Linux tutorial: Starting an audio program after boot
Turn any computer or Raspberry Pi into a dedicated audio device, music instrument or installation, with everything audio related automatically and correctly started every time the device turns on.
Introduction
Basing a music instrument or audio system on a Linux system or Raspberry Pi is a great idea, but getting it to start up properly with all audio systems and programs in the correct order can be a hassle. That said, it IS possible, and in this tutorial we show you how.
The following will show you how to set up your system so that JACK is automatically started as an audio server after the computer boots. Then, when JACK is ready to make sound via your sound interface, it runs a script that starts any audio software of your choice. An example is shown of how to do this with SuperCollider but it could be any software really.
It’s really just a bit of Linux plumbing that needs to happen when the system boots:
- Wait for all audio drivers and audio components of the boot process to finish (done using
systemd) - Then and only then start up JACK as your audio server
- Wait for JACK to be ready
- Once JACK is ready and started, run your audio software
It is crucial that these steps happen in this order – and thankfully we can do all of this automatically with a bit of setup using systemd – the program responsible for managing the bootup process and services in your Linux system.
All codeblocks below are shell commands meant to be run in a terminal on your Linux system.
Setting up user and group
I like to set up an audio group for audio software and make sure my user is in this group. We will rely on this later as well.
To do this, run the following commands while logged in as your user.
groupadd audio
usermod -a -G audio $USER
Then REBOOT your system to make sure the changes take effect, before proceeding.
Setting up Jack
Okay, now it’s time to set up jack. It’s an audio server for professional audio purposes.
Installation
We need to install both jack2 and the example tools (which contains jack_wait – needed in our systemd-service below):
Debian-based systems
sudo apt-get install jackd2 jack-example-tools
Arch based systems
Using pacman:
pacman -S jack2 jack-example-tools
Configuration file
We need a configuration file for Jack. This is where you set sample rate and other things.
If you already have a .jackdrc in your home folder, skip this part.
echo "/usr/bin/jackd -P95 -dalsa -dhw:0 -r44100 -n3 -p512" > /home/$USER/.jackdrc
Plugging into systemd
Start JACK when audio system is ready
To automatically boot JACK with the configurations file we set up earlier, we need to create a systemd-service.
This will be triggered when all audio in your system is ready.
This will wait for the sound target to be reached, then start JACK.
It will also wait for JACK for 30 seconds to be ready before continuing. If it fails, it will report an error.
echo "
# A systemd service file that will be responsible for starting jack once audio is initialized
[Unit]
After=sound.target
Description=Starts jack when audio is ready on the system
[Service]
Type=simple
User=$USER
Group=audio
Environment="JACK_NO_AUDIO_RESERVATION=1"
LimitRTPRIO=99
LimitMEMLOCK=infinity
ExecStart=/usr/bin/zsh /home/$USER/.jackdrc
ExecStartPost=/usr/bin/jack_wait -w -t 10
Restart=on-failure
TimeoutSec=30
[Install]
WantedBy=multi-user.target
" > /etc/systemd/system/start-jack.service
sudo systemctl enable start-jack.service --now
Check it worked:
systemctl status start-jack.service
Run script when JACK is ready
Now we need to create a service that will run a script when JACK is ready.
It will simply run a script called post-jack.sh in your home folder.
echo "
[Unit]
Description=After the jack audio system has started, this will be launched
After=start-jack.service
[Service]
Type=simple
User=$USER
Group=audio
CapabilityBoundingSet=CAP_SYS_NICE
AmbientCapabilities=CAP_SYS_NICE
ExecStart=/usr/bin/bash /home/$USER/post-jack.sh
[Install]
WantedBy=multi-user.target
" > /etc/systemd/system/after-jack.service
Check it worked:
systemctl status after-jack.service
The script: Actually starting your audio software
Okay, phew, we are now ready to start a piece of audio software.
Let’s create a script which will be run when the audio system has started, and JACK has been booted up and is ready to make sound.
Now, this script could be any audio software you want. Popular choices are Pure Data or SuperCollider.
I won’t go over how to install any of them here but here is an example of a script that starts SuperCollider in a headless state, running a file called startup.scd.
What is important here is that this startup script is called post-jack.sh and is placed in the home folder of your user because this is what systemd relies on.
echo "
#!/bin/bash
export QT_QPA_PLATFORM=offscreen
sclang startup.scd
" > /home/$USER/post-jack.sh