` will be called automatically when the
   mapping is modified.

   On Unix, keys and values use ``sys.getfilesystemencoding()`` and
   ``'surrogateescape'`` error handler. Use ``environb`` if you would
   like to use a different encoding.

   Note: Calling ``putenv()`` directly does not change ``os.environ``, so
     it's better to modify ``os.environ``.

   Note: On some platforms, including FreeBSD and Mac OS X, setting
     ``environ`` may cause memory leaks.  Refer to the system
     documentation for ``putenv()``.

   If ``putenv()`` is not provided, a modified copy of this mapping
   may be passed to the appropriate process-creation functions to
   cause  child processes to use a modified environment.

   If the platform supports the ``unsetenv()`` function, you can
   delete items in this mapping to unset environment variables.
   ``unsetenv()`` will be called automatically when an item is deleted
   from ``os.environ``, and when one of the ``pop()`` or ``clear()``
   methods is called.

os.environb

   Bytes version of ``environ``: a mapping object representing the
   environment as byte strings. ``environ`` and ``environb`` are
   synchronized (modify ``environb`` updates ``environ``, and vice
   versa).

   ``environb`` is only available if ``supports_bytes_environ`` is
   True.

   New in version 3.2.

os.chdir(path)
os.fchdir(fd)
os.getcwd()

   These functions are described in *Files and Directories*.

os.fsencode(filename)

   Encode *filename* to the filesystem encoding with
   ``'surrogateescape'`` error handler, or ``'strict'`` on Windows;
   return ``bytes`` unchanged.

   ``fsdecode()`` is the reverse function.

   New in version 3.2.

os.fsdecode(filename)

   Decode *filename* from the filesystem encoding with
   ``'surrogateescape'`` error handler, or ``'strict'`` on Windows;
   return ``str`` unchanged.

   ``fsencode()`` is the reverse function.

   New in version 3.2.

os.get_exec_path(env=None)

   Returns the list of directories that will be searched for a named
   executable, similar to a shell, when launching a process. *env*,
   when specified, should be an environment variable dictionary to
   lookup the PATH in. By default, when *env* is None, ``environ`` is
   used.

   New in version 3.2.

os.ctermid()

   Return the filename corresponding to the controlling terminal of
   the process.

   Availability: Unix.

os.getegid()

   Return the effective group id of the current process.  This
   corresponds to the "set id" bit on the file being executed in the
   current process.

   Availability: Unix.

os.geteuid()

   Return the current process's effective user id.

   Availability: Unix.

os.getgid()

   Return the real group id of the current process.

   Availability: Unix.

os.getgroups()

   Return list of supplemental group ids associated with the current
   process.

   Availability: Unix.

os.initgroups(username, gid)

   Call the system initgroups() to initialize the group access list
   with all of the groups of which the specified username is a member,
   plus the specified group id.

   Availability: Unix.

   New in version 3.2.

os.getlogin()

   Return the name of the user logged in on the controlling terminal
   of the process.  For most purposes, it is more useful to use the
   environment variables ``LOGNAME`` or ``USERNAME`` to find out who
   the user is, or ``pwd.getpwuid(os.getuid())[0]`` to get the login
   name of the currently effective user id.

   Availability: Unix, Windows.

os.getpgid(pid)

   Return the process group id of the process with process id *pid*.
   If *pid* is 0, the process group id of the current process is
   returned.

   Availability: Unix.

os.getpgrp()

   Return the id of the current process group.

   Availability: Unix.

os.getpid()

   Return the current process id.

   Availability: Unix, Windows.

os.getppid()

   Return the parent's process id.  When the parent process has
   exited, on Unix the id returned is the one of the init process (1),
   on Windows it is still the same id, which may be already reused by
   another process.

   Availability: Unix, Windows

   Changed in version 3.2: Added support for Windows.

os.getresuid()

   Return a tuple (ruid, euid, suid) denoting the current process's
   real, effective, and saved user ids.

   Availability: Unix.

   New in version 3.2.

os.getresgid()

   Return a tuple (rgid, egid, sgid) denoting the current process's
   real, effective, and saved group ids.

   Availability: Unix.

   New in version 3.2.

os.getuid()

   Return the current process's user id.

   Availability: Unix.

os.getenv(key, default=None)

   Return the value of the environment variable *key* if it exists, or
   *default* if it doesn't. *key*, *default* and the result are str.

   On Unix, keys and values are decoded with
   ``sys.getfilesystemencoding()`` and ``'surrogateescape'`` error
   handler. Use ``os.getenvb()`` if you would like to use a different
   encoding.

   Availability: most flavors of Unix, Windows.

os.getenvb(key, default=None)

   Return the value of the environment variable *key* if it exists, or
   *default* if it doesn't. *key*, *default* and the result are bytes.

   Availability: most flavors of Unix.

   New in version 3.2.

os.putenv(key, value)

   Set the environment variable named *key* to the string *value*.
   Such changes to the environment affect subprocesses started with
   ``os.system()``, ``popen()`` or ``fork()`` and ``execv()``.

   Availability: most flavors of Unix, Windows.

   Note: On some platforms, including FreeBSD and Mac OS X, setting
     ``environ`` may cause memory leaks. Refer to the system
     documentation for putenv.

   When ``putenv()`` is supported, assignments to items in
   ``os.environ`` are automatically translated into corresponding
   calls to ``putenv()``; however, calls to ``putenv()`` don't update
   ``os.environ``, so it is actually preferable to assign to items of
   ``os.environ``.

os.setegid(egid)

   Set the current process's effective group id.

   Availability: Unix.

os.seteuid(euid)

   Set the current process's effective user id.

   Availability: Unix.

os.setgid(gid)

   Set the current process' group id.

   Availability: Unix.

os.setgroups(groups)

   Set the list of supplemental group ids associated with the current
   process to *groups*. *groups* must be a sequence, and each element
   must be an integer identifying a group. This operation is typically
   available only to the superuser.

   Availability: Unix.

os.setpgrp()

   Call the system call ``setpgrp()`` or ``setpgrp(0, 0)()`` depending
   on which version is implemented (if any).  See the Unix manual for
   the semantics.

   Availability: Unix.

os.setpgid(pid, pgrp)

   Call the system call ``setpgid()`` to set the process group id of
   the process with id *pid* to the process group with id *pgrp*.  See
   the Unix manual for the semantics.

   Availability: Unix.

os.setregid(rgid, egid)

   Set the current process's real and effective group ids.

   Availability: Unix.

os.setresgid(rgid, egid, sgid)

   Set the current process's real, effective, and saved group ids.

   Availability: Unix.

   New in version 3.2.

os.setresuid(ruid, euid, suid)

   Set the current process's real, effective, and saved user ids.

   Availability: Unix.

   New in version 3.2.

os.setreuid(ruid, euid)

   Set the current process's real and effective user ids.

   Availability: Unix.

os.getsid(pid)

   Call the system call ``getsid()``.  See the Unix manual for the
   semantics.

   Availability: Unix.

os.setsid()

   Call the system call ``setsid()``.  See the Unix manual for the
   semantics.

   Availability: Unix.

os.setuid(uid)

   Set the current process's user id.

   Availability: Unix.

os.strerror(code)

   Return the error message corresponding to the error code in *code*.
   On platforms where ``strerror()`` returns ``NULL`` when given an
   unknown error number, ``ValueError`` is raised.

   Availability: Unix, Windows.

os.supports_bytes_environ

   True if the native OS type of the environment is bytes (eg. False
   on Windows).

   New in version 3.2.

os.umask(mask)

   Set the current numeric umask and return the previous umask.

