crts already hit the nail on the head! From your output the variable file_mod_time_secs
contains two lines because most likely after midnight a new log file is created and the variable log_file
stores two file names. Is it assigned as the output of a ls command? If this is the case you should probably sort the log files by modification time and retrieve only the last one, e.g.
log_file=$(ls -rt *.log | tail -1)
after that the stat command in
file_mod_time_secs=$(stat -c %Y $log_file)
will bring to a single value.