Storage, Filesystems, and Permissions
Storage, Filesystems, and Permissions covers block devices, filesystem creation and mounting, capacity checks, and the permission model. The exam expects you to choose the right tool and avoid permission changes that create security holes.
Devices and mounts
- lsblk shows block devices and mountpoints; blkid shows UUIDs and filesystem types; df -h shows free space per filesystem; du -sh summarizes directory usage.
- mkfs.ext4 or mkfs.xfs creates a filesystem; mount attaches it; /etc/fstab makes it persistent, preferably by UUID.
- LVM layers physical volumes (pvcreate), volume groups (vgcreate), and logical volumes (lvcreate), allowing resizing.
Permissions
- Permissions are rwx for owner, group, and other; octal 755 means rwxr-xr-x.
- chmod changes mode, chown changes owner, and chgrp changes group.
- umask 022 removes write for group and other on new files.
- setgid on a directory makes new files inherit the directory's group; the sticky bit on /tmp prevents deleting others' files.
- ACLs such as setfacl -m u:alice:rw file grant permissions beyond the owner, group, and other triple.
Worked example
A shared directory /srv/team must be writable by group devs, with new files group-owned by devs. Run chgrp devs /srv/team; chmod 2775 /srv/team; and set umask 002 for members. Reasoning: 2 is setgid, 775 gives group write, and umask 002 lets group members write new files while keeping them group-readable.
Traps
- Using chmod -R 777 to 'fix' access.
- Recursive chown that changes system files.
- Mounting by device name when UUIDs are stable.
- Confusing hard links, which share an inode on one filesystem, with symlinks.