# Get a store path:
nix-instantiate --eval -E 'with import <nixpkgs>{}; "${hello}"'

# List biggest things in /nix/store:
for x in /nix/store/*/; do echo $(nix-store --query --size $x) $x;done | sort -n

# Create a GC root:
# this will create a new root in the current dir, for the current llvm
# and now gc will never delete it, until you remove saved-llvm
nix-build '<nixpkgs>' -A llvm -o saved-llvm

# Why can't I delete something? This lists why it wont be deleted:
nix-store --query --roots /nix/store/thing