How to fix E: Type 'deb' is not known on line X in source list /etc/apt/sources.list in APT

Note the file path and line number from the error message. It will be either /etc/apt/sources.list or a file in /etc/apt/sources.list.d/ directory.

Example error:

E: Type 'deb' is not known on line 5 in source list /etc/apt/sources.list.d/elastic-7.x.list

This tells you the problem is on line 5 of /etc/apt/sources.list.d/elastic-7.x.list.

Use nano or your preferred text editor to open and examine the file:

sudo nano /etc/apt/sources.list

Or for sources.list.d files:

sudo nano /etc/apt/sources.list.d/filename.list

Navigate to the line number mentioned in the error message.

Check if the problematic line has extra quotation marks. Common mistake:

❌ Incorrect:

"deb https://repo.example.com/debian bookworm main"

✓ Correct:

deb https://repo.example.com/debian bookworm main

Delete any quotation marks at the beginning or end of the line. Keep only the actual repository definition.

Verify that the line starts with 'deb' or 'deb-src', not variations like:
- 'dep' (missing 'b')
- '.deb' (has a period)
- 'Deb' (incorrect capitalization, though this may work depending on the APT version)

Fix any typos by correcting the first word of the line to exactly 'deb' or 'deb-src'.

If the line looks correct but still causes errors, it may contain invisible characters. In nano, enable display of special characters to check:

Press Ctrl+X to exit nano (if you didn't make changes), then try:

sudo cat -A /etc/apt/sources.list.d/filename.list | head -n 5

Look for unusual characters like '^M' (carriage return) or Unicode markers at the start. If found, delete the entire problematic line and retype it manually.

If the line cannot be corrected or contains special characters, delete it entirely:

1. In nano, go to the problematic line
2. Press Ctrl+K to delete the entire line
3. If there are duplicate entries (both with and without quotes), keep only the correct one

Example: if you see:

"deb https://example.com stable main"
deb https://example.com stable main

Delete the first line with quotes and keep the second.

After editing:

1. Save the file: Press Ctrl+O, then Enter to confirm (in nano)
2. Exit the editor: Press Ctrl+X
3. Test by running:

sudo apt update

If successful, you'll see 'Reading package lists... Done' without any errors. If errors persist, check the file again or refer to the advanced notes below.