Does overlayfs redirect opened files automatically on the fly?

Question

or I should close file - then mount overlay - and then re-open file again? i.e.

#!/bin/bash
my_background_process >log.txt &
...
pkill my_background_process
mount overlay
my_background_process >>log.txt &
...

is it necessary?

Answer 1

Once a file is open, it stays open until the process closes it. Reading and writing from a file don't care whether the file is still available under its original name. The file may have been renamed, deleted, shadowed… it's still the same file.

If you open a file /somewhere/somefile then mount a filesystem at /somewhere, then after that point /somewhere/somefile will designate a file on the new filesystem. But that only matters when you open /somewhere/somefile. If a process already had /somewhere/somefile open on the original filesystem, that doesn't change.

Redirection opens a file when the shell processes the redirection operator. After that, it keeps being the same open file, even if multiple processes are involved. For example, in the snippet below, program2 writes to the file at /mnt/log.txt on the root partition, whereas program3 writes to /log.txt on /dev/something.

do_stuff () {
  program1
  mount /dev/something /mnt
  program2
  program3 >/mnt/log.txt
}
do_stuff >/mnt/log.txt

If you've already started a program and you want to change where its output goes, you need to ask the program to do it. In theory, you can force the program to do it using a debugger — this thread lists some programs that can do it. But getting down and dirty with a program like this can crash it.

If you really need to change where the program's output goes midway, you need to relay the output through a helper that is able to change its own output. Here's a tiny proof-of-concept in a shell script:

my_background_process | {
  while IFS= read -r line; do
    printf '%s\n' "$line" >>/mnt/log.txt
  done
} &

This script opens the output file for each line, so if the file designated by /mnt/log.txt changes (because the file has been moved, because a new filesystem has been mounted at /mnt, etc.) then the subsequent lines will be written to the new file. Note that you need to specify the name of the directory: with just >log.txt, this would always open the file in the current directory, so it wouldn't be affected by a mount operation (the current directory works like an open file: mounting something on /mnt doesn't affect what processes see as their current directory even if their current directory is /mnt).

Answer 2

There is no support in Linux for automatically redirecting files which were opened on an arbitrary filesystem. It is necessary to do so yourself.