files: backup file collisions

When a configuration file would be written to an existing file, rather
than failing switch (and having the user have to move or delete those
files), move the files automatically to a new path.

Closes #585

home-manager/home-manager

@@ -416,6 +416,7 @@ function doHelp() {
     echo "  -A ATTRIBUTE Optional attribute that selects a configuration"
     echo "               expression in the configuration file."
     echo "  -I PATH      Add a path to the Nix expression search path."
+    echo "  -b EXT       Move existing files to new path rather than fail."
     echo "  -v           Verbose output"
     echo "  -n           Do a dry run, only prints what actions would be taken"
     echo "  -h           Print this help"
@@ -460,7 +461,7 @@ for arg in "$@"; do
     fi
 done
 
-while getopts 2f:I:A:vnh opt; do
+while getopts 2f:I:A:b:vnh opt; do
     case $opt in
         2)
             USE_NIX2_COMMAND=1
@@ -474,6 +475,9 @@ while getopts 2f:I:A:vnh opt; do
         A)
             HOME_MANAGER_CONFIG_ATTRIBUTE="$OPTARG"
             ;;
+        b)
+            export HOME_MANAGER_BACKUP_EXT="$OPTARG"
+            ;;
         v)
             export VERBOSE=1
             ;;

modules/files.nix

@@ -59,13 +59,23 @@ in
             targetPath="$HOME/$relativePath"
             if [[ -e "$targetPath" \
                 && ! "$(readlink "$targetPath")" == ${homeFilePattern} ]] ; then
+              if [[ ! -L "$targetPath" && -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'"
+                  collision=1
+                else
+                  warnEcho "Existing file '$targetPath' is in the way, will be moved to '$backup'"
+                fi
+              else
                 errorEcho "Existing file '$targetPath' is in the way"
                 collision=1
               fi
+            fi
           done
 
           if [[ -v collision ]] ; then
-            errorEcho "Please move the above files and try again"
+            errorEcho "Please move the above files and try again or use -b <ext> to move automatically."
             exit 1
           fi
         '';
@@ -111,6 +121,10 @@ in
           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

modules/misc/news.nix

@@ -1106,6 +1106,22 @@ in
           A new module is available: 'programs.gpg'.
         '';
       }
+
+      {
+        time = "2019-06-09T12:19:18+00:00";
+        message = ''
+          Collisions between unmanaged and managed files can now be
+          automatically resolved by moving the target file to a new
+          path instead of failing the switch operation. To enable
+          this, use the new '-b' command line argument. For example,
+
+              home-manager -b bck switch
+
+          where 'bck' is the suffix to give the moved file. In this
+          case a colliding file 'foo.conf' will be moved to
+          'foo.conf.bck'.
+        '';
+      }
     ];
   };
 }