   Availability: Unix, Windows.

os.uname()

   Return a 5-tuple containing information identifying the current
   operating system.  The tuple contains 5 strings: ``(sysname,
   nodename, release, version, machine)``.  Some systems truncate the
   nodename to 8 characters or to the leading component; a better way
   to get the hostname is ``socket.gethostname()``  or even
   ``socket.gethostbyaddr(socket.gethostname())``.

   Availability: recent flavors of Unix.

os.unsetenv(key)

   Unset (delete) the environment variable named *key*. Such changes
   to the environment affect subprocesses started with
   ``os.system()``, ``popen()`` or ``fork()`` and ``execv()``.

   When ``unsetenv()`` is supported, deletion of items in
   ``os.environ`` is automatically translated into a corresponding
   call to ``unsetenv()``; however, calls to ``unsetenv()`` don't
   update ``os.environ``, so it is actually preferable to delete items
   of ``os.environ``.

   Availability: most flavors of Unix, Windows.


File Object Creation
====================

These functions create new *file objects*. (See also ``open()``.)

os.fdopen(fd[, mode[, bufsize]])

   Return an open file object connected to the file descriptor *fd*.
   The *mode* and *bufsize* arguments have the same meaning as the
   corresponding arguments to the built-in ``open()`` function.

   When specified, the *mode* argument must start with one of the
   letters ``'r'``, ``'w'``, or ``'a'``, otherwise a ``ValueError`` is
   raised.

   On Unix, when the *mode* argument starts with ``'a'``, the
   *O_APPEND* flag is set on the file descriptor (which the
   ``fdopen()`` implementation already does on most platforms).

   Availability: Unix, Windows.


File Descriptor Operations
==========================

These functions operate on I/O streams referenced using file
descriptors.

File descriptors are small integers corresponding to a file that has
been opened by the current process.  For example, standard input is
usually file descriptor 0, standard output is 1, and standard error is
2.  Further files opened by a process will then be assigned 3, 4, 5,
and so forth.  The name "file descriptor" is slightly deceptive; on
Unix platforms, sockets and pipes are also referenced by file
descriptors.

The ``fileno()`` method can be used to obtain the file descriptor
associated with a *file object* when required.  Note that using the
file descriptor directly will bypass the file object methods, ignoring
aspects such as internal buffering of data.

os.close(fd)

   Close file descriptor *fd*.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O and must be applied
     to a file descriptor as returned by ``os.open()`` or ``pipe()``.
     To close a "file object" returned by the built-in function
     ``open()`` or by ``popen()`` or ``fdopen()``, use its ``close()``
     method.

os.closerange(fd_low, fd_high)

   Close all file descriptors from *fd_low* (inclusive) to *fd_high*
   (exclusive), ignoring errors. Equivalent to:

      for fd in range(fd_low, fd_high):
          try:
              os.close(fd)
          except OSError:
              pass

   Availability: Unix, Windows.

os.device_encoding(fd)

   Return a string describing the encoding of the device associated
   with *fd* if it is connected to a terminal; else return ``None``.

os.dup(fd)

   Return a duplicate of file descriptor *fd*.

   Availability: Unix, Windows.

os.dup2(fd, fd2)

   Duplicate file descriptor *fd* to *fd2*, closing the latter first
   if necessary.

   Availability: Unix, Windows.

os.fchmod(fd, mode)

   Change the mode of the file given by *fd* to the numeric *mode*.
   See the docs for ``chmod()`` for possible values of *mode*.

   Availability: Unix.

os.fchown(fd, uid, gid)

   Change the owner and group id of the file given by *fd* to the
   numeric *uid* and *gid*.  To leave one of the ids unchanged, set it
   to -1.

   Availability: Unix.

os.fdatasync(fd)

   Force write of file with filedescriptor *fd* to disk. Does not
   force update of metadata.

   Availability: Unix.

   Note: This function is not available on MacOS.

os.fpathconf(fd, name)

   Return system configuration information relevant to an open file.
   *name* specifies the configuration value to retrieve; it may be a
   string which is the name of a defined system value; these names are
   specified in a number of standards (POSIX.1, Unix 95, Unix 98, and
   others).  Some platforms define additional names as well.  The
   names known to the host operating system are given in the
   ``pathconf_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also
   accepted.

   If *name* is a string and is not known, ``ValueError`` is raised.
   If a specific value for *name* is not supported by the host system,
   even if it is included in ``pathconf_names``, an ``OSError`` is
   raised with ``errno.EINVAL`` for the error number.

   Availability: Unix.

os.fstat(fd)

   Return status for file descriptor *fd*, like ``stat()``.

   Availability: Unix, Windows.

os.fstatvfs(fd)

   Return information about the filesystem containing the file
   associated with file descriptor *fd*, like ``statvfs()``.

   Availability: Unix.

os.fsync(fd)

   Force write of file with filedescriptor *fd* to disk.  On Unix,
   this calls the native ``fsync()`` function; on Windows, the MS
   ``_commit()`` function.

   If you're starting with a buffered Python *file object* *f*, first
   do ``f.flush()``, and then do ``os.fsync(f.fileno())``, to ensure
   that all internal buffers associated with *f* are written to disk.

   Availability: Unix, and Windows.

os.ftruncate(fd, length)

   Truncate the file corresponding to file descriptor *fd*, so that it
   is at most *length* bytes in size.

   Availability: Unix.

os.isatty(fd)

   Return ``True`` if the file descriptor *fd* is open and connected
   to a tty(-like) device, else ``False``.

   Availability: Unix.

os.lseek(fd, pos, how)

   Set the current position of file descriptor *fd* to position *pos*,
   modified by *how*: ``SEEK_SET`` or ``0`` to set the position
   relative to the beginning of the file; ``SEEK_CUR`` or ``1`` to set
   it relative to the current position; ``os.SEEK_END`` or ``2`` to
   set it relative to the end of the file. Return the new cursor
   position in bytes, starting from the beginning.

   Availability: Unix, Windows.

os.SEEK_SET
os.SEEK_CUR
os.SEEK_END

   Parameters to the ``lseek()`` function. Their values are 0, 1, and
   2, respectively. Availability: Windows, Unix.

os.open(file, flags[, mode])

   Open the file *file* and set various flags according to *flags* and
   possibly its mode according to *mode*.  The default *mode* is
   ``0o777`` (octal), and the current umask value is first masked out.
   Return the file descriptor for the newly opened file.

   For a description of the flag and mode values, see the C run-time
   documentation; flag constants (like ``O_RDONLY`` and ``O_WRONLY``)
   are defined in this module too (see *open() flag constants*).  In
   particular, on Windows adding ``O_BINARY`` is needed to open files
   in binary mode.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O.  For normal usage,
     use the built-in function ``open()``, which returns a *file
     object* with ``read()`` and ``write()`` methods (and many more).
     To wrap a file descriptor in a file object, use ``fdopen()``.

os.openpty()

   Open a new pseudo-terminal pair. Return a pair of file descriptors
   ``(master, slave)`` for the pty and the tty, respectively. For a
   (slightly) more portable approach, use the ``pty`` module.

   Availability: some flavors of Unix.

os.pipe()

   Create a pipe.  Return a pair of file descriptors ``(r, w)`` usable
   for reading and writing, respectively.

   Availability: Unix, Windows.

os.read(fd, n)

   Read at most *n* bytes from file descriptor *fd*. Return a
   bytestring containing the bytes read.  If the end of the file
   referred to by *fd* has been reached, an empty bytes object is
   returned.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O and must be applied
     to a file descriptor as returned by ``os.open()`` or ``pipe()``.
     To read a "file object" returned by the built-in function
     ``open()`` or by ``popen()`` or ``fdopen()``, or ``sys.stdin``,
     use its ``read()`` or ``readline()`` methods.

os.tcgetpgrp(fd)

   Return the process group associated with the terminal given by *fd*
   (an open file descriptor as returned by ``os.open()``).

