Solaris

I know how to check if any file has a unix process using a file by looking at 'lsof <fullpath/filename>' command.

I think using lsof is very expensive. Also to make it accurate we need to inlcude fullpath of the file.

Is there another command that can tell if a file has a truely active open file handle pointing to it ? or if a file has a runtime PID associated with it?

fuser, which is more commonly available than lsof, does this, putting the pids on stdout and the file names and flags on stderr, so you can get the pid list easily. For example:

Code:

$ for p in $( fuser . 2>/dev/null )
do
 echo
 ps -fp $p
 echo
 ptree $p
done
$

Nothing bad happens if you delete a file in use. It just persists on disk until the last thing using it quits, then is deleted for good.

There's no faster way to query the kernel about open files, than querying the kernel about open files. fuser works roughly the same way as lsof, so isn't really a slimmer/better workaround.

If you want to delete a file and not have lost space if some program holds it forever, truncate it first. However, if the program appends with a high file position, all the space is reallocated and written to zeros, so there is some virtue in ensuring nobody has it open to write before you delete it.

This is slightly incorrect, at least when using UFS or ZFS. If the writing program is seeking to the previous end of the file location, the space from the beginning of the file to that new location isn't filled with zeroes but unallocated, leading to the creation of a sparse file. If you read the unallocated data, the OS will return "virtual" zeroes but they use no space at all on the disk.

I was taken in by the zeros! You can close it, and there is no space allocated? I guess it is all done in the allocation storage. It is an interesting resource in the 64 bit world, as you could have a very huge, sparse file with just a few blocks. Consider how handy that is in conjunction with mmap64().

This space isn't allocated whether the file is closed or not.
A simpler approach is to mmap /dev/zero which is kind of an infinite sparse file by design.