How to force User in ssh config file

You can't override a user specified on the command line. Only give a default for when none is specified (if there's no default given in your ~/.ssh/config, your login as returned by whoami will be used). So it will work as desired if you use: ssh -Tv github.com

Note you can actually set Host to any value, not just one that corresponds to an actual hostname, like so:

Host gh
    Hostname gh
    User git

Then you could use ssh -Tv gh.

The %r and %h in the IdentityFile (~/.ssh/%r@%h_id_rsa) will always be the same as the remote user and remote host used to authenticate.

In the example i've changed Hostname

Host github.com bitbucket.org bitbucket.com
  IdentityFile ~/.ssh/%r@%h_id_rsa
  Hostname newhostname.com

Test

$ ssh -Tv [email protected]

Result

debug1: Authenticating to github.com:22 as 'tarranjones'
debug1: Offering RSA public key: ~/.ssh/[email protected]_id_rsa

In this example i've changed User

Host github.com bitbucket.org bitbucket.com
  IdentityFile ~/.ssh/%r@%h_id_rsa
  User git

Test

$ ssh -Tv [email protected]

Now the remote host name wont change but if it did change this would just mean that the corresponding IdentityFile name would then be wrong.

debug1: Authenticating to github.com:22 as 'git'
debug1: Offering RSA public key: ~/.ssh/[email protected]_id_rsa

Using percent_expand for the remote host name %h is not going to work for naming the identity file. This will always need to be hard coded if you want it to differ from the remote host name used to authenticate.

The best i could come up with is this.

#Set Git User Domains
Host *-github.com *-bitbucket.org *-bitbucket.com
  User git

#IdentityFile
Host tarranjones-*
  IdentityFile ~/.ssh/tarranjones@%h_id_rsa

Host otherusername-*
  IdentityFile ~/.ssh/otherusername@%h_id_rsa

#Hostnames
Host *-github.com
  Hostname github.com

Host *-bitbucket.com *-bitbucket.org
  Hostname bitbucket.org

Host *
  Protocol 2
  UseKeychain yes
  AddKeysToAgent yes
  IdentitiesOnly yes

Usage

$ ssh -Tv tarranjones-github.com

Result

debug1: Authenticating to github.com:22 as 'git'
debug1: Offering RSA public key: ~/.ssh/[email protected]_id_rsa

Please see Updated Gist

Host * may as well be deleted:

IdentityFile %d/.ssh/%r@%h_id_rsa
Protocol 2
UseKeychain yes
AddKeysToAgent yes
IdentitiesOnly yes

Host github.com bitbucket.org bitbucket.com
  User git

Don't specify the username on the command line as that will override the user name in the configuration.