   Availability: Unix.

os.tcsetpgrp(fd, pg)

   Set the process group associated with the terminal given by *fd*
   (an open file descriptor as returned by ``os.open()``) to *pg*.

   Availability: Unix.

os.ttyname(fd)

   Return a string which specifies the terminal device associated with
   file descriptor *fd*.  If *fd* is not associated with a terminal
   device, an exception is raised.

   Availability: Unix.

os.write(fd, str)

   Write the bytestring in *str* to file descriptor *fd*. Return the
   number of bytes actually written.

   Availability: Unix, Windows.

   Note: This function is intended for low-level I/O and must be applied
     to a file descriptor as returned by ``os.open()`` or ``pipe()``.
     To write a "file object" returned by the built-in function
     ``open()`` or by ``popen()`` or ``fdopen()``, or ``sys.stdout``
     or ``sys.stderr``, use its ``write()`` method.


``open()`` flag constants
-------------------------

The following constants are options for the *flags* parameter to the
``open()`` function.  They can be combined using the bitwise OR
operator ``|``.  Some of them are not available on all platforms.  For
descriptions of their availability and use, consult the *open(2)*
manual page on Unix or the MSDN on Windows.

os.O_RDONLY
os.O_WRONLY
os.O_RDWR
os.O_APPEND
os.O_CREAT
os.O_EXCL
os.O_TRUNC

   These constants are available on Unix and Windows.

os.O_DSYNC
os.O_RSYNC
os.O_SYNC
os.O_NDELAY
os.O_NONBLOCK
os.O_NOCTTY
os.O_SHLOCK
os.O_EXLOCK

   These constants are only available on Unix.

os.O_BINARY
os.O_NOINHERIT
os.O_SHORT_LIVED
os.O_TEMPORARY
os.O_RANDOM
os.O_SEQUENTIAL
os.O_TEXT

   These constants are only available on Windows.

os.O_ASYNC
os.O_DIRECT
os.O_DIRECTORY
os.O_NOFOLLOW
os.O_NOATIME

   These constants are GNU extensions and not present if they are not
   defined by the C library.


Files and Directories
=====================

os.access(path, mode)

   Use the real uid/gid to test for access to *path*.  Note that most
   operations will use the effective uid/gid, therefore this routine
   can be used in a suid/sgid environment to test if the invoking user
   has the specified access to *path*.  *mode* should be ``F_OK`` to
   test the existence of *path*, or it can be the inclusive OR of one
   or more of ``R_OK``, ``W_OK``, and ``X_OK`` to test permissions.
   Return ``True`` if access is allowed, ``False`` if not. See the
   Unix man page *access(2)* for more information.

   Availability: Unix, Windows.

   Note: Using ``access()`` to check if a user is authorized to e.g. open
     a file before actually doing so using ``open()`` creates a
     security hole, because the user might exploit the short time
     interval between checking and opening the file to manipulate it.
     It's preferable to use *EAFP* techniques. For example:

        if os.access("myfile", os.R_OK):
            with open("myfile") as fp:
                return fp.read()
        return "some default data"

     is better written as:

        try:
            fp = open("myfile")
        except IOError as e:
            if e.errno == errno.EACCES:
                return "some default data"
            # Not a permission error.
            raise
        else:
            with fp:
                return fp.read()

   Note: I/O operations may fail even when ``access()`` indicates that
     they would succeed, particularly for operations on network
     filesystems which may have permissions semantics beyond the usual
     POSIX permission-bit model.

os.F_OK

   Value to pass as the *mode* parameter of ``access()`` to test the
   existence of *path*.

os.R_OK

   Value to include in the *mode* parameter of ``access()`` to test
   the readability of *path*.

os.W_OK

   Value to include in the *mode* parameter of ``access()`` to test
   the writability of *path*.

os.X_OK

   Value to include in the *mode* parameter of ``access()`` to
   determine if *path* can be executed.

os.chdir(path)

   Change the current working directory to *path*.

   Availability: Unix, Windows.

os.fchdir(fd)

   Change the current working directory to the directory represented
   by the file descriptor *fd*.  The descriptor must refer to an
   opened directory, not an open file.

   Availability: Unix.

os.getcwd()

   Return a string representing the current working directory.

   Availability: Unix, Windows.

os.getcwdb()

   Return a bytestring representing the current working directory.

   Availability: Unix, Windows.

os.chflags(path, flags)

   Set the flags of *path* to the numeric *flags*. *flags* may take a
   combination (bitwise OR) of the following values (as defined in the
   ``stat`` module):

   * ``stat.UF_NODUMP``

   * ``stat.UF_IMMUTABLE``

   * ``stat.UF_APPEND``

   * ``stat.UF_OPAQUE``

   * ``stat.UF_NOUNLINK``

   * ``stat.UF_COMPRESSED``

   * ``stat.UF_HIDDEN``

   * ``stat.SF_ARCHIVED``

   * ``stat.SF_IMMUTABLE``

   * ``stat.SF_APPEND``

   * ``stat.SF_NOUNLINK``

   * ``stat.SF_SNAPSHOT``

   Availability: Unix.

os.chroot(path)

   Change the root directory of the current process to *path*.
   Availability: Unix.

os.chmod(path, mode)

   Change the mode of *path* to the numeric *mode*. *mode* may take
   one of the following values (as defined in the ``stat`` module) or
   bitwise ORed combinations of them:

   * ``stat.S_ISUID``

   * ``stat.S_ISGID``

   * ``stat.S_ENFMT``

   * ``stat.S_ISVTX``

   * ``stat.S_IREAD``

   * ``stat.S_IWRITE``

   * ``stat.S_IEXEC``

   * ``stat.S_IRWXU``

   * ``stat.S_IRUSR``

   * ``stat.S_IWUSR``

   * ``stat.S_IXUSR``

   * ``stat.S_IRWXG``

   * ``stat.S_IRGRP``

   * ``stat.S_IWGRP``

   * ``stat.S_IXGRP``

   * ``stat.S_IRWXO``

   * ``stat.S_IROTH``

   * ``stat.S_IWOTH``

   * ``stat.S_IXOTH``

   Availability: Unix, Windows.

   Note: Although Windows supports ``chmod()``, you can only  set the
     file's read-only flag with it (via the ``stat.S_IWRITE``  and
     ``stat.S_IREAD`` constants or a corresponding integer value).
     All other bits are ignored.

os.chown(path, uid, gid)

   Change the owner and group id of *path* to the numeric *uid* and
   *gid*. To leave one of the ids unchanged, set it to -1.

   Availability: Unix.

os.lchflags(path, flags)

   Set the flags of *path* to the numeric *flags*, like ``chflags()``,
   but do not follow symbolic links.

   Availability: Unix.

os.lchmod(path, mode)

   Change the mode of *path* to the numeric *mode*. If path is a
   symlink, this affects the symlink rather than the target. See the
   docs for ``chmod()`` for possible values of *mode*.

   Availability: Unix.

os.lchown(path, uid, gid)

   Change the owner and group id of *path* to the numeric *uid* and
   *gid*. This function will not follow symbolic links.

   Availability: Unix.

os.link(source, link_name)

   Create a hard link pointing to *source* named *link_name*.

   Availability: Unix, Windows.

   Changed in version 3.2: Added Windows support.

os.listdir(path='.')

   Return a list containing the names of the entries in the directory
   given by *path* (default: ``'.'``).  The list is in arbitrary
   order.  It does not include the special entries ``'.'`` and
   ``'..'`` even if they are present in the directory.

   This function can be called with a bytes or string argument, and
   returns filenames of the same datatype.

   Availability: Unix, Windows.

