Linux & Shell Scripting

Learn what Linux is: kernel versus distribution versus shell, which distro to learn for DevOps, and why servers and containers overwhelmingly run Linux.

Learn the Linux directory tree: what /etc, /var and /usr hold, /tmp versus /var/tmp, virtual /proc and /sys, and the places where disks tend to fill up.

Move around Linux with confidence: absolute versus relative paths, reading ls -l output, which versus type, man pages and fixing command not found errors.

Copy, move, delete and read files safely on Linux: why rm has no undo, cp -r versus cp -a, the trailing-slash trap, find by name and size, less versus tail.

Use grep, sed, awk, sort and uniq for real work: pick the right tool for each question, avoid the sed -i portability trap and see why errors vanish in pipes.

Understand Linux permissions: rwx for user, group and other, octal modes like 755 and 644, chmod versus chown, setuid, the sticky bit and umask defaults.

Manage Linux accounts safely: useradd versus adduser, why usermod -aG needs a new login, the destructive -G mistake, sudo versus su and the account files.

Find, watch and stop Linux processes: use ps and top, send SIGTERM before SIGKILL, spot zombie states, read load average and find which process owns a port.

Manage Linux services with systemctl: start versus enable, daemon-reload after edits, writing a unit file, what Type= controls and reading logs.

Diagnose Linux networking with ip and ss instead of ifconfig and netstat, learn why ping proves little, debug DNS with dig and choose rsync over scp.

Find and fix Linux disk problems: why df and du disagree, deleted-but-open files, inode exhaustion, persistent mounts by UUID in fstab and tar versus zip.

Write bash scripts that fail loudly, not silently: use set -euo pipefail, quote variables, prefer [[ ]] over [ ], clean up with trap and lint with shellcheck.

Install software on any distro: apt versus apt-get, remove versus purge, fixing dpkg lock errors, dnf and rpm on Red Hat systems, and when snap is worth it.

Learn Linux environment variables: shell versus exported variables, .bashrc versus .bash_profile, extending PATH safely and why sudo drops your environment.

Schedule jobs with crontab: the five time fields, common examples, why a script that works manually fails in cron, capturing output, and systemd timers.

Set up SSH keys properly: generate Ed25519 keys with ssh-keygen, fix permission errors that block a valid key, use ssh-agent, ~/.ssh/config and ProxyJump.

Set up a Linux firewall without locking yourself out: compare ufw, firewalld and iptables, allow SSH first, default deny, zones and persistent rules.

Find what went wrong in Linux logs: /var/log versus the systemd journal, tail -F over -f, grep filtering, logrotate, journald size caps and error triage.

Triage Linux performance in order: CPU, memory and I/O. Learn what load average counts, why free memory looks low and how to read iowait with vmstat.