Terminal and File Basics

The terminal gives precise, repeatable evidence about DNS, files, network listeners, certificates, and HTTP responses.

The Prompt and Working Directory

The shell waits at a prompt for a command. Check your current directory before creating, copying, or deleting files:

pwd

List files, including hidden files:

ls -la

Change directory:

cd my-first-site

Return to the parent directory:

cd ..

Paths

An absolute path begins from the filesystem root:

/var/www/example.dpdns.org/index.html

A relative path begins from the current directory:

./index.html

Before running a command with --delete, rm, administrator privileges, or recursive behavior, resolve both source and destination paths.

Create and Inspect Files

Create directories:

mkdir -p my-first-site/assets

Display a text file:

sed -n '1,120p' index.html

Search text recursively:

rg -n 'example.dpdns.org' .

Show file type:

file index.html

Standard Output and Errors

Commands can write normal output and error output. A command that prints nothing is not automatically successful.

Check the previous exit status in a POSIX-like shell:

echo $?

Zero normally indicates success. Scripts should verify status rather than search only for reassuring words.

Redirect output to a file:

dig NS example.dpdns.org > ns-check.txt

Redirect normal and error output:

curl -I https://example.dpdns.org > http-check.txt 2>&1

Review the file before sharing it.

Pipes

A pipe sends one command's output to another command:

dig A example.dpdns.org +short | sort -u

Pipelines can hide an earlier command failure unless the shell is configured to detect it. Use simple commands while learning and inspect the complete output during troubleshooting.

Quoting

Single quotes preserve text literally in common shells:

curl -I -H 'Host: example.dpdns.org' http://192.0.2.10

Double quotes allow variable expansion:

domain='example.dpdns.org'
dig A "$domain"

Never paste a command containing an unfamiliar variable, substitution, redirection, or encoded payload without understanding it.

Administrator Privileges

sudo runs a command with elevated privileges. Use it only for tasks that genuinely require system configuration, such as installing a package or changing a web server configuration.

Do not run an entire shell as root for convenience. Check a configuration before reloading a service.

Processes and Listening Ports

On Linux:

ps aux
sudo ss -lntup

On macOS:

ps aux
lsof -nP -iTCP -sTCP:LISTEN

Identify the program, address, and port before deciding whether a service is reachable.

Command History and Secrets

Shell history can retain commands. Avoid typing tokens and passwords directly in commands. Prefer a secret manager, protected prompt, or tool-specific credential store.

Do not include a secret in a URL. URLs may appear in history, logs, referrer data, and screenshots.

Practice Lab

mkdir -p domain-book-lab/site
cd domain-book-lab
pwd
printf '%s\n' 'example.dpdns.org' > planned-domain.txt
sed -n '1,20p' planned-domain.txt
find . -maxdepth 2 -type f -print

Expected result: one planned-domain.txt file containing the fictional domain. Remove the lab only after confirming the path:

cd ..
rm -r domain-book-lab

Review Questions

1. What command confirms your current directory?
2. Why should you inspect paths before rsync --delete?
3. What is the difference between output and exit status?
4. Why should an API token not appear directly in a shell command?

Continue to Build a Safe Practice Environment.