1:flock

From Linux Man Pages

      flock - acquire a file lock and then execute a command with the lock held
      
      flock [ --shared | --timeout=seconds ] lockfile command ..

DESCRIPTION

      Acquire  a  file  lock  using  the  flock(2)  system  call and then execute the given command with the lock held.
      Depending on the options given, the lock can be either exclusive or shared, and the behavior in the event of lock
      contention  can  be  specified  as either waiting indefinitely for the lock to become available (the default), or
      failing if the lock does not become available after a specific time, which can be specified as zero to  make  the
      command not wait at all.
 
      --shared
             Acquire  a shared lock.  Acquiring a shared lock does not stop others from acquiring a shared lock, but it
             will stop others from acquiring an exclusive lock.  Conversely, acquiring an exclusive lock (the  default)
             stops  both exclusive and shared attempts to acquire the lock.  Typically, a shared lock is used if a com-
             mand is just going to read the locked data, and an exclusive lock is used if the command  might  write  to
             it.
 
      --timeout=n
             Abort if the lock cannot be acquired before n seconds.  For a completely non-blocking attempt to acquire a
             lock, specify --timeout=0.  The timer applies only to the attempt to acquire the lock.   As  soon  as  the
             lock is acquired, the timeout is cancelled.  The command to be run is not subject to the timeout.

EXAMPLES (invoking some imaginary programs)

      flock /etc/passwd read-and-write-to-passwd
 
      flock --shared /etc/passwd just-read-something-from-passwd
 
      flock --timeout=0 /sys /usr/local/bin/update-hotplug /sys

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