How do I check to see if an apt lock file is locked?

Well I thought there would be a simple answer here but I can't find anything. First, are you 100% sure that lock file is always there? Try running

lsof /var/lib/dpkg/lock

as root to see if any process has it open.

From what I've read, apt-get does an fcntl lock, but I haven't looked at the code to verify. I guess that wuld explain why the file is there all the time, apt just locks it as needed.

What about just doing a check of the process list when your script runs, and exiting if apt is running at the same time? Would that be sufficient for your use?

Looks like this person went down the same path as you did, without much success.

I came here looking for a solution similar to what gondoi ended up using but written in Python instead of Ruby. The following seems to work well:

import fcntl

def is_dpkg_active():
    """
    Check whether ``apt-get`` or ``dpkg`` is currently active.

    This works by checking whether the lock file ``/var/lib/dpkg/lock`` is
    locked by an ``apt-get`` or ``dpkg`` process, which in turn is done by
    momentarily trying to acquire the lock. This means that the current process
    needs to have sufficient privileges.

    :returns: ``True`` when the lock is already taken (``apt-get`` or ``dpkg``
              is running), ``False`` otherwise.
    :raises: :py:exc:`exceptions.IOError` if the required privileges are not
             available.

    .. note:: ``apt-get`` doesn't acquire this lock until it needs it, for
              example an ``apt-get update`` run consists of two phases (first
              fetching updated package lists and then updating the local
              package index) and only the second phase claims the lock (because
              the second phase writes the local package index which is also
              read from and written to by ``dpkg``).
    """
    with open('/var/lib/dpkg/lock', 'w') as handle:
        try:
            fcntl.lockf(handle, fcntl.LOCK_EX | fcntl.LOCK_NB)
            return False
        except IOError:
            return True

From a shell script (see flock(1)):

flock --timeout 60 --exclusive --close /var/lib/dpkg/lock apt-get -y -o Dpkg::Options::="--force-confold" upgrade
if [ $? -ne 0 ]; then
  echo "Another process has f-locked /var/lib/dpkg/lock" 1>&2
  exit 1
fi