CODEWITHSUNDEEP
bashhardware
Danny
Danny

Reputation: 158

Killing script with hardware control

I have a script that controls an array of 12v relays depending on certain paramters. For an example: I am monitoring temperatures and pressures. If a temperature exceeds a certain value a relay will be pulled in to open vents and start fans. If the temperature drops to a certain value the relays are released and the vents will close and the fans will stop. Same with the pressures, which will open a solenoid valve and close it again depending on the pressure values.

All works fine and I am happy. The script (bash) is started at boot-up. However, sometimes the script dies mysteriously which leaves the relays in an "active" state.

Is there a way to ensure to reset the relays to "not-active" or "unenergized" when the script dies?

Upvotes: 0

Views: 24

Answers (1)

David C. Rankin
David C. Rankin

Reputation: 84662

Continuing from my comment, you can trap any of the signals your script can receive (except SIGKILL and SIGSTOP) that are shutting it down and use trap to intercept the signal received and run the required commands to reset the relays to "not-active" or "unenergized" state before the process dies.

Using trap is quite easy. You simply set a trap at the top of your script listing the commands to be executed when a signal is caught. For simply commands you can do

trap 'command1; command2` SIGTERM SIGINT EXIT

to run command1 and command2 on receipt of any of the three listed signals. If you have a series of commands you need to execute, declare a function and then have trap execute the function on signal receipt, e.g.

cleanup () {
    # any number of commands to run
}

trap cleanup SIGTERM SIGINT EXIT

See man 7 signals for addition information on standard signals. Consult man bash (or search on "using trap in bash") for additional information on trap.

Upvotes: 2

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