Watch file using multitail even if it has not been created

logsmultitail

I've a MapServer logfile; it is created in /tmp/mapserv.log, but obviously it is flushed at every reboot, and is created only when a loggable event occurs.

I'm already using Multitail, but now I need to watch the folder /tmp for a file that has not even been created, waiting for it to appear.

Any hint?

Best Answer

You could perhaps use the -iw option. It check for new files matching a given pattern at a given interval. When one is found start following it.

E.g.:

multitail -iw /tmp/mapserv.log 2

Would look for the file /tmp/mapserv.log every 2 seconds. If and when it appears follow it.

It is meant to take a wildcard as in

-iw "/tmp/map*" 2 
# Quotes needed to prevent the shell to expand the pattern.

but works for non-wildcard as well. The number is how often to check for new files matching the pattern.

Else, touch could perhaps work. Might require something like su user -c 'touch ...' or a chown etc after touch.

If MapServer deletes existing mapserv.log on first logging and not truncate or append you would perhaps also need the -f option, as in:

Follow the following filename, not the descriptor.

Related Solutions

Does multitail follow the inode or the file name by default

According to the multitail manual:

There are a few other options not fitting elsewhere, these are:
-f  This makes MultiTail follow the file. In case the original file gets
    renamed and a new file is created with the original filename, MultiTail
    will start watching the file with the original filename (the one you
    entered).

To me, this implies that by default it follows by inode / file descriptor rather than filename.

A cursory reading of the source bears this out; in exec.c:79, the follow_filename var (set in cmdline.c:889 or ui.c:966) defines whether the follow-by-filename flag (-F, --follow=name, etc.) is passed to tail.

Ubuntu – Using tail -f with Line Numbers for Logs

tail -f | nl

works for me and is the first what I thought of - that is if you really want the lines numbered from 1 and not with the real line number from the file watched. Optionally add grep if needed to the appropriate place (either before or after nl). However, remember that buffering may occur. In my particular case, grep has the --line-buffered option, but nl buffers it's output and doesn't have an option to switch that off. Hence the tail | nl | grep combo doesn't really flow nicely.

That said,

tail -f | grep -n pattern

works for me as well. Numbering starts again from the beginning of the "tailing" rather than beginning of the whole log file.

Best Answer

Related Solutions

Does multitail follow the inode or the file name by default

Ubuntu – Using tail -f with Line Numbers for Logs

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