home-manager: add generation expiration command

This adds a new command to the home-manager shell script that allows
generations to be removed that are older than an given absolute or
relative date.

This allows users to manage the expiration of their home-manager
generations separately from their system or user profiles via
nix-collect-garbage. It is most important if the user desires to have
a convenient way to have different expiration times for Home Manager
generations than other system or user profiles.

home-manager/default.nix

@@ -21,6 +21,8 @@ pkgs.stdenv.mkDerivation {
     substituteInPlace $out/bin/home-manager \
       --subst-var-by bash "${pkgs.bash}" \
       --subst-var-by coreutils "${pkgs.coreutils}" \
+      --subst-var-by findutils "${pkgs.findutils}" \
+      --subst-var-by gnused "${pkgs.gnused}" \
       --subst-var-by less "${pkgs.less}" \
       --subst-var-by HOME_MANAGER_PATH '${pathStr}'
   '';

home-manager/home-manager

@@ -1,9 +1,7 @@
 #!@bash@/bin/bash
 
-# This code explicitly requires GNU Core Utilities and we therefore
-# need to ensure they are prioritized over any other similarly named
-# tools on the system.
-PATH=@coreutils@/bin:@less@/bin${PATH:+:}$PATH
+# Prepare to use tools from Nixpkgs.
+PATH=@coreutils@/bin:@findutils@/bin:@gnused@/bin:@less@/bin${PATH:+:}$PATH
 
 set -euo pipefail
 
@@ -237,6 +235,23 @@ function doRmGenerations() {
     popd > /dev/null
 }
 
+function doExpireGenerations() {
+    local profileDir="/nix/var/nix/profiles/per-user/$USER"
+
+    local generations
+    generations="$( \
+        find "$profileDir" -name 'home-manager-*-link' -not -newermt "$1" \
+        | sed 's/^.*-\([0-9]*\)-link$/\1/' \
+    )"
+
+    if [[ -n $generations ]]; then
+        # shellcheck disable=2086
+        doRmGenerations $generations
+    elif [[ -v VERBOSE ]]; then
+        echo "No generations to expire"
+    fi
+}
+
 function doListPackages() {
     local outPath
     outPath="$(nix-env -q --out-path | grep -o '/.*home-manager-path$')"
@@ -349,6 +364,11 @@ function doHelp() {
     echo "      Remove indicated generations. Use 'generations' command to"
     echo "      find suitable generation numbers."
     echo
+    echo "  expire-generations TIMESTAMP"
+    echo "      Remove generations older than TIMESTAMP where TIMESTAMP is"
+    echo "      interpreted as in the -d argument of the date tool. For"
+    echo "      example \"-30 days\" or \"2018-01-01\"."
+    echo
     echo "  packages     List all packages installed in home-manager-path"
     echo
     echo "  news         Show news entries in a pager"
@@ -422,6 +442,9 @@ case "$cmd" in
     remove-generations)
         doRmGenerations "$@"
         ;;
+    expire-generations)
+        doExpireGenerations "$@"
+        ;;
     packages)
         doListPackages
         ;;