How to set "default" username to be used in ssh alias?

Question

I want to set an alias that would by "default" use a different username then my current one. Like so:

$ echo $USER          # outputs kamil
$ alias myssh='ssh -o User=somebody'   # non-working example of what I want to do
$ myssh server        # Uses somebody@server - all fine!
$ myssh root@server   # I want it to use root@server, but it does not. It connects to `somebody@server`!

# Easy testing:
$ myssh -v root@localhost |& grep -i 'Authenticating to'
debug1: Authenticating to localhost:22 as 'somebody'
#                                          ^^^^^^^^ - I want root!

The code above does not work - the user in root@server is overwritten by -o User=somebody. I could do something along:

myssh() {
   # parse all ssh arguments -o -F etc.
   if [[ ! "$server_to_connect_to" =~ @ ]]; then    # if the use is not specified
        # use a default username if not given
        server_to_connect_to="somebody@$server_to_connect_to"
   fi
   ssh "${opts[@]}" "$server_to_connect_to" "${rest_of_opts[@]}"
}

but the requires parsing all ssh arguments in the function to extract server name and then add username to it. The solution would be to modify ~/.ssh/config and add Host * User somebody - however I am on a machine with no write access to home directory (and no home directory at all, actually) and I can't modify the config file and I do not want to overwrite normal ssh operation anyway.

Is there a simple solution to specify "default overridable" user to connect to server without modifying ~/.ssh/config?

Answer 1

Don't use an alias, just configure your ssh client. Edit (or create, if it doesn't exist) ~/.ssh/config and add these lines:

Host rootServer
HostName server_to_connect_to
User root

Host userServer
HostName server_to_connect_to
User somebody

Save the file, and you can now run ssh rootServer to connect as root and ssh userServer to connect as somebody.

Answer 2

Maybe you could set the username using a variable, and fall back to the default:

myssh() {
  ssh -o "User=${user:-somebody}" "$@"
}

And use it like so:

myssh server  # use default user
user=root myssh -v server  # use root as the username

Answer 3

I can generate a temporary file and pass that file to ssh as configuration with -F with the Host * User somebody added to it.

myssh() {
    (
        # in a subshell, so that `trap` will not affect parent
        local tmp
        tmp=$(mktemp --tmpdir .cis-ssh-config.XXXXXXXXXXX)
        trap 'rm "$tmp"' exit
        {
            printf "%s\n" "Host *" "  User somebody"
            # silence errors, if the files doesn't exists
            cat /etc/ssh/ssh_config ~/.ssh/config 2>/dev/null ||true
        } > "$tmp"
        ssh -F "$tmp" "$@"
    )
}