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Prism Launcher allows you to execute custom commands at different stages of instance launch and exit. These commands provide powerful automation and customization capabilities.

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Command Types

There are three types of custom commands available:

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Pre-Launch Command

Runs before the Minecraft instance starts. Useful for:
  • Setting up required services or configurations
  • Copying files or resources
  • Running initialization scripts
  • Validating system requirements

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Wrapper Command

Wraps the entire Minecraft launch process. The wrapper command receives the Minecraft command as arguments. Common uses:
  • Performance tools (e.g., gamemoderun, mangohud)
  • GPU switching utilities (e.g., optirun, primusrun)
  • Debugging tools (e.g., gdb, strace)
  • Custom launchers or environment managers

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Post-Exit Command

Runs after the Minecraft instance exits. Ideal for:
  • Cleanup operations
  • Backing up world saves
  • Resetting system configurations
  • Logging session information

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Available Environment Variables

All custom commands are executed with special environment variables that provide context about the instance:
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$INST_NAME
string
The display name of the instance
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$INST_ID
string
The unique ID of the instance (typically the folder name)
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$INST_DIR
string
Absolute path to the instance root directory
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$INST_MC_DIR
string
Absolute path to the Minecraft game directory
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$INST_JAVA
string
Path to the Java binary used for launch
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$INST_JAVA_ARGS
string
Command-line parameters used for launch
This variable will not work correctly if arguments contain spaces. Use with caution.

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Configuration

Custom commands can be configured at two levels:

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Global Settings

Applies to all instances by default:
  1. Navigate to Settings → Minecraft → Custom Commands
  2. Enter your commands in the respective fields

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Per-Instance Settings

Override global settings for specific instances:
  1. Right-click an instance → Edit Instance → Settings
  2. Navigate to Custom Commands
  3. Check Override Global Settings
  4. Enter instance-specific commands

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Examples

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Pre-Launch Command Examples

#!/bin/bash
"$INST_JAVA" -jar packwiz-installer-bootstrap.jar "$INST_MC_DIR/pack.toml"

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Wrapper Command Examples

gamemoderun

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Post-Exit Command Examples

#!/bin/bash
cp -r "$INST_MC_DIR/saves" "$HOME/backups/$INST_ID-$(date +%Y%m%d)"

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Advanced Usage

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Using Scripts

For complex operations, use a script file: 
#!/bin/bash
# Save as: pre-launch.sh

INSTANCE_DIR="$INST_MC_DIR"
MOD_DIR="$INSTANCE_DIR/mods"

# Update mods from packwiz
if [ -f "$INSTANCE_DIR/pack.toml" ]; then
    "$INST_JAVA" -jar packwiz-installer-bootstrap.jar "$INSTANCE_DIR/pack.toml"
fi

# Check for shader updates
if [ -d "$INSTANCE_DIR/shaderpacks" ]; then
    echo "Shader packs directory found"
fi

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Conditional Execution

#!/bin/bash
if [ -f "$INST_MC_DIR/packwiz.jar" ]; then
    "$INST_JAVA" -jar "$INST_MC_DIR/packwiz.jar" refresh
fi

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Working Directory

All custom commands are executed in the launcher’s working directory, not the instance directory. Use the environment variables (e.g., $INST_DIR, $INST_MC_DIR) to reference instance-specific paths.

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Troubleshooting

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Command Not Executing

  1. Check permissions: Ensure scripts have execute permissions (chmod +x script.sh on Linux)
  2. Use absolute paths: Always use full paths to executables and scripts
  3. Test independently: Run the command manually to verify it works
  4. Check logs: Review the launcher console for error messages

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Arguments With Spaces

The $INST_JAVA_ARGS variable does not handle arguments containing spaces correctly. For complex argument processing, parse them manually in your script.

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Script Encoding Issues

On Windows, ensure batch files use CRLF line endings. On Linux/macOS, use LF line endings.

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Security Considerations

  • Be cautious when downloading and running commands from untrusted sources
  • Review scripts before execution to prevent malicious code
  • Avoid storing sensitive information in command strings
  • Use proper quoting to prevent command injection