On 19/04/13 00:31, Don Cragun wrote:
I'm new to this forum and hope I'm not missing basic etiquette rules here.
There is a discussion going on in the UNIX and Linux Forums "UNIX for Dummies Questions &
Answers" forum titled "Knowing when a different program modifies a file" (see
http://www.unix.com/unix-dummies-questions-answers/221357-knowing-when-different-program-modifies-file.html
for the full discussion) where it was pointed out that if someone edits a log file with vim while a
daemon has the file open for writing all subsequent output written to the log file by the daemon will
be lost until the deamon switches to a new log file (or is killed and restarted).
I understand that renaming a temp file with the changes incorporated is an atomic
operation reducing the amount of time that a large file would be in an inconsistent
state. and that it widens (or opens) a window where a file could be "damaged"
due to an anomalous event (e.g., power failure, lack of disk space, etc.). (But I assume
that in these cases, vim's recover command could restore the data after the system is
rebooted or space is made available.)
I know that when there are multiple hard links to a file, the inode is not
changed when vim saves the file updates. Given the above scenario, should vim
ever change the inode?
Clearly a sysadmin that edits a log file should be retrained or fired, and
clearly the POSIX standards state that the results produced when two processes
are writing to the same file without exchanging active handles produces
undefined results; but the fact that a system daemon would continue writing to
a file after vim has unlink()ed it and that all future diagnostics written by
that daemon could be lost is scary.
- Don
See
:help 'backupcopy'
:help FileChangedShell
I agree with you that "doctoring" a logfile is contrary to the very
purpose of the logfile itself.
Some daemons will "restart" when sent a SIGHUP signal. I'm not sure of
the details, which may vary from daemon to daemon.
Best regards,
Tony.
--
Never count your chickens before they rip your lips off
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