Add initial section of activation script

The initial section creates some useful variables for use by later
activation sections.

modules/activation-init.sh

@@ -0,0 +1,46 @@
+function setupVars() {
+    local profilesPath="/nix/var/nix/profiles/per-user/$USER"
+    local gcPath="/nix/var/nix/gcroots/per-user/$USER"
+    local greatestGenNum
+
+    greatestGenNum=$( \
+        find "$profilesPath" -name 'home-manager-*-link' \
+            | sed 's/^.*-\([0-9]*\)-link$/\1/' \
+            | sort -rn \
+            | head -1)
+
+    if [[ -n "$greatestGenNum" ]] ; then
+        oldGenNum=$greatestGenNum
+        newGenNum=$((oldGenNum + 1))
+    else
+        newGenNum=1
+    fi
+
+    if [[ -e "$gcPath/current-home" ]] ; then
+        oldGenPath="$(readlink -e "$gcPath/current-home")"
+    fi
+
+    newGenPath="@GENERATION_DIR@";
+    newGenProfilePath="$profilesPath/home-manager-$newGenNum-link"
+    newGenGcPath="$gcPath/current-home"
+}
+
+setupVars
+
+echo "Starting home manager activation"
+
+if [[ $DRY_RUN ]] ; then
+  echo "This is a dry run"
+  export DRY_RUN_CMD=echo
+else
+  echo "This is a live run"
+  unset DRY_RUN_CMD
+fi
+
+echo "Activation variables:"
+echo "  oldGenNum=$oldGenNum"
+echo "  newGenNum=$newGenNum"
+echo "  oldGenPath=$oldGenPath"
+echo "  newGenPath=$newGenPath"
+echo "  newGenProfilePath=$newGenProfilePath"
+echo "  newGenGcPath=$newGenGcPath"