tmux: update current shell's environment's ssh agent forwarding info (ie. value of $SSH_AUTH_SOCK) when reconnecting, eg via PuTTY

0.sh

# next try: `test` before `export`
val=`tmux show-environment | grep '^SSH_AUTH_SOCK='`;\
 test -n "$val" && export "$val"

# with the $_ bash-ism:
test -n `tmux show-environment | grep '^SSH_AUTH_SOCK='`\
 && export "$_";

1.sh

# use grep instead of `tmux showenv VAR` since output could possibly begin with
# '-' (for variables removed from environment)
eval `tmux show-environment | grep '^SSH_AUTH_SOCK='`; \
  export SSH_AUTH_SOCK

# as a function
function copy_one_tmux_env() {
  eval `tmux show-environment | grep "^$1="`
  export $1
}

eval $(tmux showenv -s SSH_AUTH_SOCK) is simpler