   Changed in version 3.2: The *path* parameter became optional.

os.lstat(path)

   Perform the equivalent of an ``lstat()`` system call on the given
   path. Similar to ``stat()``, but does not follow symbolic links.
   On platforms that do not support symbolic links, this is an alias
   for ``stat()``.

   Changed in version 3.2: Added support for Windows 6.0 (Vista)
   symbolic links.

os.mkfifo(path[, mode])

   Create a FIFO (a named pipe) named *path* with numeric mode *mode*.
   The default *mode* is ``0o666`` (octal).  The current umask value
   is first masked out from the mode.

   FIFOs are pipes that can be accessed like regular files.  FIFOs
   exist until they are deleted (for example with ``os.unlink()``).
   Generally, FIFOs are used as rendezvous between "client" and
   "server" type processes: the server opens the FIFO for reading, and
   the client opens it for writing.  Note that ``mkfifo()`` doesn't
   open the FIFO --- it just creates the rendezvous point.

   Availability: Unix.

os.mknod(filename[, mode=0o600[, device]])

   Create a filesystem node (file, device special file or named pipe)
   named *filename*. *mode* specifies both the permissions to use and
   the type of node to be created, being combined (bitwise OR) with
   one of ``stat.S_IFREG``, ``stat.S_IFCHR``, ``stat.S_IFBLK``, and
   ``stat.S_IFIFO`` (those constants are available in ``stat``).  For
   ``stat.S_IFCHR`` and ``stat.S_IFBLK``, *device* defines the newly
   created device special file (probably using ``os.makedev()``),
   otherwise it is ignored.

os.major(device)

   Extract the device major number from a raw device number (usually
   the ``st_dev`` or ``st_rdev`` field from ``stat``).

os.minor(device)

   Extract the device minor number from a raw device number (usually
   the ``st_dev`` or ``st_rdev`` field from ``stat``).

os.makedev(major, minor)

   Compose a raw device number from the major and minor device
   numbers.

os.mkdir(path[, mode])

   Create a directory named *path* with numeric mode *mode*. The
   default *mode* is ``0o777`` (octal).  On some systems, *mode* is
   ignored.  Where it is used, the current umask value is first masked
   out.  If the directory already exists, ``OSError`` is raised.

   It is also possible to create temporary directories; see the
   ``tempfile`` module's ``tempfile.mkdtemp()`` function.

   Availability: Unix, Windows.

os.makedirs(path, mode=0o777, exist_ok=False)

   Recursive directory creation function.  Like ``mkdir()``, but makes
   all intermediate-level directories needed to contain the leaf
   directory.  If the target directory with the same mode as specified
   already exists, raises an ``OSError`` exception if *exist_ok* is
   False, otherwise no exception is raised.  If the directory cannot
   be created in other cases, raises an ``OSError`` exception.  The
   default *mode* is ``0o777`` (octal). On some systems, *mode* is
   ignored.  Where it is used, the current umask value is first masked
   out.

   Note: ``makedirs()`` will become confused if the path elements to
     create include ``pardir``.

   This function handles UNC paths correctly.

   New in version 3.2: The *exist_ok* parameter.

os.pathconf(path, name)

   Return system configuration information relevant to a named file.
   *name* specifies the configuration value to retrieve; it may be a
   string which is the name of a defined system value; these names are
   specified in a number of standards (POSIX.1, Unix 95, Unix 98, and
   others).  Some platforms define additional names as well.  The
   names known to the host operating system are given in the
   ``pathconf_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also
   accepted.

   If *name* is a string and is not known, ``ValueError`` is raised.
   If a specific value for *name* is not supported by the host system,
   even if it is included in ``pathconf_names``, an ``OSError`` is
   raised with ``errno.EINVAL`` for the error number.

   Availability: Unix.

os.pathconf_names

   Dictionary mapping names accepted by ``pathconf()`` and
   ``fpathconf()`` to the integer values defined for those names by
   the host operating system.  This can be used to determine the set
   of names known to the system. Availability: Unix.

os.readlink(path)

   Return a string representing the path to which the symbolic link
   points.  The result may be either an absolute or relative pathname;
   if it is relative, it may be converted to an absolute pathname
   using ``os.path.join(os.path.dirname(path), result)``.

   If the *path* is a string object, the result will also be a string
   object, and the call may raise an UnicodeDecodeError. If the *path*
   is a bytes object, the result will be a bytes object.

   Availability: Unix, Windows

   Changed in version 3.2: Added support for Windows 6.0 (Vista)
   symbolic links.

os.remove(path)

   Remove (delete) the file *path*.  If *path* is a directory,
   ``OSError`` is raised; see ``rmdir()`` below to remove a directory.
   This is identical to the ``unlink()`` function documented below.
   On Windows, attempting to remove a file that is in use causes an
   exception to be raised; on Unix, the directory entry is removed but
   the storage allocated to the file is not made available until the
   original file is no longer in use.

   Availability: Unix, Windows.

os.removedirs(path)

   Remove directories recursively.  Works like ``rmdir()`` except
   that, if the leaf directory is successfully removed,
   ``removedirs()``  tries to successively remove every parent
   directory mentioned in  *path* until an error is raised (which is
   ignored, because it generally means that a parent directory is not
   empty). For example, ``os.removedirs('foo/bar/baz')`` will first
   remove the directory ``'foo/bar/baz'``, and then remove
   ``'foo/bar'`` and ``'foo'`` if they are empty. Raises ``OSError``
   if the leaf directory could not be successfully removed.

os.rename(src, dst)

   Rename the file or directory *src* to *dst*.  If *dst* is a
   directory, ``OSError`` will be raised.  On Unix, if *dst* exists
   and is a file, it will be replaced silently if the user has
   permission.  The operation may fail on some Unix flavors if *src*
   and *dst* are on different filesystems.  If successful, the
   renaming will be an atomic operation (this is a POSIX requirement).
   On Windows, if *dst* already exists, ``OSError`` will be raised
   even if it is a file; there may be no way to implement an atomic
   rename when *dst* names an existing file.

   Availability: Unix, Windows.

os.renames(old, new)

   Recursive directory or file renaming function. Works like
   ``rename()``, except creation of any intermediate directories
   needed to make the new pathname good is attempted first. After the
   rename, directories corresponding to rightmost path segments of the
   old name will be pruned away using ``removedirs()``.

   Note: This function can fail with the new directory structure made if
     you lack permissions needed to remove the leaf directory or file.

os.rmdir(path)

   Remove (delete) the directory *path*.  Only works when the
   directory is empty, otherwise, ``OSError`` is raised.  In order to
   remove whole directory trees, ``shutil.rmtree()`` can be used.

   Availability: Unix, Windows.

os.stat(path)

   Perform the equivalent of a ``stat()`` system call on the given
   path. (This function follows symlinks; to stat a symlink use
   ``lstat()``.)

   The return value is an object whose attributes correspond to the
   members of the ``stat`` structure, namely:

   * ``st_mode`` - protection bits,

   * ``st_ino`` - inode number,

   * ``st_dev`` - device,

   * ``st_nlink`` - number of hard links,

   * ``st_uid`` - user id of owner,

   * ``st_gid`` - group id of owner,

   * ``st_size`` - size of file, in bytes,

   * ``st_atime`` - time of most recent access,

   * ``st_mtime`` - time of most recent content modification,

   * ``st_ctime`` - platform dependent; time of most recent metadata
     change on Unix, or the time of creation on Windows)

   On some Unix systems (such as Linux), the following attributes may
   also be available:

   * ``st_blocks`` - number of blocks allocated for file

   * ``st_blksize`` - filesystem blocksize

   * ``st_rdev`` - type of device if an inode device

   * ``st_flags`` - user defined flags for file

   On other Unix systems (such as FreeBSD), the following attributes
   may be available (but may be only filled out if root tries to use
   them):

   * ``st_gen`` - file generation number

   * ``st_birthtime`` - time of file creation

   On Mac OS systems, the following attributes may also be available:

   * ``st_rsize``

   * ``st_creator``

   * ``st_type``

   Note: The exact meaning and resolution of the ``st_atime``,
     ``st_mtime``, and ``st_ctime`` attributes depend on the operating
     system and the file system. For example, on Windows systems using
     the FAT or FAT32 file systems, ``st_mtime`` has 2-second
     resolution, and ``st_atime`` has only 1-day resolution.  See your
     operating system documentation for details.

