Stop 03 · Linux & Scripting BasicsTopics · 2/9

Command Line Navigation & File Permissions

In short

Cloud servers don't come with a desktop. You get a black terminal and a blinking cursor. Moving around confidently and fixing 'Permission denied' are the first skills every cloud job assumes you have.

Moving around the Linux directory structure and understanding the read/write/execute permission model — the baseline fluency for configuring and troubleshooting any cloud VM.

The blob character walks down a corridor of doors holding a big ring of keys.

Think of it like this

The college building's key ring

The building has floors, corridors and rooms, like folders inside folders. Each room has rules: the HOD can enter and rearrange, staff can enter and read the notice board, students can only peek through the glass. Linux permissions work the same way.

Watch it happen

Reading -rwxr-x--- deploy.sh

1/5ls -l shows nine permission letters. They come in three groups of three: owner, group, everyone else.

The big ideas

What you need to know

Everything starts at /

Linux has one tree starting at /. Your home is /home/you (or ~), config lives in /etc, logs in /var/log.

Move with cd, look with ls

pwd shows where you are, cd moves you, ls -la lists files including hidden ones that start with a dot.

Read, write, execute

r = 4, w = 2, x = 1. Add them per group: rwx = 7, r-x = 5, r-- = 4. So 755 or 640.

Owner and group matter

chown changes who owns a file. A web server running as www-data can't read files only root can open.

See it in code

Walk through the code

bash

1pwd                      # where am I?2cd /var/log && ls -la    # go to logs, list everything3cd ~                     # back home4 5ls -l deploy.sh          # -rw-r--r-- 1 ubuntu ubuntu ...6chmod 750 deploy.sh      # owner rwx, group r-x, others none7./deploy.sh              # now it runs8 9chmod 600 ~/.ssh/my-key.pem   # private keys: owner read/write only10sudo chown www-data:www-data /var/www/site -R

1/3Know where you are before you run anything. Hidden files show up with ls -la.

The everyday moves on any cloud VM. SSH refuses to use a private key that other users can read, hence chmod 600.

Save yourself the pain

Common mistakes

Oops

Fixing 'Permission denied' with chmod 777.

Do this instead

777 lets anyone change or run the file. Find who actually needs access and give just that, like 750 or 640.

Oops

Running every command with sudo.

Do this instead

Use sudo only for system changes. Running your app or editing your own files as root hides real permission problems.

Your mission

Permission puzzle

~20 min

Quick check

A file shows -rw-r-----. What can a user who is in the file's group do?

Go deeper

1Practice cd, ls, pwd, and relative vs absolute paths until they're automatic.
2Practice reading a permission string and using chmod/chown to change it.
3Learn what lives in /etc, /var/log, and /home, and why it matters for troubleshooting.