Owner, group and umask

A file's owner and group, chown and chgrp, who may change them, and the umask, which removes permissions from 666 and 777 at creation.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

File owner: what is it?
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The user whose identity is recorded in a file's inode, and whose permission triplet applies to them.
The creator is the owner at first. Only root can give a file to another user.
What mode does f, created by touch, get under this umask?

```bash
umask 033
touch f
`
- 644
- 633
- 744
- 755
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644
You mask bit by bit, you do not subtract: 666 without the bits of 033 (x for the group, w and x for others) gives 644. A subtraction would give 633, which makes no sense.
A file is requested with mode … and a directory with mode …; the umask is then removed, bit by bit.
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A file is requested with mode 666 and a directory with mode 777; the umask is then removed, bit by bit.
touch, an editor or a > redirection ask for 666, with no x: their files are not programs. A compiler, though, asks for 777 for the executable it produces, and a directory needs x to be traversed.
Owning group: what is it?
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The group recorded in a file's inode, whose members receive the second permission triplet.
At creation it is the creator's primary group, unless the directory has the setgid bit. The owner can change it to a group they belong to.