Why does pgrep return randomly a wrong pid?

In the past I used pid-files to guarantee race-condition safe execution of scripts. But this had the downside, that the pid-file was not deleted if the kernel killed the script somehow. So sometimes manual interaction was needed (remove the pid file, killing some sub processes). Finally I came up to a solution which checks if a script is running by using pgrep and kills really old processes that shouldn't exist anymore: while read -r pid cmd; do # check if pid or parent pid belong to current script execution if [[ $pid == "$$" ]] || [[ $(ps -o ppid= -p "$pid" | xargs) == "$$" ]]; then continue fi # avoid re-execution of script within 12 hours (43200 seconds) if [[ $(date +%s --d="now - $( stat -c%X "/proc/$pid" ) seconds") -lt 43200 ]]; then echo "Error: Script is already running!" exit fi # kill outdated script executions if kill -9 "$pid"; then echo "Warning: Outdated script execution ($pid) has been killed!" fi done /dev/null; then continue fi # verify that pid is part of the list ("pstree --show-parents" returns list, even pid does not exist https://github.com/acg/psmisc/issues/5) if ! echo "$pid_list" | grep -F "($pid)" >/dev/null; then continue # process does not exist anymore fi # obtain age of pid pid_time=$( stat -c%X "/proc/$pid" 2>/dev/null ) if [[ ! $pid_time ]]; then continue # process does not exist anymore fi # kill outdated script executions (older than 12 hours / 43200 seconds) if [[ $(date +%s --d="now - $pid_time seconds") -gt 43200 ]]; then if kill -9 "$pid"; then echo "Warning: Outdated script execution ($pid) has been killed!" fi # we are facing a race condition else echo "Error: Script is already running!" exit 1 fi done < <(pgrep -f "^/bin/bash $script_path") # obtain pids that belong to this script