Why must I add a colon for scp to work?

Question

This command does nothing.

scp doc.txt user@mywebsite.com

This command, however, copies doc.txt into the home directory of my website.

scp doc.txt user@mywebsite.com:

Why doesn't the first command (without a colon) work?

Answer 1

According to the man page for scp:

[user@]host1:]file1

The host needs a colon to indicate that it's remote.

Answer 2

scp can be used locally. The colon is how it distinguishes between a local copy and a remote copy. BTW that first one probably does something - copy doc.txt to a file called "user@mywebsite.com"

Answer 3

The name user@mywebsite.com is a valid name on the local filesystem. scp like rcp before it uses the colon character to seperate the remote user and hostname from the remote path.

Or to put it differently, that is just the way the program was written.