scripts/update: implement -j option to control the number of parallel update jobs

scripts/update

@@ -2,6 +2,8 @@
 #!nix-shell -i bash -p findutils -p nix-update
 
 NIX_FILES_TOP=/home/colin/nixos
+# each update job has to do an entire nix eval, which can be memory intensive; be careful when tuning this
+PARALLELISM=8
 
 usage() {
   echo "update: update rev/hash for one or more packages"
@@ -10,6 +12,7 @@ usage() {
   echo "options:"
   echo "- --dry-run"
   echo "- --verbose"
+  echo "- -j <n>  (default: $PARALLELISM)"
   echo ""
   echo "examples:"
   echo "- update nixpkgs: update only the nixpkgs input"
@@ -82,8 +85,8 @@ updateOnePkg() {
 updatePkgsInParallel() {
   debug "updating packages in parallel using xargs"
   debug "- $@"
-  debug "- xargs -n 1 -P 4 $0 ${scriptFlags[*]}"
-  echo "$@" | xargs -n 1 -P 4 "$0" "${scriptFlags[@]}"
+  debug "- xargs -n 1 -P $PARALLELISM $0 ${scriptFlags[*]}"
+  echo "$@" | xargs -n 1 -P "$PARALLELISM" "$0" "${scriptFlags[@]}"
 }
 
 scriptFlags=()
@@ -107,6 +110,10 @@ parseArgs() {
         scriptFlags+=(--verbose)
         verbose=1
         ;;
+      (-j)
+        PARALLELISM=$1
+        shift
+        ;;
       (--*)
         nixFlags+=("$arg")
         ;;