One other way to make sure a single instance of bash script runs:

#!/bin/bash

# Check if another instance of script is running
pidof -o %PPID -x $0 >/dev/null && echo "ERROR: Script $0 already running" && exit 1

...

pidof -o %PPID -x $0 gets the PID of the existing script if its already running or exits with error code 1 if no other script is running

One other way to make sure a single instance of bash script runs:

#! /bin/bash -

# Check if another instance of script is running
if pidof -o %PPID -x -- "$0" >/dev/null; then
  printf >&2 '%s\n' "ERROR: Script $0 already running" 
  exit 1
fi

...

pidof -o %PPID -x -- "$0" gets the PID of the existing script¹ if it's already running or exits with error code 1 if no other script is running

^{¹ Well, any process with the same name...}

One other way to make sure a single instance of bash script runs:

#!/bin/bash

# Check if another instance of script is running
pidof -x $0 >/dev/null && echo "ERROR: Script $0 already running" && exit 1

...

pidof -x $0 gets the PID of the existing script if its already running

One other way to make sure a single instance of bash script runs:

#!/bin/bash

# Check if another instance of script is running
pidof -o %PPID -x $0 >/dev/null && echo "ERROR: Script $0 already running" && exit 1

...

pidof -o %PPID -x $0 gets the PID of the existing script if its already running or exits with error code 1 if no other script is running