Linux

Big picture: the general model of who owns a file and who can touch it, on one machine and then across machines. Later sections are learnings that still apply outside the situation that produced them.

Reading order

1. Users, groups, and permissions: owners, groups, modes, and how access is checked.
2. umask: what mode new files get.
3. UID and GID ranges: why a UID is 1000 on Linux and 501 on a Mac.
4. Cross-machine identity: what happens to ownership with scp, rsync, NFS, and SMB.
5. NFS security: what sec=sys trusts, and where the boundary actually is.

flowchart LR
    P[Permissions] --> U[umask]
    P --> R[UID/GID ranges]
    R --> X[Cross-machine identity]
    U --> X
    X --> N[NFS security]

Related

• Container namespaces: user namespaces remap UIDs inside containers, the same idea as ID mapping.