   For backward compatibility, the return value of ``stat()`` is also
   accessible as a tuple of at least 10 integers giving the most
   important (and portable) members of the ``stat`` structure, in the
   order ``st_mode``, ``st_ino``, ``st_dev``, ``st_nlink``,
   ``st_uid``, ``st_gid``, ``st_size``, ``st_atime``, ``st_mtime``,
   ``st_ctime``. More items may be added at the end by some
   implementations.

   The standard module ``stat`` defines functions and constants that
   are useful for extracting information from a ``stat`` structure.
   (On Windows, some items are filled with dummy values.)

   Example:

      >>> import os
      >>> statinfo = os.stat('somefile.txt')
      >>> statinfo
      posix.stat_result(st_mode=33188, st_ino=7876932, st_dev=234881026,
      st_nlink=1, st_uid=501, st_gid=501, st_size=264, st_atime=1297230295,
      st_mtime=1297230027, st_ctime=1297230027)
      >>> statinfo.st_size
      264

   Availability: Unix, Windows.

os.stat_float_times([newvalue])

   Determine whether ``stat_result`` represents time stamps as float
   objects. If *newvalue* is ``True``, future calls to ``stat()``
   return floats, if it is ``False``, future calls return ints. If
   *newvalue* is omitted, return the current setting.

   For compatibility with older Python versions, accessing
   ``stat_result`` as a tuple always returns integers.

   Python now returns float values by default. Applications which do
   not work correctly with floating point time stamps can use this
   function to restore the old behaviour.

   The resolution of the timestamps (that is the smallest possible
   fraction) depends on the system. Some systems only support second
   resolution; on these systems, the fraction will always be zero.

   It is recommended that this setting is only changed at program
   startup time in the *__main__* module; libraries should never
   change this setting. If an application uses a library that works
   incorrectly if floating point time stamps are processed, this
   application should turn the feature off until the library has been
   corrected.

os.statvfs(path)

   Perform a ``statvfs()`` system call on the given path.  The return
   value is an object whose attributes describe the filesystem on the
   given path, and correspond to the members of the ``statvfs``
   structure, namely: ``f_bsize``, ``f_frsize``, ``f_blocks``,
   ``f_bfree``, ``f_bavail``, ``f_files``, ``f_ffree``, ``f_favail``,
   ``f_flag``, ``f_namemax``.

   Two module-level constants are defined for the ``f_flag``
   attribute's bit-flags: if ``ST_RDONLY`` is set, the filesystem is
   mounted read-only, and if ``ST_NOSUID`` is set, the semantics of
   setuid/setgid bits are disabled or not supported.

   Changed in version 3.2: The ``ST_RDONLY`` and ``ST_NOSUID``
   constants were added.

   Availability: Unix.

os.symlink(source, link_name)
os.symlink(source, link_name, target_is_directory=False)

   Create a symbolic link pointing to *source* named *link_name*.

   On Windows, symlink version takes an additional optional parameter,
   *target_is_directory*, which defaults to ``False``.

   On Windows, a symlink represents a file or a directory, and does
   not morph to the target dynamically.  If *target_is_directory* is
   set to ``True``, the symlink will be created as a directory
   symlink, otherwise as a file symlink (the default).

   Symbolic link support was introduced in Windows 6.0 (Vista).
   ``symlink()`` will raise a ``NotImplementedError`` on Windows
   versions earlier than 6.0.

   Note: The *SeCreateSymbolicLinkPrivilege* is required in order to
     successfully create symlinks. This privilege is not typically
     granted to regular users but is available to accounts which can
     escalate privileges to the administrator level. Either obtaining
     the privilege or running your application as an administrator are
     ways to successfully create symlinks.``OSError`` is raised when
     the function is called by an unprivileged user.

   Availability: Unix, Windows.

   Changed in version 3.2: Added support for Windows 6.0 (Vista)
   symbolic links.

os.unlink(path)

   Remove (delete) the file *path*.  This is the same function as
   ``remove()``; the ``unlink()`` name is its traditional Unix name.

   Availability: Unix, Windows.

os.utime(path, times)

   Set the access and modified times of the file specified by *path*.
   If *times* is ``None``, then the file's access and modified times
   are set to the current time. (The effect is similar to running the
   Unix program **touch** on the path.)  Otherwise, *times* must be a
   2-tuple of numbers, of the form ``(atime, mtime)`` which is used to
   set the access and modified times, respectively. Whether a
   directory can be given for *path* depends on whether the operating
   system implements directories as files (for example, Windows does
   not).  Note that the exact times you set here may not be returned
   by a subsequent ``stat()`` call, depending on the resolution with
   which your operating system records access and modification times;
   see ``stat()``.

   Availability: Unix, Windows.

os.walk(top, topdown=True, onerror=None, followlinks=False)

   Generate the file names in a directory tree by walking the tree
   either top-down or bottom-up. For each directory in the tree rooted
   at directory *top* (including *top* itself), it yields a 3-tuple
   ``(dirpath, dirnames, filenames)``.

   *dirpath* is a string, the path to the directory.  *dirnames* is a
   list of the names of the subdirectories in *dirpath* (excluding
   ``'.'`` and ``'..'``). *filenames* is a list of the names of the
   non-directory files in *dirpath*. Note that the names in the lists
   contain no path components.  To get a full path (which begins with
   *top*) to a file or directory in *dirpath*, do
   ``os.path.join(dirpath, name)``.

   If optional argument *topdown* is ``True`` or not specified, the
   triple for a directory is generated before the triples for any of
   its subdirectories (directories are generated top-down).  If
   *topdown* is ``False``, the triple for a directory is generated
   after the triples for all of its subdirectories (directories are
   generated bottom-up).

   When *topdown* is ``True``, the caller can modify the *dirnames*
   list in-place (perhaps using ``del`` or slice assignment), and
   ``walk()`` will only recurse into the subdirectories whose names
   remain in *dirnames*; this can be used to prune the search, impose
   a specific order of visiting, or even to inform ``walk()`` about
   directories the caller creates or renames before it resumes
   ``walk()`` again.  Modifying *dirnames* when *topdown* is ``False``
   is ineffective, because in bottom-up mode the directories in
   *dirnames* are generated before *dirpath* itself is generated.

   By default, errors from the ``listdir()`` call are ignored.  If
   optional argument *onerror* is specified, it should be a function;
   it will be called with one argument, an ``OSError`` instance.  It
   can report the error to continue with the walk, or raise the
   exception to abort the walk.  Note that the filename is available
   as the ``filename`` attribute of the exception object.

   By default, ``walk()`` will not walk down into symbolic links that
   resolve to directories. Set *followlinks* to ``True`` to visit
   directories pointed to by symlinks, on systems that support them.

   Note: Be aware that setting *followlinks* to ``True`` can lead to
     infinite recursion if a link points to a parent directory of
     itself. ``walk()`` does not keep track of the directories it
     visited already.

   Note: If you pass a relative pathname, don't change the current working
     directory between resumptions of ``walk()``.  ``walk()`` never
     changes the current directory, and assumes that its caller
     doesn't either.

