This task is to determine if there is only one instance of an application running. If the program discovers that an instance of it is already running, then it should display a message indicating that it is already running and exit.

package require Tcl 8.6
try {
    # Pick a port number based on the name of the main script executing
    socket -server {apply {{chan args} {close $chan}}} -myaddr localhost \
            [expr {1024 + [zlib crc32 [file normalize $::argv0]] % 30000}]
} trap {POSIX EADDRINUSE} {} {
    # Generate a nice error message
    puts stderr "Application $::argv0 already running?"
    exit 1
}