The "protocol error: unexpected <newline>" message typically occurs when there's an issue with how SCP interprets the remote server's response. This isn't a syntax problem with your command - scp user@server:/path/to/file . is fundamentally correct.

Common culprits include:

1. Corrupted shell initialization files (.bashrc, .profile)
2. Non-standard shell configurations
3. Echoed messages during login
4. Incompatible SSH/SCP versions

First, verify your SSH connection works properly:

ssh user@server "echo test"

If you see any output before "test", that's the root cause. Try these commands:

ssh -t user@server "/bin/sh"
scp -T user@server:/path/to/file .

On the server, check your shell initialization files:

grep -r "echo" ~/.bashrc ~/.profile ~/.bash_profile

Remove any echo statements or commands that produce output. Alternatively, modify your SCP command:

scp -o "LogLevel=ERROR" user@server:/path/to/file .

For stubborn cases, try forcing the legacy SCP protocol:

scp -O user@server:/path/to/file .

Or specify a different shell:

scp -S /bin/bash user@server:/path/to/file .

When SCP fails persistently, consider these alternatives:

# Using rsync
rsync -avz -e ssh user@server:/path/to/file .

# Using SFTP
sftp user@server
get /path/to/file

When executing SCP transfers between Linux systems, the protocol error: unexpected <newline> message typically indicates a malformed SSH/SCP handshake. The error occurs before file transfer begins, suggesting authentication succeeded but protocol negotiation failed.

From troubleshooting multiple servers, I've found these frequent culprits:

# 1. Corrupted shell initialization files on remote server
scp user@problem-server:/etc/profile .  # Test if system files are readable

# 2. Strict SSH configurations
ssh -v user@server  # Check for debugging output during connection

# 3. Output generation during login
scp -T user@server:/path/file .  # Try disabling pseudo-terminal allocation

First verify basic SSH functionality:

ssh user@server "echo 'Test command'" > /dev/null
if [ $? -ne 0 ]; then
    echo "SSH execution failed - check .bashrc/.profile"
fi

Then test raw SCP protocol:

echo "exec /usr/bin/scp -t /tmp" | ssh user@server
# Should return nothing if working properly

For production systems where you can't modify user profiles, use these workarounds:

# Method 1: Force direct shell command
scp -S ssh user@server:"/path/with spaces/file" .

# Method 2: Use sftp instead
sftp user@server <

# 1. Clean problematic shell files
sudo mv /etc/profile.d/motd.sh{,.bak}

# 2. Configure SSHd to skip PTY
echo "ForceCommand /usr/bin/scp" >> /etc/ssh/sshd_config
systemctl restart sshd

# 3. Create dedicated SCP user
useradd -m -s /usr/bin/scp scpuser