Linux Commands, Configuration Files, and Components

Linux as a toolbox

A Linux session is a set of small tools chained together. The exam lists the tools and asks what each does, plus the files and components that hold the system together. Organize them by category and keep the quirks (case sensitivity, no recycle bin, numeric permissions) next to them.

Files and directories

command   does                             example
ls        list                             ls -la  (long, all)
pwd       print working directory
cd        change directory                 cd ~  home;  cd ..  up
mv        move or rename                   mv old.txt new.txt
cp        copy                             cp -r dir backup  (recursive)
rm        remove                           rm -rf dir  (recursive, forced; no undo)
chmod     change permissions               chmod 755 run.sh;  chmod +x run.sh
chown     change owner and group           chown -R alice:staff project
grep      search text                      grep -ri "error" /var/log
find      find files                       find / -name "*.conf" 2>/dev/null
cat       print a file                     cat /etc/hosts

Permissions decoded

-rwxr-xr--  1 alice staff  1234 Sep  7 file.sh
 ^^^^^^^^^
 owner rwx = 7;  group r-x = 5;  others r-- = 4   ->  chmod 754
r = 4, w = 2, x = 1; add them per class; owner, group, others in that order
755: owner everything, everyone else read and execute (scripts, programs)
644: owner read and write, everyone else read (documents)
700: owner only (private keys)

A directory needs x to be entered. chown user:group changes ownership; only root (via sudo) can give files away.

Disks and mounts

fsck checks and repairs a filesystem and must run on an unmounted volume (boot to recovery for the root filesystem). mount attaches a device to a directory (mount /dev/sdb1 /mnt/usb); umount detaches; mount with no arguments lists everything mounted. df -h shows free space per mounted filesystem; du -sh path shows what a directory consumes. /etc/fstab lists what mounts at boot.

Root, su, sudo

tool     what happens                                         when
root     the superuser, UID 0, can do anything                never log in as root for daily work
su       switch user; su alone becomes root with root's password; the whole shell is root   rare; many distributions lock root's password
sudo     run one command as root with your own password; logged; membership in sudo or wheel group   the normal way: sudo apt update

Packages

apt (Debian, Ubuntu, Mint): sudo apt update refreshes the index, sudo apt upgrade applies updates, sudo apt install name, sudo apt remove name. dnf (Fedora, RHEL, Rocky; yum's successor): sudo dnf install name, sudo dnf update. Packages are signed and dependencies resolved from repositories; snap and flatpak are cross-distribution alternatives.

Network

command      does
ip addr      addresses (ip a);  ip route  gateway and routes;  ip link  interfaces up or down
ping         reachability; continuous by default; -c 4 for four packets
curl         fetch a URL: curl https://example.com; test an API; download with -O
dig          DNS query with full detail: dig example.com;  dig @8.8.8.8 example.com MX
traceroute   hops to a destination

Processes and space

top shows live processes sorted by CPU (q quits); ps aux lists all; kill PID ends one (kill -9 forces). du and df as above; a full disk stops logins and services, so df -h is the first command on a sick server.

Editors and files

nano: arrow keys, Ctrl+O write out, Ctrl+X exit, Ctrl+W search; the shortcuts are printed at the bottom. vi/vim: i to insert, Esc, :wq to save and quit, :q! to abandon.

file                 holds                                                     note
/etc/passwd          users: name:x:uid:gid:comment:home:shell                  readable by all; the x means the hash is in shadow
/etc/shadow          hashed passwords and aging                                root only
/etc/hosts           static name-to-IP mappings                                checked before DNS; malware and overrides live here
/etc/fstab           filesystems to mount at boot                              a bad line can stop the boot
/etc/resolv.conf     DNS servers (nameserver lines) and search domain           may be generated by a resolver service

Components

The kernel manages hardware, memory, and processes (uname -r shows the version). The bootloader, GRUB on most systems, loads the kernel and offers a menu (older kernels, recovery mode). systemd starts and supervises services: systemctl status sshd, systemctl restart sshd, systemctl enable sshd (start at boot), systemctl disable; journalctl -u sshd reads its log.

Worked examples

• "A script will not run: Permission denied." chmod +x script.sh, then ./script.sh.
• "A web server on Ubuntu needs a package." sudo apt update && sudo apt install nginx; sudo systemctl enable --now nginx.
• "The server is out of space." df -h to find the full filesystem, du -sh /var/log/* to find the culprit, then rotate or delete.
• "A hostname resolves to the wrong address only on this machine." cat /etc/hosts; remove the stale line; cat /etc/resolv.conf to confirm the DNS server.
• "A service is down after reboot." systemctl status name; journalctl -u name; systemctl enable name if it was never enabled.

How the exam asks it

• "Which command changes file permissions?" chmod. "Which changes the owner?" chown.
• "Which command lets a user run one command as root?" sudo.
• "Which package manager is used on Ubuntu?" apt. "On Fedora?" dnf.
• "Which command shows IP addresses on a Linux system?" ip (ip addr).
• "Which command shows free disk space?" df. "Disk usage of a folder?" du.
• "Which file contains hashed passwords?" /etc/shadow.
• "Which file defines filesystems mounted at boot?" /etc/fstab.
• "Which component manages services on modern Linux?" systemd.

What to memorize

• The command tables and the chmod numbers.
• su versus sudo versus root; apt versus dnf.
• The five /etc files; kernel, bootloader (GRUB), systemd and systemctl.