s6-svc Command Linux: Complete Guide to Managing s6 Services

The s6-svc command is a powerful utility in the s6 process supervision suite that allows system administrators to control and manage s6 services effectively. As part of the s6 init system, this command provides granular control over service states, making it essential for modern Linux system management.

What is s6-svc?

The s6-svc command is the primary interface for sending control signals to services managed by the s6 supervision system. It communicates with the s6-supervise process to change service states, restart services, and manage service dependencies. Unlike traditional init systems, s6 provides real-time service supervision with automatic restarts and dependency management.

Basic Syntax and Usage

The basic syntax of the s6-svc command follows this pattern:

s6-svc [options] servicedir

Where servicedir is the path to the service directory containing the service configuration and run scripts.

Common Options

Starting Services

To start a service using s6-svc, use the -u option:

$ s6-svc -u /var/service/nginx

This command instructs the s6-supervise process to start the nginx service. The service will be brought up according to its run script configuration.

Example Output

$ s6-svc -u /var/service/nginx
$ s6-svstat /var/service/nginx
up (pid 1234) 5 seconds

The service is now running with process ID 1234 and has been up for 5 seconds.

Stopping Services

To stop a running service, use the -d option:

$ s6-svc -d /var/service/nginx

This sends a termination signal to the service and prevents automatic restart by s6-supervise.

Verification

$ s6-svstat /var/service/nginx
down 10 seconds, normally up

The output confirms the service has been stopped and is in a “normally up” state, meaning it would restart automatically if the system were rebooted.

Restarting Services

The restart option -r provides a clean way to restart services:

$ s6-svc -r /var/service/apache2

This command stops the current process and immediately starts a new instance, useful for applying configuration changes.

Restart Process Flow

$ s6-svstat /var/service/apache2
up (pid 5678) 120 seconds
$ s6-svc -r /var/service/apache2
$ s6-svstat /var/service/apache2
up (pid 9012) 2 seconds

Notice how the process ID changed from 5678 to 9012, indicating a complete restart.

Sending Signals to Services

s6-svc allows sending various signals to services for different purposes:

Graceful Reload with SIGHUP

$ s6-svc -h /var/service/nginx

Many services interpret SIGHUP as a signal to reload configuration without stopping:

$ s6-svc -h /var/service/nginx
$ tail -f /var/log/nginx/error.log
2025/08/26 01:55:12 [notice] 1234#0: signal 1 (SIGHUP) received, reconfiguring
2025/08/26 01:55:12 [notice] 1234#0: configuration file test successful

Graceful Shutdown with SIGTERM

$ s6-svc -t /var/service/database

SIGTERM allows processes to perform cleanup operations before terminating.

Advanced Service Control

Killing Unresponsive Services

When a service doesn’t respond to normal termination signals, use the kill option:

$ s6-svc -k /var/service/stuck-process

This sends SIGKILL, forcibly terminating the process:

$ s6-svstat /var/service/stuck-process
up (pid 3456) 300 seconds
$ s6-svc -k /var/service/stuck-process
$ s6-svstat /var/service/stuck-process
up (pid 7890) 1 second

Process Suspension and Continuation

Temporarily suspend a service:

$ s6-svc -p /var/service/cpu-intensive

Resume the suspended service:

$ s6-svc -c /var/service/cpu-intensive

Managing Multiple Services

s6-svc can operate on multiple services simultaneously:

$ s6-svc -u /var/service/nginx /var/service/mysql /var/service/redis

This starts all three services with a single command. You can verify the status:

$ s6-svstat /var/service/nginx /var/service/mysql /var/service/redis
/var/service/nginx: up (pid 1111) 15 seconds
/var/service/mysql: up (pid 2222) 15 seconds
/var/service/redis: up (pid 3333) 15 seconds

Service Directory Structure

Understanding the s6 service directory structure is crucial for effective service management:

/var/service/myapp/
├── run                 # Main service script
├── finish             # Cleanup script (optional)
├── down              # Prevents auto-start if present
└── supervise/        # s6-supervise working directory
    ├── control       # Control FIFO
    ├── lock          # Supervisor lock file
    ├── pid           # Process ID file
    └── status        # Service status file

Practical Examples

Web Server Management

Managing a web server cluster:

# Start web servers
$ s6-svc -u /var/service/nginx /var/service/apache2

# Reload configurations
$ s6-svc -h /var/service/nginx /var/service/apache2

# Check status
$ s6-svstat /var/service/nginx /var/service/apache2
/var/service/nginx: up (pid 4444) 60 seconds
/var/service/apache2: up (pid 5555) 45 seconds

Database Service Control

Controlled database shutdown and startup:

# Graceful shutdown
$ s6-svc -t /var/service/postgresql
$ sleep 5
$ s6-svstat /var/service/postgresql
down 3 seconds, normally up

# Restart database
$ s6-svc -u /var/service/postgresql
$ s6-svstat /var/service/postgresql
up (pid 6666) 2 seconds

Error Handling and Troubleshooting

Common Issues

When s6-svc fails to control a service, check these common issues:

$ s6-svc -u /var/service/broken-service
s6-svc: fatal: unable to control /var/service/broken-service: supervisor not running

This error indicates the s6-supervise process isn’t running. Start supervision:

$ s6-supervise /var/service/broken-service &

Permission Issues

$ s6-svc -u /var/service/restricted-service
s6-svc: fatal: unable to control /var/service/restricted-service: access denied

Ensure proper permissions on the service directory and run as appropriate user.

Integration with System Management

Service Dependencies

Create startup scripts that handle dependencies:

#!/bin/bash
# Start database first
s6-svc -u /var/service/mysql
sleep 2

# Then start application
s6-svc -u /var/service/webapp

# Finally start reverse proxy
s6-svc -u /var/service/nginx

Monitoring Integration

Combine s6-svc with monitoring scripts:

#!/bin/bash
# Health check and restart if needed
if ! s6-svstat /var/service/critical-app | grep -q "up"; then
    echo "Service down, restarting..."
    s6-svc -u /var/service/critical-app
fi

Best Practices

• Use appropriate signals: Always try SIGTERM before SIGKILL
• Monitor service states: Regularly check service status with s6-svstat
• Handle dependencies: Start services in correct order
• Log operations: Keep records of service management actions
• Test thoroughly: Verify service behavior after control operations

Comparison with Other Init Systems

Conclusion

The s6-svc command provides comprehensive control over s6-supervised services with its rich set of options and reliable operation. Its ability to send precise signals, manage multiple services, and integrate with system scripts makes it an essential tool for system administrators using the s6 supervision suite. By mastering s6-svc, you gain fine-grained control over service management in modern Linux environments.

Regular practice with s6-svc commands, combined with understanding of service dependencies and proper signal handling, will enable you to build robust and maintainable service management workflows in your Linux systems.