How to run a script when accessing a specific path?-CodePudding

I have some path I have to access, which is the result of mounting.

I would like the mounting to be automatic, via a script, and I want that script to run just before an error is thrown from not being able to access the path.

For example, assume the script is

echo scripting!
mkdir -p /non_existing_path

and I want it to run when trying to access (in any way) to the path /non_existing_path.

So when I do for example

cd /non_existing_path

touch /non_existing_path/my_file.txt

It would always succeed, with the output scripting!. In reality, the script would be more elaborated than that.

Is this possible at all?

CodePudding user response：

Yes, the important case is that 3rd parties (such as a new C program, command line, or other scripts) that would call for example cd should also be affected, and a call by them to cd as they normally would, should invoke the hooked script beforehand.

Out of kernel:

Write a fuse filesystem that would mount on top of other filesystem, that would upon open() syscall run fork() execve() a custom script.

In kernel:

Write kernel filesystem that would expose /proc/some/interface and would create a filesystem "on-top" of underlying existing handle. Such kernel module would execute a special command upon open() system call and forward all others. In open() system call, the kernel would write some data to /proc/some/interface and wait for an answer. After receiving the answer, open() syscall would continue.

Write a user-space demon that would for example poll() on events on a /proc/some/interface waiting for events, then read() the events, parse them and execute custom script. Then after the script completion, it would write to /proc/some/interface on the same file descriptor to notify the kernel module that the operation has compleated.

CodePudding user response：

Why don't you use autofs?

autofs is a program for automatically mounting directories on an as-needed basis.

https://help.ubuntu.com/community/Autofs

CodePudding user response：

Not sure I understand.

Do you want the script to run even if the path is not accessible?
Do you want the script to run only if the path is not accessible?
Is the script supposed to mount the "not accessible" path?

In any case, I think you should just use an if else statement

The script would look like:

#!/bin/bash

if [ -d "/non_existing_path" ]
then
    bash $1
else
    bash $1
    mkdir -p /non_existing_path
fi

Lets assume this script's name is "myScript.sh" and the external script's name is "extScript.sh". You would call:
bash myScript.sh /home/user/extScript.sh

This script will check if path exist.
If yes, execute bash /home/user/extScript.sh
If no, execute bash /home/user/extScript.sh and mkdir...

Again, I'm not sure to get you goal, but you can adapt it to your needs.