Deep-dive into the simulation launcher

In this tutorial, we will dive into how a standalone script launches the simulator and configures it using CLI arguments and environment variables (envars). Particularly, we will demonstrate how to use app.add_launcher_args and app.launch_simulation to enable livestreaming and to configure the isaacsim.simulation_app.SimulationApp instance that runs Isaac Sim, while also allowing user-provided options.

Launching is split into two steps. First, app.add_launcher_args appends the launch-related options to the script’s own argparse.ArgumentParser. Then, the app.launch_simulation context manager takes the simulation configuration and the parsed arguments, and starts only the runtime that they need. The default PhysX physics backend, Kit-based RTX cameras, the Kit visualizer (--visualizer kit) and livestreaming all run inside Isaac Sim (Kit), so for these ~app.launch_simulation starts a ~isaacsim.simulation_app.SimulationApp. A kitless configuration, for example Newton physics without a Kit visualizer, starts no Isaac Sim at all. The runtime is closed automatically when the with block exits.

The ~isaacsim.simulation_app.SimulationApp has many extensions that must be loaded to enable different capabilities, and some of these extensions are order- and inter-dependent. Additionally, there are startup options such as headless which must be set at instantiation time, and which have an implied relationship with some extensions, e.g. the livestreaming extensions. The launcher handles these extensions and startup options in a portable manner across a variety of use cases. To achieve this, we offer CLI and envar flags which can be merged with user-defined CLI args, while passing forward arguments intended for ~isaacsim.simulation_app.SimulationApp.

The Code

The tutorial corresponds to the launch_app.py script in the scripts/tutorials/00_sim directory.

# Copyright (c) 2022-2026, The Isaac Lab Project Developers (https://github.com/isaac-sim/IsaacLab/blob/main/CONTRIBUTORS.md).
# All rights reserved.
#
# SPDX-License-Identifier: BSD-3-Clause
"""
This script demonstrates how to configure the simulator launch through command-line arguments.
~~~bash
# Usage
uv run python scripts/tutorials/00_sim/launch_app.py
~~~
"""
"""Parse the command-line arguments first."""
import argparse
from isaaclab.app import add_launcher_args, launch_simulation
# create argparser
parser = argparse.ArgumentParser(description="Tutorial on configuring the simulator launch.")
parser.add_argument("--size", type=float, default=1.0, help="Side-length of cuboid")
# SimulationApp arguments https://docs.omniverse.nvidia.com/py/isaacsim/source/isaacsim.simulation_app/docs/index.html?highlight=simulationapp#isaacsim.simulation_app.SimulationApp
parser.add_argument(
    "--width", type=int, default=1280, help="Width of the viewport and generated images. Defaults to 1280"
)
parser.add_argument(
    "--height", type=int, default=720, help="Height of the viewport and generated images. Defaults to 720"
)
# append simulation launcher cli args
add_launcher_args(parser)
# parse the arguments
args_cli = parser.parse_args()
"""Rest everything follows."""
import isaaclab.sim as sim_utils
def design_scene():
    """Designs the scene by spawning ground plane, light, objects and meshes from usd files."""
    # Ground-plane
    cfg_ground = sim_utils.GroundPlaneCfg()
    cfg_ground.func("/World/defaultGroundPlane", cfg_ground)
    # spawn distant light
    cfg_light_distant = sim_utils.DistantLightCfg(
        intensity=3000.0,
        color=(0.75, 0.75, 0.75),
    )
    cfg_light_distant.func("/World/lightDistant", cfg_light_distant, translation=(1, 0, 10))
    # spawn a cuboid
    cfg_cuboid = sim_utils.CuboidCfg(
        size=[args_cli.size] * 3,
        visual_material=sim_utils.PreviewSurfaceCfg(diffuse_color=(1.0, 1.0, 1.0)),
    )
    # Spawn cuboid, altering translation on the z-axis to scale to its size
    cfg_cuboid.func("/World/Object", cfg_cuboid, translation=(0.0, 0.0, args_cli.size / 2))
def main():
    """Main function."""
    # Configure the simulation
    sim_cfg = sim_utils.SimulationCfg(device=args_cli.device, dt=0.01)
    # Launch the simulator runtime that the configuration needs; the launcher arguments
    # (including --width and --height) configure it, and it is closed when the block exits
    with launch_simulation(sim_cfg, args_cli):
        # Initialize the simulation context
        sim = sim_utils.SimulationContext(sim_cfg)
        # Set main camera
        sim.set_camera_view([2.0, 0.0, 2.5], [-0.5, 0.0, 0.5])
        # Design scene by adding assets to it
        design_scene()
        # Play the simulator
        sim.reset()
        # Now we are ready!
        print("[INFO]: Setup complete...")
        # Simulate physics
        while sim.is_running():
            # perform step
            sim.step()
if __name__ == "__main__":
    # run the main function
    main()

The Code Explained

Adding arguments to the argparser

The launcher is designed to be compatible with custom CLI args that users need for their own scripts, while still providing a portable CLI interface.

In this tutorial, a standard argparse.ArgumentParser is instantiated and given the script-specific --size argument, as well as the arguments --height and --width. The latter are ingested by ~isaacsim.simulation_app.SimulationApp.

