Background commands

Running commands in the background

Normally, commands in a payload are run until they complete, then the next command runs. This blocks the rest of the payload from executing while a long-running command runs.

Often this is desirable: This functionality is what allows a payload to pause until a button is pressed, for instance.

At other times, a payload may need a command to run in the background; to run multiple KILLSTREAM commands with different patterns, monitor status of a command, or for other reasons.

To solve this, commands can be run concurrently, called "backgrounding". Any number of commands or functions can be run in the background at once; when backgrounded, the payload immediately executes the next command.

Appending an & to the end of a command tells Bash to run this command in the background.

To run multiple commands, simply run all of them with an & after each:

#!/bin/bash 

# Title: Killstream multi example
#
# Description: Kill multiple types of streams

NETMODE BRIDGE
LED R SINGLE


KILLSTREAM br-lan ANY 'Authorization: Basic' 80 &
KILLSTREAM br-lan ANY 'Foobar' 80 &
KILLSTREAM br-lan ANY '[0-9}{3}-[0-9]{2}-[0-9]{4}' 80 &

wait

All the commands will be run, regardless if the commands are successful.

Waiting for commands

In the example above, the wait command is used. This keeps the payload running until all backgrounded commands have finished.

Without the wait command, the payload above would run the KILLSTREAM command, but then immediately complete: Backgrounded commands will not keep a payload running!

Using the wait command tells the payload to continue running. In this case, it will run indefinitely, as the KILLSTREAM command runs forever.

Running groups of commands in the background

A group of commands can be made using the { and } symbols, and run in the background.

This can easily be combined with the while true loop to run forever, and something like the BUTTON command to wait for user input, for example:

#!/bin/bash

# Title: Button blinky
# 
# Description: Blink the LED whenever the button is pressed

NETMODE NAT

# Group the commands inside {} and run them in the background with &
{
    # Repeat forever
    while true; do
        # Wait for the button
        BUTTON
        
        # Set the LED to blue
        LED B SOLID
        
        # Wait a second
        sleep 1
    done
} &

# Do whatever else in the payload here

wait