   This example displays the number of bytes taken by non-directory
   files in each directory under the starting directory, except that
   it doesn't look under any CVS subdirectory:

      import os
      from os.path import join, getsize
      for root, dirs, files in os.walk('python/Lib/email'):
          print(root, "consumes", end=" ")
          print(sum(getsize(join(root, name)) for name in files), end=" ")
          print("bytes in", len(files), "non-directory files")
          if 'CVS' in dirs:
              dirs.remove('CVS')  # don't visit CVS directories

   In the next example, walking the tree bottom-up is essential:
   ``rmdir()`` doesn't allow deleting a directory before the directory
   is empty:

      # Delete everything reachable from the directory named in "top",
      # assuming there are no symbolic links.
      # CAUTION:  This is dangerous!  For example, if top == '/', it
      # could delete all your disk files.
      import os
      for root, dirs, files in os.walk(top, topdown=False):
          for name in files:
              os.remove(os.path.join(root, name))
          for name in dirs:
              os.rmdir(os.path.join(root, name))


Process Management
==================

These functions may be used to create and manage processes.

The various ``exec*()`` functions take a list of arguments for the new
program loaded into the process.  In each case, the first of these
arguments is passed to the new program as its own name rather than as
an argument a user may have typed on a command line.  For the C
programmer, this is the ``argv[0]`` passed to a program's ``main()``.
For example, ``os.execv('/bin/echo', ['foo', 'bar'])`` will only print
``bar`` on standard output; ``foo`` will seem to be ignored.

os.abort()

   Generate a ``SIGABRT`` signal to the current process.  On Unix, the
   default behavior is to produce a core dump; on Windows, the process
   immediately returns an exit code of ``3``.  Be aware that calling
   this function will not call the Python signal handler registered
   for ``SIGABRT`` with ``signal.signal()``.

   Availability: Unix, Windows.

os.execl(path, arg0, arg1, ...)
os.execle(path, arg0, arg1, ..., env)
os.execlp(file, arg0, arg1, ...)
os.execlpe(file, arg0, arg1, ..., env)
os.execv(path, args)
os.execve(path, args, env)
os.execvp(file, args)
os.execvpe(file, args, env)

   These functions all execute a new program, replacing the current
   process; they do not return.  On Unix, the new executable is loaded
   into the current process, and will have the same process id as the
   caller.  Errors will be reported as ``OSError`` exceptions.

   The current process is replaced immediately. Open file objects and
   descriptors are not flushed, so if there may be data buffered on
   these open files, you should flush them using
   ``sys.stdout.flush()`` or ``os.fsync()`` before calling an
   ``exec*()`` function.

   The "l" and "v" variants of the ``exec*()`` functions differ in how
   command-line arguments are passed.  The "l" variants are perhaps
   the easiest to work with if the number of parameters is fixed when
   the code is written; the individual parameters simply become
   additional parameters to the ``execl*()`` functions.  The "v"
   variants are good when the number of parameters is variable, with
   the arguments being passed in a list or tuple as the *args*
   parameter.  In either case, the arguments to the child process
   should start with the name of the command being run, but this is
   not enforced.

   The variants which include a "p" near the end (``execlp()``,
   ``execlpe()``, ``execvp()``, and ``execvpe()``) will use the
   ``PATH`` environment variable to locate the program *file*.  When
   the environment is being replaced (using one of the ``exec*e()``
   variants, discussed in the next paragraph), the new environment is
   used as the source of the ``PATH`` variable. The other variants,
   ``execl()``, ``execle()``, ``execv()``, and ``execve()``, will not
   use the ``PATH`` variable to locate the executable; *path* must
   contain an appropriate absolute or relative path.

   For ``execle()``, ``execlpe()``, ``execve()``, and ``execvpe()``
   (note that these all end in "e"), the *env* parameter must be a
   mapping which is used to define the environment variables for the
   new process (these are used instead of the current process'
   environment); the functions ``execl()``, ``execlp()``, ``execv()``,
   and ``execvp()`` all cause the new process to inherit the
   environment of the current process.

   Availability: Unix, Windows.

os._exit(n)

   Exit the process with status *n*, without calling cleanup handlers,
   flushing stdio buffers, etc.

   Availability: Unix, Windows.

   Note: The standard way to exit is ``sys.exit(n)``.  ``_exit()`` should
     normally only be used in the child process after a ``fork()``.

The following exit codes are defined and can be used with ``_exit()``,
although they are not required.  These are typically used for system
programs written in Python, such as a mail server's external command
delivery program.

Note: Some of these may not be available on all Unix platforms, since
  there is some variation.  These constants are defined where they are
  defined by the underlying platform.

os.EX_OK

   Exit code that means no error occurred.

   Availability: Unix.

os.EX_USAGE

   Exit code that means the command was used incorrectly, such as when
   the wrong number of arguments are given.

   Availability: Unix.

os.EX_DATAERR

   Exit code that means the input data was incorrect.

   Availability: Unix.

os.EX_NOINPUT

   Exit code that means an input file did not exist or was not
   readable.

   Availability: Unix.

os.EX_NOUSER

   Exit code that means a specified user did not exist.

   Availability: Unix.

os.EX_NOHOST

   Exit code that means a specified host did not exist.

   Availability: Unix.

os.EX_UNAVAILABLE

   Exit code that means that a required service is unavailable.

   Availability: Unix.

os.EX_SOFTWARE

   Exit code that means an internal software error was detected.

   Availability: Unix.

os.EX_OSERR

   Exit code that means an operating system error was detected, such
   as the inability to fork or create a pipe.

   Availability: Unix.

os.EX_OSFILE

   Exit code that means some system file did not exist, could not be
   opened, or had some other kind of error.

   Availability: Unix.

os.EX_CANTCREAT

   Exit code that means a user specified output file could not be
   created.

   Availability: Unix.

os.EX_IOERR

   Exit code that means that an error occurred while doing I/O on some
   file.

   Availability: Unix.

os.EX_TEMPFAIL

   Exit code that means a temporary failure occurred.  This indicates
   something that may not really be an error, such as a network
   connection that couldn't be made during a retryable operation.

   Availability: Unix.

os.EX_PROTOCOL

   Exit code that means that a protocol exchange was illegal, invalid,
   or not understood.

   Availability: Unix.

os.EX_NOPERM

   Exit code that means that there were insufficient permissions to
   perform the operation (but not intended for file system problems).

   Availability: Unix.

os.EX_CONFIG

   Exit code that means that some kind of configuration error
   occurred.

   Availability: Unix.

os.EX_NOTFOUND

   Exit code that means something like "an entry was not found".

   Availability: Unix.

os.fork()

   Fork a child process.  Return ``0`` in the child and the child's
   process id in the parent.  If an error occurs ``OSError`` is
   raised.

   Note that some platforms including FreeBSD <= 6.3, Cygwin and OS/2
   EMX have known issues when using fork() from a thread.

   Availability: Unix.

os.forkpty()

   Fork a child process, using a new pseudo-terminal as the child's
   controlling terminal. Return a pair of ``(pid, fd)``, where *pid*
   is ``0`` in the child, the new child's process id in the parent,
   and *fd* is the file descriptor of the master end of the pseudo-
   terminal.  For a more portable approach, use the ``pty`` module.
   If an error occurs ``OSError`` is raised.

   Availability: some flavors of Unix.

os.kill(pid, sig)

   Send signal *sig* to the process *pid*.  Constants for the specific
   signals available on the host platform are defined in the
   ``signal`` module.

   Windows: The ``signal.CTRL_C_EVENT`` and
   ``signal.CTRL_BREAK_EVENT`` signals are special signals which can
   only be sent to console processes which share a common console
   window, e.g., some subprocesses. Any other value for *sig* will
   cause the process to be unconditionally killed by the
   TerminateProcess API, and the exit code will be set to *sig*. The
   Windows version of ``kill()`` additionally takes process handles to
   be killed.

