Paul

Me hacking away at things.

1 minute read

nixos-rebuild switch started printing warnings about UID changes it refused to apply:

warning: not applying UID change of user 'gdm-greeter-2' (60580 -> 60579) in /etc/passwd
warning: not applying UID change of user 'gdm-greeter-3' (60581 -> 60580) in /etc/passwd
warning: not applying UID change of user 'gdm-greeter-4' (60582 -> 60581) in /etc/passwd

GDM had left stale gdm-greeter-N entries in /etc/passwd and /var/lib/nixos/uid-map, including a duplicate UID. The plan: drop them all, let NixOS recreate them cleanly. I used sed for the passwd files and jq to strip the greeter keys from uid-map.

The next rebuild failed:

activating the configuration...
, or } expected while parsing object/hash, at character offset 2
(before "(end of string)")
at /nix/store/.../update-users-groups.pl line 11.
Activation script snippet 'users' failed (255)

The new uid-map was valid JSON — but jq had pretty-printed it. NixOS’s Perl reader doesn’t handle multi-line uid-map: it ends up parsing only the first line, which after pretty-printing was just {.

Final solution — compact JSON with jq -c:

sudo sed -i '/^gdm-greeter[-0-9]*:/d' /etc/passwd /etc/shadow /etc/group
sudo jq -c 'with_entries(select(.key | startswith("gdm-greeter") | not))' \
    /var/lib/nixos/uid-map | sudo sponge /var/lib/nixos/uid-map
sudo nixos-rebuild switch

Pretty-printing is not free.

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