Continuously monitor logs with tail that are occasionally rotated

Question

We're using tail to continuously monitor several logs, but when a log is rotated the tail for that file will cease.

As far as I understand, the problem is that when the log is rotated, there is a new file created, and the running tail process doesn't know anything about that new file handle.

Solution 1

Ah, there's a flag for this.

instead of using tail -f /var/log/file we should be using tail -F /var/log/file

---

tail -F translates to tail --follow=name --retry as in;

• --follow=name: follow the name of the file instead of the file descriptor
• --retry: if the file is inaccessible, try again later instead of dying

Solution 2

# tail --follow=mylog.log

From man tail:

With --follow (-f), tail defaults to  following  the  file  descriptor,
       which  means that even if a tail’ed file is renamed, tail will continue
       to track its end.  This default behavior  is  not  desirable  when  you
       really want to track the actual name of the file, not the file descrip‐
       tor (e.g., log rotation).  Use --follow=name in that case.  That causes
       tail  to track the named file by reopening it periodically to see if it
       has been removed and recreated by some other program.

So in this case using the -F option would be correct.

-F     same as --follow=name --retry

Solution 3

The exact answer depends on your OS - but in many cases, tail -F will do the right thing.

Solution 4

tail -F or tail --follow=name

Solution 5

IMHO, it's a little odd to change your log file by SIZE rather than by date. Most system logs (in unix or linux) rotate on a weekly or monthly basis, and not based on size...This is something I like for various reasons, and also something which, if implemented, would solve your problem.

Eight years later, I don't know what the hell I was talking about here: there are tons of places where you want to rotate by size, because daily/weekly/monthly rotations can yield MASSIVE files which can cause serious issues.

From a more experienced perspective, the real question is why you'd want to sit and continuously tail a file that's growing so fast that you're rotating it more than daily...It'd be like watching the Matrix stream by.

These days you'd be better looking into some big data log aggregation like Splunk or Sumologic, where it can filter log events into classes and trigger based on specific log values...No need for watching live logs at all.

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Comments

[user202729]

Sometimes even -F doesn't work when file is repeatedly truncated. See overwrite - Is there a tail -f equivalent for when a file is overwritten (instead of appended to)? - Server Fault and logging - tail -f sometimes stops updating - and the file hasn't moved - Server Fault

[chris]

If tail -F doesn't work, compile a version of tail -F that does. The other alternative is a short road to crazy town.

[David]

I agree and this is one thing I realized I should look into. Hopefully log4j supports it. Thanks!

[Jim Zajkowski]

One clear advantage of rotation by size is you can be sure the entire set of logs will be less than some known size.

[Dominic Cerisano]

Why would they even have the "-f" option? This should be the default!