fix(1722): use safe expansion for SSH_CONNECTION and SHLVL variable

Author: Lars Gerber

src/action/macos/configure_remote_building.rs

@@ -23,7 +23,7 @@ impl ConfigureRemoteBuilding {
             r#"
 # Set up Nix only on SSH connections
 # See: https://github.com/DeterminateSystems/nix-installer/pull/714
-if [ -e '{PROFILE_NIX_FILE_SHELL}' ] && [ -n "${{SSH_CONNECTION}}" ] && [ "${{SHLVL}}" -eq 1 ]; then
+if [ -e '{PROFILE_NIX_FILE_SHELL}' ] && [ -n "${{SSH_CONNECTION:-}}" ] && [ "${{SHLVL:-0}}" -eq 1 ]; then
     . '{PROFILE_NIX_FILE_SHELL}'
 fi
 # End Nix

tests/fixtures/macos/macos.json

@@ -1188,7 +1188,7 @@
             "user": null,
             "group": null,
             "mode": 420,
-            "buf": "\n# Set up Nix only on SSH connections\n# See: https://github.com/DeterminateSystems/nix-installer/pull/714\nif [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ] && [ -n \"${SSH_CONNECTION}\" ] && [ \"${SHLVL}\" -eq 1 ]; then\n    . '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'\nfi\n# End Nix\n",
+            "buf": "\n# Set up Nix only on SSH connections\n# See: https://github.com/DeterminateSystems/nix-installer/pull/714\nif [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ] && [ -n \"${SSH_CONNECTION:-}\" ] && [ \"${SHLVL:-0}\" -eq 1 ]; then\n    . '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'\nfi\n# End Nix\n",
             "position": "Beginning"
           },
           "state": "Completed"