files: clean up

Specifically, replace the `link` and `clean` scripts by inline Bash
functions. This avoids a number of Bash invocations and simplifies the
code a little. This should also make it slightly easier to implement
support for arbitrary file locations.

modules/files.nix

@@ -106,7 +106,8 @@ in
               $VERBOSE_ECHO "Skipping collision check for $targetPath"
             elif [[ -e "$targetPath" \
                 && ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
-              if [[ ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
+              if [[ ! -L "$targetPath" \
+                  && -v HOME_MANAGER_BACKUP_EXT && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
                 backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
                 if [[ -e "$backup" ]]; then
                   errorEcho "Existing file '$backup' would be clobbered by backing up '$targetPath'"
@@ -160,34 +161,16 @@ in
     # and a failure during the intermediate state FA ∩ FB will not
     # result in lost links because this set of links are in both the
     # source and target generation.
-    home.activation.linkGeneration = hm.dag.entryAfter ["writeBoundary"] (
-      let
-        link = pkgs.writeShellScript "link" ''
-          newGenFiles="$1"
-          shift
-          for sourcePath in "$@" ; do
-            relativePath="''${sourcePath#$newGenFiles/}"
-            targetPath="$HOME/$relativePath"
-            if [[ -e "$targetPath" && ! -L "$targetPath" && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
-              backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
-              $DRY_RUN_CMD mv $VERBOSE_ARG "$targetPath" "$backup" || errorEcho "Moving '$targetPath' failed!"
-            fi
-            $DRY_RUN_CMD mkdir -p $VERBOSE_ARG "$(dirname "$targetPath")"
-            $DRY_RUN_CMD ln -nsf $VERBOSE_ARG "$sourcePath" "$targetPath"
-          done
-        '';
-
-        cleanup = pkgs.writeShellScript "cleanup" ''
-          . ${./lib-bash/color-echo.sh}
-
+    home.activation.linkGeneration = hm.dag.entryAfter ["writeBoundary"] (''
+      function clean() {
         # A symbolic link whose target path matches this pattern will be
         # considered part of a Home Manager generation.
+        local homeFilePattern
         homeFilePattern="$(readlink -e ${escapeShellArg builtins.storeDir})/*-home-manager-files/*"
 
-          newGenFiles="$1"
-          shift 1
-          for relativePath in "$@" ; do
-            targetPath="$HOME/$relativePath"
+        local newGenFiles="$1"
+        local relativePath="$2"
+        local targetPath="$HOME/$relativePath"
         if [[ -e "$newGenFiles/$relativePath" ]] ; then
           $VERBOSE_ECHO "Checking $targetPath: exists"
         elif [[ ! "$(readlink "$targetPath")" == $homeFilePattern ]] ; then
@@ -211,17 +194,6 @@ in
             popd > /dev/null
           fi
         fi
-          done
-        '';
-      in
-        ''
-          function linkNewGen() {
-            echo "Creating home file links in $HOME"
-
-            local newGenFiles
-            newGenFiles="$(readlink -e "$newGenPath/home-files")"
-            find "$newGenFiles" \( -type f -or -type l \) \
-              -exec bash ${link} "$newGenFiles" {} +
       }
 
       function cleanOldGen() {
@@ -239,7 +211,35 @@ in
         # generation. The find command below will print the
         # relative path of the entry.
         find "$oldGenFiles" '(' -type f -or -type l ')' -printf '%P\0' \
-              | xargs -0 bash ${cleanup} "$newGenFiles"
+          | while IFS= read -r -d "" relativePath ; do \
+              clean "$newGenFiles" "$relativePath"; \
+            done
+      }
+
+      function link() {
+        local newGenFiles="$1"
+        local sourcePath="$2"
+        local relativePath="''${sourcePath#$newGenFiles/}"
+        local targetPath="$HOME/$relativePath"
+        if [[ -e "$targetPath" && ! -L "$targetPath" \
+            && -v HOME_MANAGER_BACKUP_EXT && -n "$HOME_MANAGER_BACKUP_EXT" ]] ; then
+          local backup="$targetPath.$HOME_MANAGER_BACKUP_EXT"
+          $DRY_RUN_CMD mv $VERBOSE_ARG "$targetPath" "$backup" \
+            || errorEcho "Moving '$targetPath' failed!"
+        fi
+        $DRY_RUN_CMD mkdir -p $VERBOSE_ARG "$(dirname "$targetPath")"
+        $DRY_RUN_CMD ln -nsf $VERBOSE_ARG "$sourcePath" "$targetPath"
+      }
+
+      function linkNewGen() {
+        echo "Creating home file links in $HOME"
+
+        local newGenFiles
+        newGenFiles="$(readlink -e "$newGenPath/home-files")"
+        find "$newGenFiles" \( -type f -or -type l \) -print0 \
+          | while IFS= read -r -d "" sourcePath ; do \
+              link "$newGenFiles" "$sourcePath"; \
+            done
       }
 
       cleanOldGen
@@ -253,8 +253,9 @@ in
       fi
 
       linkNewGen
-        ''
-    );
+
+      unset -f clean cleanOldGen link linkNewGen
+    '');
 
     home.activation.checkFilesChanged = hm.dag.entryBefore ["linkGeneration"] (
       let