   New in version 3.2: Windows support.

os.killpg(pgid, sig)

   Send the signal *sig* to the process group *pgid*.

   Availability: Unix.

os.nice(increment)

   Add *increment* to the process's "niceness".  Return the new
   niceness.

   Availability: Unix.

os.plock(op)

   Lock program segments into memory.  The value of *op* (defined in
   ``<sys/lock.h>``) determines which segments are locked.

   Availability: Unix.

os.popen(...)

   Run child processes, returning opened pipes for communications.
   These functions are described in section *File Object Creation*.

os.spawnl(mode, path, ...)
os.spawnle(mode, path, ..., env)
os.spawnlp(mode, file, ...)
os.spawnlpe(mode, file, ..., env)
os.spawnv(mode, path, args)
os.spawnve(mode, path, args, env)
os.spawnvp(mode, file, args)
os.spawnvpe(mode, file, args, env)

   Execute the program *path* in a new process.

   (Note that the ``subprocess`` module provides more powerful
   facilities for spawning new processes and retrieving their results;
   using that module is preferable to using these functions.  Check
   especially the *Replacing Older Functions with the subprocess
   Module* section.)

   If *mode* is ``P_NOWAIT``, this function returns the process id of
   the new process; if *mode* is ``P_WAIT``, returns the process's
   exit code if it exits normally, or ``-signal``, where *signal* is
   the signal that killed the process.  On Windows, the process id
   will actually be the process handle, so can be used with the
   ``waitpid()`` function.

   The "l" and "v" variants of the ``spawn*()`` functions differ in
   how command-line arguments are passed.  The "l" variants are
   perhaps the easiest to work with if the number of parameters is
   fixed when the code is written; the individual parameters simply
   become additional parameters to the ``spawnl*()`` functions.  The
   "v" variants are good when the number of parameters is variable,
   with the arguments being passed in a list or tuple as the *args*
   parameter.  In either case, the arguments to the child process must
   start with the name of the command being run.

   The variants which include a second "p" near the end
   (``spawnlp()``, ``spawnlpe()``, ``spawnvp()``, and ``spawnvpe()``)
   will use the ``PATH`` environment variable to locate the program
   *file*.  When the environment is being replaced (using one of the
   ``spawn*e()`` variants, discussed in the next paragraph), the new
   environment is used as the source of the ``PATH`` variable.  The
   other variants, ``spawnl()``, ``spawnle()``, ``spawnv()``, and
   ``spawnve()``, will not use the ``PATH`` variable to locate the
   executable; *path* must contain an appropriate absolute or relative
   path.

   For ``spawnle()``, ``spawnlpe()``, ``spawnve()``, and
   ``spawnvpe()`` (note that these all end in "e"), the *env*
   parameter must be a mapping which is used to define the environment
   variables for the new process (they are used instead of the current
   process' environment); the functions ``spawnl()``, ``spawnlp()``,
   ``spawnv()``, and ``spawnvp()`` all cause the new process to
   inherit the environment of the current process.  Note that keys and
   values in the *env* dictionary must be strings; invalid keys or
   values will cause the function to fail, with a return value of
   ``127``.

   As an example, the following calls to ``spawnlp()`` and
   ``spawnvpe()`` are equivalent:

      import os
      os.spawnlp(os.P_WAIT, 'cp', 'cp', 'index.html', '/dev/null')

      L = ['cp', 'index.html', '/dev/null']
      os.spawnvpe(os.P_WAIT, 'cp', L, os.environ)

   Availability: Unix, Windows.  ``spawnlp()``, ``spawnlpe()``,
   ``spawnvp()`` and ``spawnvpe()`` are not available on Windows.
   ``spawnle()`` and ``spawnve()`` are not thread-safe on Windows; we
   advise you to use the ``subprocess`` module instead.

os.P_NOWAIT
os.P_NOWAITO

   Possible values for the *mode* parameter to the ``spawn*()`` family
   of functions.  If either of these values is given, the ``spawn*()``
   functions will return as soon as the new process has been created,
   with the process id as the return value.

   Availability: Unix, Windows.

os.P_WAIT

   Possible value for the *mode* parameter to the ``spawn*()`` family
   of functions.  If this is given as *mode*, the ``spawn*()``
   functions will not return until the new process has run to
   completion and will return the exit code of the process the run is
   successful, or ``-signal`` if a signal kills the process.

   Availability: Unix, Windows.

os.P_DETACH
os.P_OVERLAY

   Possible values for the *mode* parameter to the ``spawn*()`` family
   of functions.  These are less portable than those listed above.
   ``P_DETACH`` is similar to ``P_NOWAIT``, but the new process is
   detached from the console of the calling process. If ``P_OVERLAY``
   is used, the current process will be replaced; the ``spawn*()``
   function will not return.

   Availability: Windows.

os.startfile(path[, operation])

   Start a file with its associated application.

   When *operation* is not specified or ``'open'``, this acts like
   double-clicking the file in Windows Explorer, or giving the file
   name as an argument to the **start** command from the interactive
   command shell: the file is opened with whatever application (if
   any) its extension is associated.

   When another *operation* is given, it must be a "command verb" that
   specifies what should be done with the file. Common verbs
   documented by Microsoft are ``'print'`` and  ``'edit'`` (to be used
   on files) as well as ``'explore'`` and ``'find'`` (to be used on
   directories).

   ``startfile()`` returns as soon as the associated application is
   launched. There is no option to wait for the application to close,
   and no way to retrieve the application's exit status.  The *path*
   parameter is relative to the current directory.  If you want to use
   an absolute path, make sure the first character is not a slash
   (``'/'``); the underlying Win32 ``ShellExecute()`` function doesn't
   work if it is.  Use the ``os.path.normpath()`` function to ensure
   that the path is properly encoded for Win32.

   Availability: Windows.

os.system(command)

   Execute the command (a string) in a subshell.  This is implemented
   by calling the Standard C function ``system()``, and has the same
   limitations. Changes to ``sys.stdin``, etc. are not reflected in
   the environment of the executed command. If *command* generates any
   output, it will be sent to the interpreter standard output stream.

   On Unix, the return value is the exit status of the process encoded
   in the format specified for ``wait()``.  Note that POSIX does not
   specify the meaning of the return value of the C ``system()``
   function, so the return value of the Python function is system-
   dependent.

   On Windows, the return value is that returned by the system shell
   after running *command*.  The shell is given by the Windows
   environment variable ``COMSPEC``: it is usually **cmd.exe**, which
   returns the exit status of the command run; on systems using a non-
   native shell, consult your shell documentation.

   The ``subprocess`` module provides more powerful facilities for
   spawning new processes and retrieving their results; using that
   module is preferable to using this function.  See the *Replacing
   Older Functions with the subprocess Module* section in the
   ``subprocess`` documentation for some helpful recipes.

   Availability: Unix, Windows.

os.times()

   Return a 5-tuple of floating point numbers indicating accumulated
   (processor or other) times, in seconds.  The items are: user time,
   system time, children's user time, children's system time, and
   elapsed real time since a fixed point in the past, in that order.
   See the Unix manual page *times(2)* or the corresponding Windows
   Platform API documentation. On Windows, only the first two items
   are filled, the others are zero.

   Availability: Unix, Windows

os.wait()

   Wait for completion of a child process, and return a tuple
   containing its pid and exit status indication: a 16-bit number,
   whose low byte is the signal number that killed the process, and
   whose high byte is the exit status (if the signal number is zero);
   the high bit of the low byte is set if a core file was produced.

   Availability: Unix.

os.waitpid(pid, options)

   The details of this function differ on Unix and Windows.

