Mini Shell

import os
import sys
import stat
import select
import time
import errno

try:
    InterruptedError
except NameError:
    # Alias Python2 exception to Python3
    InterruptedError = select.error

if sys.version_info[0] >= 3:
    string_types = (str,)
else:
    string_types = (unicode, str)


def is_executable_file(path):
    """Checks that path is an executable regular file, or a symlink towards one.

    This is roughly ``os.path isfile(path) and os.access(path, os.X_OK)``.
    """
    # follow symlinks,
    fpath = os.path.realpath(path)

    if not os.path.isfile(fpath):
        # non-files (directories, fifo, etc.)
        return False

    mode = os.stat(fpath).st_mode

    if (sys.platform.startswith('sunos')
            and os.getuid() == 0):
        # When root on Solaris, os.X_OK is True for *all* files, irregardless
        # of their executability -- instead, any permission bit of any user,
        # group, or other is fine enough.
        #
        # (This may be true for other "Unix98" OS's such as HP-UX and AIX)
        return bool(mode & (stat.S_IXUSR |
                            stat.S_IXGRP |
                            stat.S_IXOTH))

    return os.access(fpath, os.X_OK)


def which(filename, env=None):
    '''This takes a given filename; tries to find it in the environment path;
    then checks if it is executable. This returns the full path to the filename
    if found and executable. Otherwise this returns None.'''

    # Special case where filename contains an explicit path.
    if os.path.dirname(filename) != '' and is_executable_file(filename):
        return filename
    if env is None:
        env = os.environ
    p = env.get('PATH')
    if not p:
        p = os.defpath
    pathlist = p.split(os.pathsep)
    for path in pathlist:
        ff = os.path.join(path, filename)
        if is_executable_file(ff):
            return ff
    return None


def split_command_line(command_line):

    '''This splits a command line into a list of arguments. It splits arguments
    on spaces, but handles embedded quotes, doublequotes, and escaped
    characters. It's impossible to do this with a regular expression, so I
    wrote a little state machine to parse the command line. '''

    arg_list = []
    arg = ''

    # Constants to name the states we can be in.
    state_basic = 0
    state_esc = 1
    state_singlequote = 2
    state_doublequote = 3
    # The state when consuming whitespace between commands.
    state_whitespace = 4
    state = state_basic

    for c in command_line:
        if state == state_basic or state == state_whitespace:
            if c == '\\':
                # Escape the next character
                state = state_esc
            elif c == r"'":
                # Handle single quote
                state = state_singlequote
            elif c == r'"':
                # Handle double quote
                state = state_doublequote
            elif c.isspace():
                # Add arg to arg_list if we aren't in the middle of whitespace.
                if state == state_whitespace:
                    # Do nothing.
                    None
                else:
                    arg_list.append(arg)
                    arg = ''
                    state = state_whitespace
            else:
                arg = arg + c
                state = state_basic
        elif state == state_esc:
            arg = arg + c
            state = state_basic
        elif state == state_singlequote:
            if c == r"'":
                state = state_basic
            else:
                arg = arg + c
        elif state == state_doublequote:
            if c == r'"':
                state = state_basic
            else:
                arg = arg + c

    if arg != '':
        arg_list.append(arg)
    return arg_list


def select_ignore_interrupts(iwtd, owtd, ewtd, timeout=None):

    '''This is a wrapper around select.select() that ignores signals. If
    select.select raises a select.error exception and errno is an EINTR
    error then it is ignored. Mainly this is used to ignore sigwinch
    (terminal resize). '''

    # if select() is interrupted by a signal (errno==EINTR) then
    # we loop back and enter the select() again.
    if timeout is not None:
        end_time = time.time() + timeout
    while True:
        try:
            return select.select(iwtd, owtd, ewtd, timeout)
        except InterruptedError:
            err = sys.exc_info()[1]
            if err.args[0] == errno.EINTR:
                # if we loop back we have to subtract the
                # amount of time we already waited.
                if timeout is not None:
                    timeout = end_time - time.time()
                    if timeout < 0:
                        return([], [], [])
            else:
                # something else caused the select.error, so
                # this actually is an exception.
                raise


def poll_ignore_interrupts(fds, timeout=None):
    '''Simple wrapper around poll to register file descriptors and
    ignore signals.'''

    if timeout is not None:
        end_time = time.time() + timeout

    poller = select.poll()
    for fd in fds:
        poller.register(fd, select.POLLIN | select.POLLPRI | select.POLLHUP | select.POLLERR)

    while True:
        try:
            timeout_ms = None if timeout is None else timeout * 1000
            results = poller.poll(timeout_ms)
            return [afd for afd, _ in results]
        except InterruptedError:
            err = sys.exc_info()[1]
            if err.args[0] == errno.EINTR:
                # if we loop back we have to subtract the
                # amount of time we already waited.
                if timeout is not None:
                    timeout = end_time - time.time()
                    if timeout < 0:
                        return []
            else:
                # something else caused the select.error, so
                # this actually is an exception.
                raise