Tuesday, September 15, 2015

Sleeping while you work

Sleeping: restful, relaxing, restorative. But sleeping in a computer script is pausing the script for a specified amount of time. As of version 4.9.0 a native sleep command, using a colon, has been added.
Counting the seconds

"Why wait until expect-lite has been around for 10 years before adding sleep?" you may ask. Because I have seen sleep abused in other scripting languages, usually a scripter will add a 60 to 600 second sleep rather than check for the event with a polling loop.

Polling with a sleep

But polling loops are an excellent example of where to use a sleep. It is usually unnecessary to check for a state change (ethernet interface up, for example) on a mili-second time basis. More likely if the interface comes up in a couple of seconds, that is good enough. The sleep will slow down the polling loop so the loop does not put an undo load on the machine.

# using a polling loop to check when eth0 is up
[ $int_state != UP
    ip link show dev $intf
    # sleep 2 seconds to slow down loop

Wait a sec :01

The colon ":" indicates a sleep. In the above example :2 is used to sleep (or pause) the script for 2 seconds. Sleep is always in seconds, but mili-seconds are also supported such as 5 mili-seconds:

The native sleep (using the colon) also gives indication that the script is sleeping. There is nothing more frustrating that debugging a script and wondering is it hung or is it sleeping. expect-lite will print dots to show the progress of the sleep. A 12 second sleep would output:
Sleeping: 12 

Each dot represents a second, with the plus every 5 seconds, and a number every 10 seconds. This output will go to stdout, and also be logged to a file with the *LOG command so that it can be observed later. The output can be disabled with the *NOINFO command if you want less clutter.

Transparency in sleeping, making scripting easier

A goal of expect-lite is transparency, showing you what it is doing, to help you debug script errors, or determine actual problems with the device you are testing. And now, you can sleep on it.