Danmaxis Danmaxis - 25 days ago 6
Linux Question

How do I use the linux flock command to prevent another root process from deleting a file?

I would like to prevent one of my root processes from deleting a certain file. So I came across the flock command, it seems to fit my need, but I didn't get its syntax.

If I only indicate a shared lock, it doesn't work:

flock -s "./file.xml"


If I add a timeout parameter, it still doesn't work

flock -s -w5 "./file.xml"


It seems that way, it fits in
flock [-sxun][-w #] fd#
way.

(What is this fd# parameter?)


So, I tried the
flock [-sxon][-w #] file [-c] command


Using
flock -s -w5 "./file.xml" -c "tail -3 ./file.xml"
and it worked, tail command at ./file.xml was executed.
But I would like to know, does the lock end after the command or does it last 5 seconds after the end of the command execution? My main question is, how can I prevent another root process from deleting a file in linux?

Answer

No, flock does NOT prevent anyone from doing anything. Unix locks are ADVISORY, which means that they prevent other processes from also calling flock (or in the case of a shared lock, prevent another process using an exclusive one).

It doesn't stop root, or anyone else, from reading, writing or deleting the file.

In any case, even if it was a mandatory lock, it wouldn't stop the file being deleted, as it's the file being locked not the directory entry.