Detect Linux Runlevel

[Tim Chase]

On 2016-12-05 18:26, Wildman via Python-list wrote:
> On Mon, 05 Dec 2016 16:08:57 -0600, Tim Chase wrote:
> 
> > On 2016-12-05 14:58, Wildman via Python-list wrote:
> >> I there a way to detect what the Linux runlevel is from
> >> within a Python program?  I would like to be able to do
> >> it without the use of an external program such as 'who'
> >> or 'runlevel'.
> > 
> > You can use something like
> > 
> > https://gist.github.com/likexian/f9da722585036d372dca
> > 
> > to parse the /var/run/utmp contents.  Based on some source-code
> > scrounging, it looks like you want the first field to be "1" for
> > the "runlevel" account.  To extract the actual runlevel, you can
> > take the PID value from the second column ("53" in my example
> > here) and take it's integer value mod 256 (AKA "& 0xff") to get
> > the character value.  So chr(int("53") & 0xff) returns "5" in my
> > case, which is my runlevel.
> > 
> > Additional links I found helpful while searching:
> > 
> > https://casper.berkeley.edu/svn/trunk/roach/sw/busybox-1.10.1/miscutils/runlevel.c
> > https://github.com/garabik/python-utmp
> 
> That is exactly the kind of thing I was looking for.  Thank you.
> Now all I have to do is get it to work with Python3.

This works based on my poking at it in both Py2 and Py3:

  import struct
  from collections import namedtuple

  try:
    basestring
  except NameError:
    basestring = str

  UTMP = namedtuple("UTMP", [
      "ut_type", # Type of record
      "ut_pid", # PID of login process
      "ut_line", # Device name of tty - "/dev/"
      "ut_id", # Terminal name suffix, or inittab(5) ID
      "ut_user", # Username
      "ut_host", # Hostname for remote login, or kernel version for run-level messages
      "e_termination", # Process termination status
      "e_exit", # Process exit status
      "ut_session", # Session ID (getsid(2)), used for windowing
      "tv_sec", # Seconds
      "tv_usec", # Microseconds
      "ut_addr_v6a", # Internet address of remote host; IPv4 address uses just ut_addr_v6[0]
      "ut_addr_v6b", # Internet address of remote host; IPv4 address uses just ut_addr_v6[0]
      "ut_addr_v6c", # Internet address of remote host; IPv4 address uses just ut_addr_v6[0]
      "ut_addr_v6d", # Internet address of remote host; IPv4 address uses just ut_addr_v6[0]
      #"__unused", # Reserved for future use
      ])

  XTMP_STRUCT = "hi32s4s32s256shhiiiiiii20x"
  XTMP_STRUCT_SIZE = struct.calcsize(XTMP_STRUCT)

  # ut_types
  EMPTY = 0
  RUN_LVL = 1
  BOOT_TIME = 2
  OLD_TIME = 3
  NEW_TIME = 4
  INIT_PROCESS = 5 # Process spawned by "init"
  LOGIN_PROCESS = 6 # A "getty" process

  DEFAULT_UTMP = "/var/run/utmp"

  def parse_utmp(utmp_fname=DEFAULT_UTMP):
      with open(utmp_fname, "rb") as f:
          while True:
              bytes = f.read(XTMP_STRUCT_SIZE)
              if not bytes:
                  break
              bits = struct.unpack(XTMP_STRUCT, bytes)
              bits = [
                  bit.rstrip('\0') if isinstance(bit, basestring) else bit
                  for bit
                  in bits
                  ]
              yield UTMP(*bits)

  def filter(ut_type, utmp_fname=DEFAULT_UTMP):
      for utmp in parse_utmp(utmp_fname):
          if utmp.ut_type == ut_type:
              yield utmp

  def get_runlevel(utmp_fname=DEFAULT_UTMP):
      return chr(next(filter(RUN_LVL, utmp_fname)).ut_pid & 0xFF)

  if __name__ == "__main__":
      print("Runlevel: %s" % get_runlevel())


-tkc