Add SSH With A Specific Key as a unix til.

README.md

@@ -210,6 +210,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Saying Yes](unix/saying-yes.md)
 - [Search History](unix/search-history.md)
 - [Securely Remove Files](unix/securely-remove-files.md)
+- [SSH With A Specific Key](unix/ssh-with-a-specific-key.md)
 - [SSH With Port Forwarding](unix/ssh-with-port-forwarding.md)
 - [Switch Versions of a Brew Formula](unix/switch-versions-of-a-brew-formula.md)
 - [Watch The Difference](unix/watch-the-difference.md)

unix/ssh-with-a-specific-key.md

@@ -0,0 +1,9 @@
+# SSH With A Specific Key
+
+When you SSH into another machine using public key authentication, the
+key pair from either `~/.ssh/id_dsa`, `~/.ssh/id_ecdsa`, or `~/.ssh/id_rsa`
+is used by default. This is generally what you want. But what if the target
+server is expecting to identify you with a different SSH key pair?
+
+The `-i` option can be used with `ssh` to specify a different _identity
+file_ when the default isn't what you want.