The
shlock command can create or verify a lock file on behalf of a shell or other script program. When it attempts to create a lock file, if one already exists,
shlock verifies that it is or is not valid. If valid,
shlock will exit with a non-zero exit code. If invalid,
shlock will remove the lock file, and create a new one.
shlock uses the
link(2) system call to make the final target lock file, which is an atomic operation (i.e. "dot locking", so named for this mechanism's original use for locking system mailboxes). It puts the process ID ("PID") from the command line into the requested lock file.
shlock verifies that an extant lock file is still valid by using
kill(2) with a zero signal to check for the existence of the process that holds the lock.
The
-d option causes
shlock to be verbose about what it is doing.
The
-f argument with
lockfile is always required.
The
-p option with
PID is given when the program is to create a lock file; when absent,
shlock will simply check for the validity of the lock file.
The
-u option causes
shlock to read and write the PID as a binary pid_t, instead of as ASCII, to be compatible with the locks created by UUCP.