Linux Fundamentals · 3.4

🔏 Linux File Permissions

rwxrwxrwx — reading the permission bits⏱ ~2 min

Every file and directory in Linux has three permission sets: one for the Owner, one for the Group, and one for Others. Each set has three bits: read (r), write (w), and execute (x).

Reading Permission Strings

bash

ls -l pythagoras.sh
# Output: -rwxr-x--- 1 cyberpatriot users 2048 Jun 18 08:00 pythagoras.sh
#         ^ type
#          ^^^  owner permissions (rwx = read, write, execute)
#             ^^^  group permissions (r-x = read, execute; no write)
#                ^^^  other permissions (--- = no access)
# Owner: cyberpatriot   Group: users

Symbol	Meaning	Notes
r	Read	Files: view contents. Directories: list contents with ls
w	Write	Files: modify. Directories: create/delete files inside
x	Execute	Files: run as a program. Directories: enter with cd
-	No permission	That permission is not granted

Changing Permissions (chmod)

bash

# Symbolic mode — easier to understand
chmod u+x script.sh       # Add execute for the owner (user)
chmod g-w file.txt        # Remove write from the group
chmod o-rx file.txt       # Remove read+execute from others
chmod a+r public.txt      # Add read for All (owner+group+others)
 
# Numeric (octal) mode — faster once you know it
# r=4, w=2, x=1 — add up the digits for each group
chmod 755 script.sh       # owner: rwx(7), group: r-x(5), others: r-x(5)
chmod 644 config.txt      # owner: rw-(6), group: r--(4), others: r--(4)
chmod 600 secret.key      # owner: rw-(6), group: ---(0), others: ---(0)
chmod 700 private/        # owner: rwx(7), group: ---(0), others: ---(0)

Changing Ownership (chown, chgrp)

bash

sudo chown alice file.txt          # Change owner to alice
sudo chown alice:staff file.txt    # Change owner AND group
sudo chgrp developers project/     # Change group only
sudo chown -R alice mydir/         # Recursive: change owner of all files inside

🔒 SecurityWorld-writable files (others have write permission) are serious vulnerabilities. Find them with: find / -perm -o+w -not -path '/proc/*' -not -path '/sys/*' 2>/dev/null. Any sensitive file with world-write is a security finding in competition.

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What does chmod 777 file.txt do — and should you?

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