Allow disabling of automatic SSH key adding

The `DISABLE_SSH_AGENT_AUTOADD` environmental variable has been added
which stops `ssh-agent.sh` from always trying to load keys by itself.
When this flag is set, users can still manually run `agent_setup` to add
keys at any time.

shell/common/ssh-agent.sh

@@ -12,12 +12,11 @@
 [[ ${#KEY_FILES[@]} = 0 ]] && KEY_FILES=(~/.ssh/id_rsa ~/.ssh/id_dsa ~/.ssh/id_ecdsa ~/.ssh/identity)
 
 function agent_setup() {
+  echo 'Running `agent_setup` ...'
+
   # Run if ssh is installed.
   command -v ssh >/dev/null || return
 
-  # Make sure the folder for the symlink exists.
-  mkdir -p ~/.ssh
-
   # Try to symlink an auth socket, to connect any existing agent.
   link_socket
 
@@ -34,6 +33,8 @@ function agent_setup() {
 
 # Create a symlink to an existing auth socket.
 function link_socket() {
+  # Make sure the folder for the symlink exists.
+  mkdir -p ~/.ssh
   local link="$HOME/.ssh/authsock"
   if [[ -n $SSH_AUTH_SOCK && $SSH_AUTH_SOCK != $link ]]; then
     ln -sf $SSH_AUTH_SOCK $link &>/dev/null
@@ -64,4 +65,6 @@ function start_agent() {
 }
 
 # Go!
-agent_setup
+# Automatic key loading can be disabled by setting `$DISABLE_SSH_AGENT_AUTOADD`
+# to `1` (running `agent_setup` manually would still work).
+[[ $DISABLE_SSH_AGENT_AUTOADD == 1 ]] && link_socket || agent_setup