Having spent many hours following the white rabbit into various alternate universes, I found the following to be the only reliable method is:

# If called a second time, kill the first version already running
kill $(pgrep -f "${0##*/}" | grep -v ^$$)

If you are interested in testing this see Lock Screen Timer code in Ask Ubuntu at: (Application that will lock screen after a set amount of time for Ubuntu)

Production version code snippet (TL;DR)

The pertinent code snippet from the lock-screen-timer program is this:

# Check if lock screen timer already running
pID=$(pgrep -f "${0##*/}") # All PIDs matching lock-screen-timer name
PREVIOUS=$(echo "$pID" | grep -v ^"$$") # Strip out this running copy ($$$)
if [ $PREVIOUS != "" ]; then
    zenity --info --title="Lock screen timer already running" --text="Previous lock screen timer has been terminated."
    kill "$PREVIOUS"
fi

pgrep -f "${0##*/}"

This finds all occurrences of the same named running program ID ${0##*/}. Although the executable file is called ~/bin/lock-screen-timer a desktop shortcut is used to call it. That can be named “Lock Screen Timer” or “Lock screen timer” or “Remind me of laundry cycle”. It can’t be hard-coded into the program as in the original question.

The resulting list of process id’s are put in the variable $pID

echo "$pID" | grep -v ^"$$"

This takes the contents of $pID (all running occurrences of lock-screen-timer, or it’s renamed desktop short cut) and pipes the list of process id’s into the next command using the pipe (|) character.

The next command grep -v removes process ID’s that match $$ which is the current running process ID. The carrot (^) tells grep to match the whole word not the character string. For example, the current process id may be 1518 and the previous version may be 11518, 21518 or 31518. In that case just matching on 4 digits the process id makes those 3 matches because 1518 is within 11518. The carrot matches on words so 1518 <> 11518. In the process id list the words are separated by spaces (in a variable) or new line characters (when ps -aux command displays them on screen).

The result of these two commands is the process ID of the previously running lock-screen-timer script. The process ID is put into the variable $PREVIOUS. If there wasn’t a previous ID the value will be “” (null field).

if [ $PREVIOUS != "" ]; then

This tests if $PREVIOUS is not equal to (!=) a null / empty field "". Obviously we can only kill a previously running process ID if we have one!

zenity --info --title="Lock screen timer already running" ...

When running a desktop shortcut you can’t echo messages to the user because the GUI won’t display them. They end up in /var/log/syslog and you have to display them with cat or gedit, etc.

zenity is a nice little program to display dialog boxes and forms from bash to the GUI (Graphical User Interface), aka Desktop. The message text goes on to say Previous lock screen timer has been terminated.. This allows the user to start a new timer countdown or simply cancel. In essence calling the script a second time and aborting is how you can kill the first script already running.

kill "$PREVIOUS"

This simply kills the previously running version which we want to do whether we start a new lock-screen-timer countdown or not. This is substantially different from the original question because we’ve put the results of two cryptic commands into the single variable called $PREVIOUS.