# View file permissions
ls -la /path/to/file
 
# View current user and groups
id

sudo COMMAND

-rwxr-xr-- 1 user group 4096 Mar 24 10:00 file.txt
|---|---|---|
|  |   |  +-- Other users: r-- (read)
|  |   +----- Group: r-x (read+execute)
|  +--------- Owner: rwx (read+write+execute)
+------------- File type (- regular file, d directory, l symlink)

# Files in /etc belong to root
sudo nano /etc/xxx/config.conf
 
# Or temporarily change permissions (not recommended for system files)
sudo chmod 666 /path/to/file

# Add execute permission
chmod +x script.sh
 
# Owner only can execute
chmod u+x script.sh

# Typical web directory permission setup
sudo chown -R www-data:www-data /var/www/html
sudo chmod -R 755 /var/www/html
sudo chmod -R 644 /var/www/html/*.html

# View current mounts
mount | grep /dev/sd
 
# Remount NTFS as writable
sudo mount -o rw,uid=$(id -u),gid=$(id -g) /dev/sdX1 /mnt/data
 
# Configure in /etc/fstab
# UUID=xxx /mnt/data ntfs-3g defaults,uid=1000,gid=1000 0 0

chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
chmod 600 ~/.ssh/authorized_keys
chmod 644 ~/.ssh/config

# Restore home directory permissions
sudo chown -R $USER:$USER ~/
chmod 755 ~/

# Need to log in as root or use recovery mode
su - root
usermod -aG sudo USERNAME
# User needs to log out and log back in

# Use pkexec to fix (no sudo needed)
pkexec visudo
 
# Or fix from recovery mode
# Make sure /etc/sudoers contains:
# %sudo   ALL=(ALL:ALL) ALL

# Set SUID (execute as file owner)
chmod u+s /path/to/file
 
# Set SGID (new files inherit the directory's group)
chmod g+s /path/to/directory
 
# Set Sticky Bit (only owner can delete files in the directory)
chmod +t /path/to/directory

# Install ACL tools
sudo apt install acl
 
# Set permissions for a specific user
setfacl -m u:USERNAME:rwx /path/to/file
 
# View ACL
getfacl /path/to/file
 
# Remove ACL
setfacl -b /path/to/file

# View AppArmor status
sudo aa-status
 
# Temporarily set a program to complain mode
sudo aa-complain /usr/bin/PROGRAM_NAME
 
# Disable a profile
sudo aa-disable /etc/apparmor.d/PROFILE_NAME