home-manager/modules/lib-bash/activation-init.sh

# Moves the existing profile from /nix or $XDG_STATE_HOME/home-manager to
# $XDG_STATE_HOME/nix to match changed behavior in Nix 2.14. See
# https://github.com/NixOS/nix/pull/5226.
function migrateProfile() {
    declare -r stateHome="${XDG_STATE_HOME:-$HOME/.local/state}"
    declare -r userNixStateDir="$stateHome/nix"
    declare -r hmStateDir="$stateHome/home-manager"

    declare -r globalNixStateDir="${NIX_STATE_DIR:-/nix/var/nix}"
    declare -r globalProfilesDir="$globalNixStateDir/profiles/per-user/$USER"

    if [[ -e $globalProfilesDir/home-manager ]]; then
        declare -r oldProfilesDir="$globalProfilesDir"
    elif [[ -e $hmStateDir/profiles/home-manager ]]; then
        declare -r oldProfilesDir="$hmStateDir/profiles"
    fi

    declare -r newProfilesDir="$userNixStateDir/profiles"

    if [[ -v oldProfilesDir && -e $newProfilesDir ]]; then
        if [[ ! -e $newProfilesDir/home-manager ]]; then
            _i 'Migrating profile from %s to %s' "$oldProfilesDir" "$newProfilesDir"
            for p in "$oldProfilesDir"/home-manager-*; do
                declare name="${p##*/}"
                nix-store --realise "$p" --add-root "$newProfilesDir/$name" > /dev/null
            done
            cp -P "$oldProfilesDir/home-manager" "$newProfilesDir"
        fi

        rm "$oldProfilesDir/home-manager" "$oldProfilesDir"/home-manager-*
    fi
}

function setupVars() {
    declare -r stateHome="${XDG_STATE_HOME:-$HOME/.local/state}"
    declare -r userNixStateDir="$stateHome/nix"
    declare -r hmGcrootsDir="$stateHome/home-manager/gcroots"

    declare -r globalNixStateDir="${NIX_STATE_DIR:-/nix/var/nix}"
    declare -r globalProfilesDir="$globalNixStateDir/profiles/per-user/$USER"
    declare -r globalGcrootsDir="$globalNixStateDir/gcroots/per-user/$USER"

    # If the user Nix profiles path exists, then place the HM profile there.
    # Otherwise, if the global Nix per-user state directory exists then use
    # that. If neither exists, then we give up.
    #
    # shellcheck disable=2174
    if [[ -d $userNixStateDir/profiles ]]; then
        declare -r profilesDir="$userNixStateDir/profiles"
    elif [[ -d $globalProfilesDir ]]; then
        declare -r profilesDir="$globalProfilesDir"
    else
        _iError 'Could not find suitable profile directory, tried %s and %s' \
                "$userNixStateDir/profiles" "$globalProfilesDir" >&2
        exit 1
    fi

    declare -gr genProfilePath="$profilesDir/home-manager"
    declare -gr newGenPath="@GENERATION_DIR@";
    declare -gr newGenGcPath="$hmGcrootsDir/current-home"
    declare -gr legacyGenGcPath="$globalGcrootsDir/current-home"

    declare greatestGenNum
    greatestGenNum=$( \
        nix-env --list-generations --profile "$genProfilePath" \
            | tail -1 \
            | sed -E 's/ *([[:digit:]]+) .*/\1/')

    if [[ -n $greatestGenNum ]] ; then
        declare -gr oldGenNum=$greatestGenNum
        declare -gr newGenNum=$((oldGenNum + 1))
    else
        declare -gr newGenNum=1
    fi

    if [[ -e $genProfilePath ]] ; then
        declare -g oldGenPath
        oldGenPath="$(readlink -e "$genProfilePath")"
    fi

    $VERBOSE_RUN _i "Sanity checking oldGenNum and oldGenPath"
    if [[ -v oldGenNum && ! -v oldGenPath
            || ! -v oldGenNum && -v oldGenPath ]]; then
        _i $'The previous generation number and path are in conflict! These\nmust be either both empty or both set but are now set to\n\n    \'%s\' and \'%s\'\n\nIf you don\'t mind losing previous profile generations then\nthe easiest solution is probably to run\n\n   rm %s/home-manager*\n   rm %s/current-home\n\nand trying home-manager switch again. Good luck!' \
           "${oldGenNum:-}" "${oldGenPath:-}" \
           "$profilesDir" "$hmGcrootsDir"
        exit 1
    fi
}

if [[ -v VERBOSE ]]; then
    export VERBOSE_ECHO=echo
    export VERBOSE_ARG="--verbose"
    export VERBOSE_RUN=""
else
    export VERBOSE_ECHO=true
    export VERBOSE_ARG=""
    export VERBOSE_RUN=true
fi

_i "Starting Home Manager activation"

# Verify that we can connect to the Nix store and/or daemon. This will
# also create the necessary directories in profiles and gcroots.
$VERBOSE_RUN _i "Sanity checking Nix"
nix-build --expr '{}' --no-out-link

# Also make sure that the Nix profiles path is created.
nix-env -q > /dev/null 2>&1 || true

migrateProfile
setupVars

if [[ -v DRY_RUN ]] ; then
    _i "This is a dry run"
    export DRY_RUN_CMD=echo
    export DRY_RUN_NULL=/dev/stdout
else
    $VERBOSE_RUN _i "This is a live run"
    export DRY_RUN_CMD=""
    export DRY_RUN_NULL=/dev/null
fi

if [[ -v VERBOSE ]]; then
    _i 'Using Nix version: %s' "$(nix-env --version)"
fi

$VERBOSE_RUN _i "Activation variables:"
if [[ -v oldGenNum ]] ; then
    $VERBOSE_ECHO "  oldGenNum=$oldGenNum"
    $VERBOSE_ECHO "  oldGenPath=$oldGenPath"
else
    $VERBOSE_ECHO "  oldGenNum undefined (first run?)"
    $VERBOSE_ECHO "  oldGenPath undefined (first run?)"
fi
$VERBOSE_ECHO "  newGenPath=$newGenPath"
$VERBOSE_ECHO "  newGenNum=$newGenNum"
$VERBOSE_ECHO "  genProfilePath=$genProfilePath"
$VERBOSE_ECHO "  newGenGcPath=$newGenGcPath"
$VERBOSE_ECHO "  legacyGenGcPath=$legacyGenGcPath"