Vrftjkry

For example,

git@github.com:JohnCoates/Aerial.git

in this command, I think

git: username in GitHub Unix server

github.com: host name

/Aerial.git: directory

.

So, what is :JohnCoates? Generally, there would be a port number, but JohnCoatesdoesn't look like a port number.

P.S.
I found this answer and by reading this, JohnCoates looks like the part of the directory.
But why isn't there /?
https://stackoverflow.com/questions/5279624/whats-the-purpose-of-the-colon-in-this-git-repository-url

You aren't looking at an URL here – you are looking at a rsh/rcp-style address, similar to what's still being used by scp and rsync these days. So the rsh syntax is different from URLs: everything after the colon is the directory path, and there is no provision for port specification.

You are still correct that the git in git@... is a Unix username on the server, and github.com is the hostname. I think this is probably something that URLs inherited from rsh-style addresses.

(The original rsh had an unusual these days security model and did not allow using a different port; ssh uses a separate command-line option -p to change it if needed.)

So the entire JohnCoates/Aerial.git is a directory path. Why does it not start with a /? First of all, again, because this isn't an URL-format address and uses : as the field separator. Second, because the server does not require the leading slash in paths.

• 
  

  On plain Unix SSH servers:

  
  
  

  The path doesn't need to start with / when it's relative to the current directory. In Git addresses, that means it's relative to the user's home directory.

  
  
  

  Because RSH/SSH are meant for logging into a system account, they have a concept of "home directory" exactly like local logins do. When you log in to the system, your commands start at /home/UserName (until you cd elsewhere), so running ls Projects is the same as ls /home/UserName/Projects.

  
  
  

  All software which uses SSH transport – including SFTP, Git, and Rsync – can also use this feature. For example, if you tried to clone from a simple SSH server, using the address fred@example.com:Projects/App.git, that's the same as cloning fred@example.com:/home/fred/Projects/App.git.

  
  
  

  (As you can see, absolute paths do start with a / – but they are still separated using a : in rcp addresses.)

  
  


• 
  

  On GitHub, GitLab, and other custom systems:

  
  
  

  GitHub runs a custom git which does not directly map repository names to filesystem paths. Instead, it manually parses the received path against a regex such as ^/?(.+)/(.+).git$ and looks up the result in a SQL database.

  
  
  

  So a GitHub "path" is really just a custom string that doesn't need to follow any of the usual path rules. As one example of that, GitHub accepts paths both with and without the leading slash, as you'll see in the URL examples below.

  
  

That being said, Git also supports URL-style addresses. Your example can also be written as:

ssh://git@github.com/JohnCoates/Aerial.git

Now the path is /JohnCoates/Aerial.git as you'd expect (in URLs it is always absolute).

Note that although the path is different, GitHub still accepts it the same way – but a standard Unix server wouldn't. (It would require the full path, with home directory, to be specified.) To continue my earlier example:

fred@example.com:/home/fred/Projects/App.git

                    ↓

ssh://fred@example.com/home/fred/Projects/App.git