The argument --size is not used by the launcher, but merges seamlessly with the launcher interface. In-script arguments are merged with the launcher interface via the ~app.add_launcher_args function, which appends the launcher arguments to the given ~argparse.ArgumentParser. This can then be processed into an argparse.Namespace using the standard argparse.ArgumentParser.parse_args method.

import argparse
from isaaclab.app import add_launcher_args, launch_simulation
# create argparser
parser = argparse.ArgumentParser(description="Tutorial on configuring the simulator launch.")
parser.add_argument("--size", type=float, default=1.0, help="Side-length of cuboid")
# SimulationApp arguments https://docs.omniverse.nvidia.com/py/isaacsim/source/isaacsim.simulation_app/docs/index.html?highlight=simulationapp#isaacsim.simulation_app.SimulationApp
parser.add_argument(
    "--width", type=int, default=1280, help="Width of the viewport and generated images. Defaults to 1280"
)
parser.add_argument(
    "--height", type=int, default=720, help="Height of the viewport and generated images. Defaults to 720"
)
# append simulation launcher cli args
add_launcher_args(parser)
# parse the arguments
args_cli = parser.parse_args()

Launching the simulator

The parsed arguments are passed, together with the simulation configuration, to ~app.launch_simulation. The configuration tells it which runtime is needed (here, the default PhysX physics backend, which runs inside Isaac Sim), and the arguments configure that runtime. Everything that uses the simulator runs inside the with block.

# Configure the simulation
sim_cfg = sim_utils.SimulationCfg(device=args_cli.device, dt=0.01)
# Launch the simulator runtime that the configuration needs; the launcher arguments
# (including --width and --height) configure it, and it is closed when the block exits
with launch_simulation(sim_cfg, args_cli):
    # Initialize the simulation context
    sim = sim_utils.SimulationContext(sim_cfg)

Understanding the output of —help

While executing the script, we can pass the --help argument and see the combined outputs of the custom arguments and the launcher options (abbreviated below).

uv run python scripts/tutorials/00_sim/launch_app.py --help
usage: launch_app.py [-h] [--size SIZE] [--width WIDTH] [--height HEIGHT] [--livestream {0,1,2}] [--xr]
                     [--device DEVICE] [--visualizer VISUALIZER] [--verbose] [--info] [--experience EXPERIENCE]
                     ...
Tutorial on configuring the simulator launch.
options:
  -h, --help            show this help message and exit
  --size SIZE           Side-length of cuboid
  --width WIDTH         Width of the viewport and generated images. Defaults to 1280
  --height HEIGHT       Height of the viewport and generated images. Defaults to 720
launcher arguments:
  Arguments for the KitLauncher. For more details, please check the documentation.
  --livestream {0,1,2}  Force enable livestreaming. Mapping corresponds to that for the `LIVESTREAM` environment
                        variable.
  --xr                  Enable XR mode for VR/AR applications.
  --device DEVICE       The device to run the simulation on. Can be "cpu", "cuda", "cuda:N", where N is the device ID
  --visualizer VISUALIZER, --viz VISUALIZER
                        Visualizer backends to enable as CSV (e.g., kit,newton,rerun,viser).
  --verbose             Enable verbose-level log output from the SimulationApp.
  --info                Enable info-level log output from the SimulationApp.
  --experience EXPERIENCE
                        The experience file to load when launching the SimulationApp. If an empty string is provided,
                        the experience file is determined from the resolved visualizer and XR settings. If a relative
                        path is provided, it is resolved relative to the `apps` folder in Isaac Sim and Isaac Lab (in
                        that order).
  ...

This readout details the --size, --height, and --width arguments defined in the script directly, as well as the launcher arguments.

Script arguments whose name and type match an argument of ~isaacsim.simulation_app.SimulationApp, in this case --height and --width, are forwarded to the ~isaacsim.simulation_app.SimulationApp instance when ~app.launch_simulation starts Isaac Sim. Please refer to the specification for such arguments for more examples.

Using environment variables

As noted in the help message, launcher arguments such as --livestream have corresponding environment variables (envar) as well. These are detailed in isaaclab.app documentation. Providing any of these arguments through CLI is equivalent to running the script in a shell environment where the corresponding envar is set.

The support for launcher envars is simply a convenience to provide session-persistent configurations, and can be set in the user’s ${HOME}/.bashrc for persistent settings between sessions. In the case where these arguments are provided from the CLI, they will override their corresponding envar, as we will demonstrate later in this tutorial.

These arguments can be used with any script that starts the simulation using ~app.launch_simulation. Camera and offscreen rendering support is configured automatically.

The Code Execution

We will now run the example script:

LIVESTREAM=2 uv run python scripts/tutorials/00_sim/launch_app.py --size 0.5

This will spawn a 0.5m\ ³ volume cuboid in the simulation. No GUI will appear, equivalent to omitting --visualizer in this setup because headlessness is implied by our LIVESTREAM envar. If a visualization is desired, we could get one via Isaac’s WebRTC Livestreaming. Streaming is currently the only supported method of visualization from within the container. The process can be killed by pressing Ctrl+C in the launching terminal.

Now, let’s look at how the launcher handles conflicting commands:

LIVESTREAM=0 uv run python scripts/tutorials/00_sim/launch_app.py --size 0.5 --livestream 2

This will cause the same behavior as in the previous run, because although we have set LIVESTREAM=0 in our envars, CLI args such as --livestream take precedence in determining behavior. The process can be killed by pressing Ctrl+C in the launching terminal.

Finally, we will examine passing arguments to ~isaacsim.simulation_app.SimulationApp through ~app.launch_simulation:

LIVESTREAM=2 uv run python scripts/tutorials/00_sim/launch_app.py --size 0.5 --width 1920 --height 1080

This will cause the same behavior as before, but now the viewport will be rendered at 1920x1080p resolution. This can be useful when we want to gather high-resolution video, or we can specify a lower resolution if we want our simulation to be more performant. The process can be killed by pressing Ctrl+C in the launching terminal.

For more details on headless mode and launching visualizers, see /source/migration/migrating_to_isaaclab_3-0.