rotate-logs.sh

#!/bin/bash
set -euo pipefail

# usage: myprogram.py | rotate-logs /tmp/myprogram.log [1000000]

file=$1
max_lines=${2:-1000000}

touch $file
line_count=$(cat $file | wc -l)

while read line; do
    if (($line_count > $max_lines * 2)); then
        mv $file $file.old
        tail -n $max_lines $file > $file
        rm $file.old
        line_count=$max_lines
    fi
    echo $line >> $file
    line_count=$(($line_count + 1))
done