Determine if only one instance is running: Difference between revisions

=={{header|Wren}}==

This works by trying to create a special file with exclusive access.

If this fails then the error is captured, a suitable message is printed and the script exits.

Otherwise, the script completes normally and the special file is deleted just before it exits.

<lang ecmascript>import "io" for File, FileFlags

var specialFile = "wren-exclusive._sp"

var checkOneInstanceRunning = Fn.new {

// attempt to create the special file with exclusive access

var ff = FileFlags.create | FileFlags.exclusive

File.openWithFlags(specialFile, ff) { |file| } // closes automatically if successful

}

// check if the current instance is the only one running

var fiber = Fiber.new {

checkOneInstanceRunning.call()

}

var error = fiber.try()

if (error) {

System.print("An instance is already running.")

return

}

// do something that takes a while for testing purposes

var sum = 0

for (i in 1...1e8) {

sum = sum + i

}

System.print(sum)

File.delete(specialFile) // clean up</lang>

{{out}}

<pre>

(in window 1)

$ wren one_instance_running.wren

4.99999995e+15

$

(in window2, started before the first instance completes)

$ wren one_instance_running.wren

An instance is already running.

$

</pre>