   On Unix: Wait for completion of a child process given by process id
   *pid*, and return a tuple containing its process id and exit status
   indication (encoded as for ``wait()``).  The semantics of the call
   are affected by the value of the integer *options*, which should be
   ``0`` for normal operation.

   If *pid* is greater than ``0``, ``waitpid()`` requests status
   information for that specific process.  If *pid* is ``0``, the
   request is for the status of any child in the process group of the
   current process.  If *pid* is ``-1``, the request pertains to any
   child of the current process.  If *pid* is less than ``-1``, status
   is requested for any process in the process group ``-pid`` (the
   absolute value of *pid*).

   An ``OSError`` is raised with the value of errno when the syscall
   returns -1.

   On Windows: Wait for completion of a process given by process
   handle *pid*, and return a tuple containing *pid*, and its exit
   status shifted left by 8 bits (shifting makes cross-platform use of
   the function easier). A *pid* less than or equal to ``0`` has no
   special meaning on Windows, and raises an exception. The value of
   integer *options* has no effect. *pid* can refer to any process
   whose id is known, not necessarily a child process. The ``spawn()``
   functions called with ``P_NOWAIT`` return suitable process handles.

os.wait3([options])

   Similar to ``waitpid()``, except no process id argument is given
   and a 3-element tuple containing the child's process id, exit
   status indication, and resource usage information is returned.
   Refer to ``resource``.``getrusage()`` for details on resource usage
   information.  The option argument is the same as that provided to
   ``waitpid()`` and ``wait4()``.

   Availability: Unix.

os.wait4(pid, options)

   Similar to ``waitpid()``, except a 3-element tuple, containing the
   child's process id, exit status indication, and resource usage
   information is returned. Refer to ``resource``.``getrusage()`` for
   details on resource usage information.  The arguments to
   ``wait4()`` are the same as those provided to ``waitpid()``.

   Availability: Unix.

os.WNOHANG

   The option for ``waitpid()`` to return immediately if no child
   process status is available immediately. The function returns ``(0,
   0)`` in this case.

   Availability: Unix.

os.WCONTINUED

   This option causes child processes to be reported if they have been
   continued from a job control stop since their status was last
   reported.

   Availability: Some Unix systems.

os.WUNTRACED

   This option causes child processes to be reported if they have been
   stopped but their current state has not been reported since they
   were stopped.

   Availability: Unix.

The following functions take a process status code as returned by
``system()``, ``wait()``, or ``waitpid()`` as a parameter.  They may
be used to determine the disposition of a process.

os.WCOREDUMP(status)

   Return ``True`` if a core dump was generated for the process,
   otherwise return ``False``.

   Availability: Unix.

os.WIFCONTINUED(status)

   Return ``True`` if the process has been continued from a job
   control stop, otherwise return ``False``.

   Availability: Unix.

os.WIFSTOPPED(status)

   Return ``True`` if the process has been stopped, otherwise return
   ``False``.

   Availability: Unix.

os.WIFSIGNALED(status)

   Return ``True`` if the process exited due to a signal, otherwise
   return ``False``.

   Availability: Unix.

os.WIFEXITED(status)

   Return ``True`` if the process exited using the *exit(2)* system
   call, otherwise return ``False``.

   Availability: Unix.

os.WEXITSTATUS(status)

   If ``WIFEXITED(status)`` is true, return the integer parameter to
   the *exit(2)* system call.  Otherwise, the return value is
   meaningless.

   Availability: Unix.

os.WSTOPSIG(status)

   Return the signal which caused the process to stop.

   Availability: Unix.

os.WTERMSIG(status)

   Return the signal which caused the process to exit.

   Availability: Unix.


Miscellaneous System Information
================================

os.confstr(name)

   Return string-valued system configuration values. *name* specifies
   the configuration value to retrieve; it may be a string which is
   the name of a defined system value; these names are specified in a
   number of standards (POSIX, Unix 95, Unix 98, and others).  Some
   platforms define additional names as well. The names known to the
   host operating system are given as the keys of the
   ``confstr_names`` dictionary.  For configuration variables not
   included in that mapping, passing an integer for *name* is also
   accepted.

   If the configuration value specified by *name* isn't defined,
   ``None`` is returned.

   If *name* is a string and is not known, ``ValueError`` is raised.
   If a specific value for *name* is not supported by the host system,
   even if it is included in ``confstr_names``, an ``OSError`` is
   raised with ``errno.EINVAL`` for the error number.

   Availability: Unix

os.confstr_names

   Dictionary mapping names accepted by ``confstr()`` to the integer
   values defined for those names by the host operating system. This
   can be used to determine the set of names known to the system.

   Availability: Unix.

os.getloadavg()

   Return the number of processes in the system run queue averaged
   over the last 1, 5, and 15 minutes or raises ``OSError`` if the
   load average was unobtainable.

   Availability: Unix.

os.sysconf(name)

   Return integer-valued system configuration values. If the
   configuration value specified by *name* isn't defined, ``-1`` is
   returned.  The comments regarding the *name* parameter for
   ``confstr()`` apply here as well; the dictionary that provides
   information on the known names is given by ``sysconf_names``.

   Availability: Unix.

os.sysconf_names

   Dictionary mapping names accepted by ``sysconf()`` to the integer
   values defined for those names by the host operating system. This
   can be used to determine the set of names known to the system.

   Availability: Unix.

The following data values are used to support path manipulation
operations.  These are defined for all platforms.

Higher-level operations on pathnames are defined in the ``os.path``
module.

os.curdir

   The constant string used by the operating system to refer to the
   current directory. This is ``'.'`` for Windows and POSIX. Also
   available via ``os.path``.

os.pardir

   The constant string used by the operating system to refer to the
   parent directory. This is ``'..'`` for Windows and POSIX. Also
   available via ``os.path``.

os.sep

   The character used by the operating system to separate pathname
   components. This is ``'/'`` for POSIX and ``'\\'`` for Windows.
   Note that knowing this is not sufficient to be able to parse or
   concatenate pathnames --- use ``os.path.split()`` and
   ``os.path.join()`` --- but it is occasionally useful. Also
   available via ``os.path``.

os.altsep

   An alternative character used by the operating system to separate
   pathname components, or ``None`` if only one separator character
   exists.  This is set to ``'/'`` on Windows systems where ``sep`` is
   a backslash. Also available via ``os.path``.

os.extsep

   The character which separates the base filename from the extension;
   for example, the ``'.'`` in ``os.py``. Also available via
   ``os.path``.

os.pathsep

   The character conventionally used by the operating system to
   separate search path components (as in ``PATH``), such as ``':'``
   for POSIX or ``';'`` for Windows. Also available via ``os.path``.

os.defpath

   The default search path used by ``exec*p*()`` and ``spawn*p*()`` if
   the environment doesn't have a ``'PATH'`` key. Also available via
   ``os.path``.

os.linesep

   The string used to separate (or, rather, terminate) lines on the
   current platform.  This may be a single character, such as ``'\n'``
   for POSIX, or multiple characters, for example, ``'\r\n'`` for
   Windows. Do not use *os.linesep* as a line terminator when writing
   files opened in text mode (the default); use a single ``'\n'``
   instead, on all platforms.

os.devnull

   The file path of the null device. For example: ``'/dev/null'`` for
   POSIX, ``'nul'`` for Windows.  Also available via ``os.path``.


Miscellaneous Functions
=======================

os.urandom(n)

   Return a string of *n* random bytes suitable for cryptographic use.

   This function returns random bytes from an OS-specific randomness
   source.  The returned data should be unpredictable enough for
   cryptographic applications, though its exact quality depends on the
   OS implementation.  On a UNIX-like system this will query
   /dev/urandom, and on Windows it will use CryptGenRandom. If a
   randomness source is not found, ``NotImplementedError`` will be
   raised.
