What's the equivalent to ${var:-defaultvalue} in fish?

Hello I am trying to translate my .bashrc to fish format almost done, mostly is clear on the documentation but this part is giving me a headache.. is so my gnupg works with my yubikey ssh etc etc.. The fish version is latest 3.0 under Arch GNU/Linux

original on BASH:

# Set SSH to use gpg-agent
unset SSH_AGENT_PID
if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]; then
  export SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"
fi
echo "UPDATESTARTUPTTY" | gpg-connect-agent > /dev/null 2&>1

Mine half converted into fish:

set -e SSH_AGENT_PID
if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]
  set -x  SSH_AUTH_SOCK="/run/user/$UID/gnupg/S.gpg-agent.ssh"
end
echo "UPDATESTARTUPTTY" | gpg-connect-agent > /dev/null 2>&1

so as you see above I have so far converted the stdin and stderror pine and the unset variable with set -e the error I am having is a bit more obscure to me:

~/.config/fish/config.fish (line 33): ${ is not a valid variable in fish.
if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]
       ^
from sourcing file ~/.config/fish/config.fish
        called during startup

Any help will be much appreciated, BTW will be nice a migrate too :) are there any out there?

[edit] ok got this working thanks to the response below, now all my bash environment, profile, bashrc etc is translated to fish and using it solely as my shell 100%

Solution

You should not change your login shell until you have a much better understanding of fish syntax and behavior. For example, in fish the equivalent of $$ is %self or $fish_pid depending on which fish version you are using. You should always specify the version of the program you are having problems with.

Assuming you're using fish 2.x that would be written as

if not set -q gnupg_SSH_AUTH_SOCK_by
or test $gnupg_SSH_AUTH_SOCK_by -ne %self
    set -gx SSH_AUTH_SOCK "/run/user/$UID/gnupg/S.gpg-agent.ssh"
end

Also, notice that there is no equal-sign between the var name and value in the set -x.