Preserve original post-install instructions for non-Fish shells

Follow-up to
#881 (review).

install.sh

@@ -1046,11 +1046,20 @@ then
 EOS
   fi
 else
-  cat <<EOS
+  if [[ "${SHELL}" == *"/fish"* ]]
+  then
+    cat <<EOS
 - Run these two commands in your terminal to add Homebrew to your ${tty_bold}PATH${tty_reset}:
     printf '%s\n' '' 'eval "\$(${HOMEBREW_PREFIX}/bin/brew shellenv)"' >> ${shell_rcfile}
     eval "\$(${HOMEBREW_PREFIX}/bin/brew shellenv)"
 EOS
+  else
+    cat <<EOS
+- Run these two commands in your terminal to add Homebrew to your ${tty_bold}PATH${tty_reset}:
+    (echo; echo 'eval "\$(${HOMEBREW_PREFIX}/bin/brew shellenv)"') >> ${shell_rcfile}
+    eval "\$(${HOMEBREW_PREFIX}/bin/brew shellenv)"
+EOS
+  fi
 fi
 
 if [[ -n "${non_default_repos}" ]]