Does multitail follow the inode or the file name by default

logsmultitailtail

For logrotated files one usually uses tail -F instead of tail -f to follow the log file contents. Does multitail(1) automatically follow the name instead of the inode (which likely changes with the log rotation)?

I sifted through the documentation and also searched on the web, but wasn't able to come up with a conclusive answer.

Thanks to the accepted answer I found that the man page does mention the option I wanted, but uses the term "descriptor" instead of "inode". So this question and the answer should be the glue for the next person researching the topic.

-f Follow the following filename, not the descriptor.

--follow-all
  For all files after this switch: follow the following filename, not the descriptor.

Best Answer

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.

Related Solutions

Watch file using multitail even if it has not been created

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.

Does tail Read the Whole File? – Understanding tail Command Behavior

No, tail doesn't read the whole file, it seeks to the end then read blocks backwards until the expected number of lines have been reached, then it displays the lines in the proper direction until the end of the file, and possibly stays monitoring the file if the -f option is used.

Note however that tail has no choice but to read the whole data if provided a non seekable input, for example when reading from a pipe.

Similarily, when asked to look for lines starting from the beginning of the file, with using the tail -n +linenumber syntax or tail +linenumber non standard option when supported, tail obviously reads the whole file (unless interrupted).

Best Answer

Related Solutions

Watch file using multitail even if it has not been created

Does tail Read the Whole File? – Understanding tail Command Behavior

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