Backup / log "rotation" keeping more more recent files

bkup.sh

TARGET=trilium-data
BKUP_DIR=/home/tbrown02/data/trilium-backup
mkdir -p $BKUP_DIR/cull
OUTFILE=$(date '+%Y-%m-%d-%H-%M-trilium_bkup.tar.xz')

cd ~
nice tar caf $BKUP_DIR/$OUTFILE $TARGET

# After https://unix.stackexchange.com/a/669081/319850
# but using sort to handle multiple bkups per day.

all=$(ls $BKUP_DIR/*.xz)
ten_daily=$(ls $BKUP_DIR/*.xz | sort -r | sort -u -k1,3 -t- | tail -10)
twelve_months=$(ls $BKUP_DIR/*.xz | sort -r | sort -u -k1,2 -t- | tail -12)
yearly=$(ls $BKUP_DIR/*.xz | sort -r | sort -u -k1,1 -t-)
five_days=$(ls $BKUP_DIR/*.xz | sort | tail -$[5*3])
keep=" $ten_daily $twelve_months $yearly $five_days "

for item in $all
do
    if ! [[ "$keep" =~ "$item" ]]
    then
        mv $item $BKUP_DIR/cull
    fi
done

# Demo
# Most recent from all years
# for BKUP in $(seq 0 1000); do date -d "+$[BKUP*96] hours" '+%Y-%m-%d-%H-%M-trilium_bkup.tar.xz'; done | sort -r | sort -u -k1,1 -t-
# Most recent from each of last 12 months
# for BKUP in $(seq 0 1000); do date -d "+$[BKUP*12] hours" '+%Y-%m-%d-%H-%M-trilium_bkup.tar.xz'; done | sort -r | sort -u -k1,2 -t- | tail -12
# Most recent from each of last 10 days
# for BKUP in $(seq 0 1000); do date -d "+$[BKUP*12] hours" '+%Y-%m-%d-%H-%M-trilium_bkup.tar.xz'; done | sort -r | sort -u -k1,3 -t- | tail -10
# All for last 5 days at three bkups per day
# NOTE if you skip days running the backup, e.g. cron job doesn't run Sat/Sun,
# this may include files from more than '5' days ago, but they will be culled eventually.
# for BKUP in $(seq 0 1000); do date -d "+$[BKUP*8] hours" '+%Y-%m-%d-%H-%M-trilium_bkup.tar.xz'; done | sort | tail -$[5*3]