Every Ubuntu administrator's nightmare - you make a simple edit to /etc/sudoers, save it, and suddenly find yourself locked out of sudo privileges with that dreaded syntax error message:

$ sudo nano /etc/sudoers
>>> /etc/sudoers: syntax error near line 39 <<<
sudo: parse error in /etc/sudoers near line 39
sudo: no valid sudoers sources found, quitting

The sudoers file is parsed strictly when sudo commands execute. Unlike many configuration files that are loaded at boot or via a service restart, sudo reads the file directly for each command execution. This means:

• No syntax checking before saving
• No fallback mechanism
• Immediate lockout on error

Method 1: Using Root Shell Access

If you've set a root password (not default in Ubuntu), this is simplest:

su -
(enter root password)
visudo

The visudo command provides syntax checking before saving.

Method 2: Single User Mode (No Root Password)

When you don't have root credentials:

1. Reboot and hold Shift to access GRUB menu
2. Select recovery mode
3. Choose "root" option
4. Run: mount -o remount,rw /
5. Execute visudo
6. Reboot normally

Method 3: Alternative Superuser Session

For systems with console access:

Ctrl+Alt+F1 (or F2-F6) to switch to TTY
Login as root if possible
Use visudo to fix the file

Always use visudo for edits - it validates syntax before saving:

sudo visudo

For complex environments, consider maintaining sudoers via configuration management:

# Example Ansible task
- name: Ensure sudoers configuration
  ansible.builtin.copy:
    src: files/sudoers
    dest: /etc/sudoers
    validate: /usr/sbin/visudo -cf %s

For extreme cases where the system won't boot:

1. Boot from Ubuntu Live USB
2. Mount your root partition: sudo mount /dev/sdXN /mnt
3. Edit the file: sudo nano /mnt/etc/sudoers
4. Unmount: sudo umount /mnt

Remember that modifying system files carries risk - always have backups of critical configuration files before editing.

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When you encounter errors like /etc/sudoers: syntax error near line X or no valid sudoers sources found, it typically means you've introduced syntax errors while editing the sudoers file. Common causes include:

• Missing commas in user/group lists
• Incorrect privilege specifications
• Unbalanced parentheses
• Using tabs instead of spaces

Method 1: Using pkexec (PolicyKit)

If your system has PolicyKit installed (default on modern Ubuntu), try:

pkexec visudo

This will prompt for your user password and allow editing the sudoers file through the proper validation mechanism.

Method 2: Root Shell via su

If you've set a root password (not default on Ubuntu):

su -
# Now you're root
visudo
# Make your corrections
exit

Method 3: Recovery Using Existing sudo Session

If you happen to have an active sudo session in another terminal:

# In the working terminal:
sudo -i
# Now in root shell
visudo
# Fix the file

Using a Live CD/USB

For systems where the above methods fail:

1. Boot from Ubuntu installation media
2. Mount your root partition: sudo mount /dev/sda1 /mnt
3. Edit the file: sudo nano /mnt/etc/sudoers

To avoid this situation in the future:

• Always use visudo instead of direct editors
• Set up a backup alias: alias visudo='cp /etc/sudoers /etc/sudoers.bak && visudo'
• Consider version control: sudo cp /etc/sudoers /etc/sudoers.git && cd /etc && sudo git init && sudo git add sudoers.git

The sudoers file isn't loaded into memory for several technical reasons:

• Security: Memory can be modified by attackers
• Flexibility: Allows runtime changes without restarting services
• Performance: File I/O is negligible compared to privilege checks