Levels 15–20: Power Tools

Level 15: xargs & find -exec

find locates files. xargs and -exec act on them.

Syntax	What it does
`find . -name '*.log'`	Find by filename glob
`find . -type f`	Files only (`d`=dirs, `l`=symlinks)
`find . -size +10M`	Larger than 10 MB
`find . -mtime -1`	Modified in the last 1 day
`find . -mmin -60`	Modified in the last 60 minutes
`find . -exec cmd {} \;`	Run cmd once per found file
`find . -exec cmd {} +`	Run cmd with all files batched
`find . -print0 \| xargs -0 cmd`	Safe with filenames containing spaces
`find . -name '*.log' -delete`	Delete all found files

xargs patterns:

# Basic: pass all found files to wc at once
find . -name '*.txt' | xargs wc -l

# With placeholder (-I{}): use filename anywhere
find . -name '*.conf' | xargs -I{} cp {} {}.bak

# Safe for filenames with spaces
find . -name '*.log' -print0 | xargs -0 grep ERROR

# Parallel execution (-P N = N parallel processes)
find . -name '*.png' -print0 | xargs -0 -P 4 convert -resize 50%

find size units: c=bytes, k=KB, M=MB, G=GB. + = greater than, - = less than.

Level 16: Disk & Storage

A full disk silently kills services. Know these commands cold.

Command	What it does
`df -h`	Disk free space, all filesystems, human-readable
`df -h /var`	Free space for a specific filesystem
`du -sh dir/`	Total size of a directory
`du -sh *`	Size of all items in current directory
`du -sh * \| sort -rh`	Largest items first
`find . -size +100M -ls`	Find files over 100 MB
`lsblk`	List block devices (disks, partitions)
`findmnt`	Show mounted filesystems (tree view)
`mount`	Show all mounts

Emergency disk full procedure:

# 1. Which filesystem is full?
df -h

# 2. Which directory is the culprit?
du -sh /var/* | sort -rh | head -10

# 3. Drill down
du -sh /var/log/* | sort -rh | head -10

# 4. Find the specific large files
find /var/log -size +100M -ls

# 5. Clear old compressed logs (if safe)
find /var/log -name '*.gz' -mtime +30 -delete

Level 17: System Information

Establish situational awareness on any machine in 60 seconds.

Command	What it shows	Linux/macOS
`uname -a`	Kernel, hostname, architecture	Both
`lscpu`	CPU cores, speed, architecture	Linux
`sysctl -n hw.ncpu`	CPU core count	macOS
`free -h`	RAM usage (total/used/free/available)	Linux
`vm_stat`	Memory statistics	macOS
`uptime`	System uptime and load averages	Both
`who`	Currently logged-in users	Both
`w`	Logged-in users + what they’re doing	Both
`lsof`	All open files and sockets	Both
`lsof -i :80`	What’s using port 80	Both
`lsof -p PID`	All files open by a process	Both
`hostname`	System hostname	Both
`hostname -f`	Fully qualified domain name	Linux

Load averages (from uptime):

Three numbers: 1 min, 5 min, 15 min average
Rule of thumb: if load > number of CPU cores, the system is under pressure
A 15-min average rising toward a 5-min average = load is increasing

Level 18: User Management

Linux security is built on users and groups.

Command	What it does
`useradd -m username`	Create user with home directory
`passwd username`	Set user password
`usermod -aG group user`	Add user to group (append: never omit `-a`)
`usermod -L username`	Lock account (prefix `!` to password hash)
`usermod -U username`	Unlock account
`userdel -r username`	Delete user and home directory
`groups username`	Show user’s group memberships
`id username`	Show UID, GID, all groups
`su - username`	Switch user (full login shell)
`sudo command`	Run one command as root
`visudo`	Safely edit /etc/sudoers

/etc/passwd format:

username:x:UID:GID:GECOS:home:shell
root:x:0:0:root:/root:/bin/bash

Fields: username, password placeholder, UID, GID, comment, home directory, login shell.

Level 19: SSH & Keys

Key-based auth is the industry standard. Passwords are for humans, keys are for machines.

Command	What it does
`ssh-keygen -t ed25519 -C 'email'`	Generate ED25519 key pair
`chmod 600 ~/.ssh/id_ed25519`	Fix private key permissions (SSH requires this)
`chmod 700 ~/.ssh`	Fix .ssh directory permissions
`ssh-copy-id user@host`	Copy public key to server’s authorized_keys
`ssh user@host`	Connect using default key
`ssh -i ~/.ssh/key user@host`	Connect using specific key
`ssh -L 8080:localhost:80 user@host`	Local port forwarding (tunnel)
`ssh -R 9090:localhost:80 user@host`	Remote port forwarding
`cat ~/.ssh/id_ed25519.pub`	View your public key (safe to share)

~/.ssh/config example:

Host webserver
    Hostname 192.168.1.50
    User admin
    IdentityFile ~/.ssh/deploy_key
    Port 22

Host bastion
    Hostname bastion.example.com
    User ec2-user

After adding this, ssh webserver works without typing the full details.

Key types:

Type	Recommendation
`ed25519`	Use this. Modern, fast, secure.
`rsa 4096`	Acceptable. Larger keys, slower operations.
`rsa 2048`	Minimum acceptable for RSA.
`dsa`	Deprecated. Do not use.
`ecdsa`	Acceptable but ed25519 is preferred.

Level 20: Environment & Shell Config

Your shell environment controls how everything works.

Command	What it does
`echo $PATH`	Show command search path
`export PATH=/new/dir:$PATH`	Prepend to PATH for this session
`export VAR=value`	Set variable for child processes
`alias ll='ls -lah'`	Create a command shortcut
`alias`	List all defined aliases
`unalias ll`	Remove an alias
`source ~/.bashrc`	Apply config changes without relogging
`. ~/.bashrc`	Same (dot = source)
`export PS1='\u@\h:\w\$ '`	Set shell prompt

Config file load order (bash):

File	When loaded
`~/.bash_profile` or `~/.profile`	Login shells (SSH, TTY login)
`~/.bashrc`	Interactive non-login shells (new terminal window)
`/etc/environment`	System-wide variables (Debian/Ubuntu)
`/etc/profile`	System-wide login shell config

Useful PS1 escape sequences:

Escape	Expands to
`\u`	Username
`\h`	Short hostname
`\H`	Full hostname (FQDN)
`\w`	Working directory (full path)
`\W`	Working directory (basename only)
`\$`	`$` for users, `#` for root
`\t`	Current time (HH:MM:SS)
`\d`	Date (Day Mon DD)