How to fix E: Malformed line 1 in source list /etc/apt/sources.list in APT

The error message tells you which file and line number has the problem:

E: Malformed line 5 in source list /etc/apt/sources.list

or

E: Malformed line 1 in source list /etc/apt/sources.list.d/docker.list

Note the line number and file path from the error message.

Open the problematic sources file with a text editor:

sudo nano /etc/apt/sources.list

Or if the error mentions a file in sources.list.d/:

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

Use nano, vim, or your preferred text editor.

Look at the line number mentioned in the error. Check for these common issues:

Correct format:

deb http://archive.ubuntu.com/ubuntu focal main restricted universe multiverse

Common mistakes to fix:

1. Extra slashes or incorrect URLs:

# WRONG - extra slash
   deb http://archive.ubuntu.com//ubuntu focal main

   # CORRECT
   deb http://archive.ubuntu.com/ubuntu focal main

2. Missing components or spaces:

# WRONG - missing space between URL and distribution
   deb http://archive.ubuntu.comainter focal main

   # CORRECT
   deb http://archive.ubuntu.com/ubuntu focal main

3. Line wrapped incorrectly:

# WRONG - split across lines
   deb http://archive.ubuntu.com/ubuntu
   focal main restricted

   # CORRECT - single line
   deb http://archive.ubuntu.com/ubuntu focal main restricted

4. Extra or stray characters:

# WRONG - extra text at end
   deb http://archive.ubuntu.com/ubuntu focal main restricted] EXTRA

   # CORRECT
   deb http://archive.ubuntu.com/ubuntu focal main restricted

If you can't determine what's wrong, you can safely comment out the problematic line by adding # at the beginning:

# deb http://archive.ubuntu.com/ubuntu focal main

Save your changes:
- In nano: Press Ctrl+X, then Y to confirm, then Enter to save
- In vim: Press Esc, type :wq, and press Enter

Then test that APT can read the file correctly:

sudo apt update

If the error is gone and apt update completes successfully, the problem is fixed.

If the error is in the main sources.list file but you've fixed all lines, check for problematic files in sources.list.d/:

ls -la /etc/apt/sources.list.d/

Look for files with incorrect extensions (like .backup, .bak, .old). APT only recognizes:
- .list files (old format)
- .sources files (new DEB822 format)

Delete or rename any backup files:

sudo rm /etc/apt/sources.list.d/old-repo.backup

Then try sudo apt update again.