{activation-scripts,activate-system}: purify environment

This ensures that system activation does not depend on various
details of its process environment, ensuring uniformity across various
invocation contexts and with the `activate-system` daemon. This becomes
more important in a post‐user‐activation world to avoid problematic
dependencies like `$SUDO_USER`, but is a good idea in general.

The `sudoers(5)` defaults on my Sequoia system are:

    Defaults	env_reset
    Defaults	env_keep += "BLOCKSIZE"
    Defaults	env_keep += "COLORFGBG COLORTERM"
    Defaults	env_keep += "__CF_USER_TEXT_ENCODING"
    Defaults	env_keep += "CHARSET LANG LANGUAGE LC_ALL LC_COLLATE LC_CTYPE"
    Defaults	env_keep += "LC_MESSAGES LC_MONETARY LC_NUMERIC LC_TIME"
    Defaults	env_keep += "LINES COLUMNS"
    Defaults	env_keep += "LSCOLORS"
    Defaults	env_keep += "SSH_AUTH_SOCK"
    Defaults	env_keep += "TZ"
    Defaults	env_keep += "DISPLAY XAUTHORIZATION XAUTHORITY"
    Defaults	env_keep += "EDITOR VISUAL"
    Defaults	env_keep += "HOME MAIL"

Of these preserved environment variables, the ones that are set in
practice when I run `sudo env` that aren’t set in the activation
script here are:

* `$COLORTERM`
* `$DISPLAY`
* `$EDITOR`
* `$MAIL`
* `$SSH_AUTH_SOCK`
* `$TERM`
* `$__CF_USER_TEXT_ENCODING`

Most of these seem either pointless or actively harmful to set for
the purpose of the system activation script.

This will mean that tools run during activation won’t print output
in the user’s preferred language, but that’s probably the right
trade‐off overall, as that is likely to break activation scripts
that parse command output anyway.

modules/services/activate-system/default.nix

@@ -10,7 +10,14 @@
       script = ''
         set -e
         set -o pipefail
+
         export PATH="${pkgs.gnugrep}/bin:${pkgs.coreutils}/bin:@out@/sw/bin:/usr/bin:/bin:/usr/sbin:/sbin"
+        export USER=root
+        export LOGNAME=root
+        export HOME=~root
+        export SHELL=$BASH
+        export LANG=C
+        export LC_CTYPE=UTF-8
 
         systemConfig=$(cat ${config.system.profile}/systemConfig)
 

modules/system/activation-scripts.nix

@@ -37,16 +37,33 @@ in
   config = {
 
     system.activationScripts.script.text = ''
-      #! ${stdenv.shell}
+      #!/usr/bin/env -i ${stdenv.shell}
+      # shellcheck shell=bash
+      # shellcheck disable=SC2096
+
       set -e
       set -o pipefail
+
       export PATH="${pkgs.gnugrep}/bin:${pkgs.coreutils}/bin:@out@/sw/bin:/usr/bin:/bin:/usr/sbin:/sbin"
+      export USER=root
+      export LOGNAME=root
+      export HOME=~root
+      export SHELL=$BASH
+      export LANG=C
+      export LC_CTYPE=UTF-8
 
       systemConfig=@out@
 
       # Ensure a consistent umask.
       umask 0022
 
+      cd /
+
+      if [[ $(id -u) -ne 0 ]]; then
+        printf >&2 '\e[1;31merror: `activate` must be run as root\e[0m\n'
+        exit 2
+      fi
+
       ${cfg.activationScripts.preActivation.text}
 
       # We run `etcChecks` again just in case someone runs `activate`