This guide revisits Linux file structure, path basics, and core user management commands (including usermod) so you can navigate and administer systems with confidence.

Linux file structure (quick tour)

• /: Root of the file system tree.
• /home: User home directories (e.g., /home/alice).
• /bin, /usr/bin: User binaries; /sbin, /usr/sbin for system binaries.
• /etc: System configuration.
• /var: Variable data (logs, spool, caches); /var/log for logs.
• /tmp: Temporary files (often cleared on reboot).
• /dev: Device nodes; /proc and /sys are virtual FS for kernel/process info.
• /opt: Optional software; /srv for service data; /mnt//media for mounts.

Paths: absolute vs relative

• Absolute path: Starts at / and points to a single location, e.g., /etc/ssh/sshd_config. Works from anywhere.
• Relative path: Based on current directory, e.g., ../logs/app.log. Shorter for quick navigation.
• Helpers: . (current dir), .. (parent), ~ (home).

Navigation essentials

pwd                     # show current directory
ls -la                  # list files (long, all)
cd /path/to/dir         # absolute
cd ../sibling           # relative

User management revision

• Check a user: id alice · list users: getent passwd · list groups: getent group
• Create group: sudo groupadd developers
• Create user (low-level): sudo useradd -m -s /bin/bash -G developers alice then sudo passwd alice
• Create user (interactive helper): sudo adduser bob then sudo usermod -aG developers bob

usermod and related commands

• Add secondary groups: sudo usermod -aG docker alice
• Change primary group: sudo usermod -g developers alice
• Change shell: sudo chsh -s /bin/bash alice
• Lock/unlock account: sudo usermod -L alice · sudo usermod -U alice
• Change password: sudo passwd alice
• Delete user: sudo userdel alice · with home removal: sudo userdel -r alice

Permissions refresher (tie-in with groups)

• Ownership: user:group. Change with sudo chown user:group file.
• Permissions: read/write/execute for owner, group, others. Change with chmod (e.g., chmod 640 file).
• Use groups to grant shared access; avoid over-permissive “others.”

Good practices

• Use absolute paths in scripts; relative paths for quick CLI work.
• Keep one account per person/service; lock or remove unused accounts.
• Set sane shells; use /usr/sbin/nologin for service accounts that shouldn’t log in.
• Rely on groups for access control; keep .dockerignore/.gitignore clean to avoid leaking files into builds.

With a clear view of the file tree, path handling, and user/group commands (especially usermod), you can navigate, automate, and administer Linux systems confidently.