Hello. I have been wondering about the issues of user permissions on removable media.

Let's say I have an external HD (ext3 formatted, let's assume) and I create some files/folders with read/write permissions just for myself. Later, I plug that drive in another computer, for example, a lab PC at the university. With no root access, would I be able to read/write those files/folders? The user (id, name...) is not the same.

And another question related to this. Let's say that I have the same drive. If I give it to a friend, can he simply create a user with the same name in his PC and be able to access those files/folders? What I mean is, what is exactly taken into account when checking for permissions in Linux? Just the username? The UID?

I know that in case I don't want anyone to peek into my folders I should take more advanced measures such as encryption, but these questions make me wonder if the whole users/permissions "thing" (forgive me linux wise men makes sense when using removable media.

Thank you in advance.

The file system keep track of file permissions with the UID. So in order for that to work, I guess you would need to use the same UID at home and at your school. But, if that's a problem, you can always override with permissions. You can set 'others' to rw(x) to give your account at school/work the permission to do what you like. Or you could add a group on your home computer with the same GID as your primary group at school.

So, to summarize;the file system use UID, and not names to identify users. But of course your friend with a GNU/Linux box can make a new user with what ever UID he likes..

Thanks Dinithion. That is what I thought. As a security measure it is not very effective, permissions can be easily overriden. If I need to use the drive in another computer with a different user I'd simply give read/write permissions to the files. I was simply unsure about how Linux handled the permissions.