How to Suspend the yes& Command

1. Overview

Managing background processes efficiently is important in Linux environments. When dealing with the yes& command, a process that continually outputs y, it’s important to know how to suspend and manage it effectively without causing terminal disruptions.

In this tutorial, we’ll learn how to suspend the yes& command using the TOSTOP setting. Additionally, we’ll discuss how to manage the suspended process and perform actions like bringing it to the foreground and killing the process.

2. The yes& Command

The yes command in Linux is a versatile utility designed to output a string repeatedly until it’s terminated:

$ yes
y
y
y
y
y
...

When paired with an ampersand (&), it runs as a background process, allowing it to operate independently of the foreground processes:

$ yes&
y
y
y
y
y
...

The command continuously sends the y string to the standard output, allowing other processes to read from it as needed. This makes it useful for automating tasks, testing software behavior under constant input, and creating sample logs or data.

When executed without modifications, the yes& command operates silently in the background. One of the challenges with background processes like yes& is their interaction with the terminal. Unlike when running yes in the foreground, we can’t terminate yes& by sending the SIGINT interrupt signal.

By default, the process continues to run uninterrupted, which can lead to the terminal appearing unresponsive. This behavior is due to the absence of the TOSTOP output mode, which would otherwise suspend background processes when they attempt to write to the terminal.

We can regain control of the terminal by foregrounding the process and then interrupting it. We won’t see the input, but we can type fg and hit Enter. Afterward, we can stop the process by sending a SIGINT signal with Ctrl+C.

In case we have multiple background processes, we can specify the job number when using fg:

$ fg %1

Notably, we won’t see this command in the terminal while typing it, but it should work.

3. The TOSTOP Output Mode

The TOSTOP output mode is an essential setting that determines the behavior of background processes when they interact with the terminal. By default, background processes can write to the terminal without any interruption, continuing their execution even if they’re running in the background. However, with the TOSTOP mode enabled, these background processes get suspended when they try to write output to the terminal.

Therefore, the TOSTOP output mode provides a mechanism to control and manage background processes by allowing them to be suspended when they try to write to the terminal. This feature enhances the overall flexibility and control we have over background tasks, ensuring smoother operation and better resource management.

3. Suspending yes& With TOSTOP

We’ve experienced how our terminal output got flooded while running yes&. Let’s try it again with the TOSTOP mode set:

$ stty tostop

This command enables TOSTOP mode in the terminal.

Next, let’s run the yes& command again:

$ yes&
[1] 12375

[1]  + suspended (tty output)  yes

The first line of output indicates that the yes process was started and assigned the PID 12375. The number 1 in square brackets denotes the job number.

In summary, the output shows that the yes command was started in the background but was then suspended because it attempted to output to the terminal.

4. Managing the Suspended Process

After suspending a process, such as the yes command, there are various actions we can take to manage it.

4.1. Resuming the Suspended Process

To resume a suspended process, we can bring it back to the foreground:

$ fg %1
[1]  + continued  yes
y
y
y
y
...

In this instance, the fg %1 command brings the suspended yes command back to the foreground, allowing it to continue its execution.

To identify the job number of a process when multiple background processes are running, we can utilize the jobs command:

$ jobs
[1]  + suspended (tty output)  yes

This command will display a list of background processes, each labeled with a job number along with its status – running, suspended, and so on.

Once we’ve identified the job number of the process we want to interact with, we can kill the process with Ctrl+C.

4.2. Killing the Suspended Process

If we’re certain we no longer need a suspended process, we can terminate it using the kill command:

$ kill %1
[1]  + terminated  yes

This command sends a termination signal to the suspended job with job number 1, causing it to stop execution immediately.

4.3. Bringing the Terminal Back to Normal

After managing the suspended process, we might want to bring the terminal back to its normal state. We can do this by resetting the terminal settings, if necessary, and ensuring that the prompt is responsive to further commands.

One way to reset the terminal settings is by using the stty command. For example, to reset all terminal settings to their default values, we can run the ssty sane command:

$ stty sane

This command restores the terminal settings to the default state, ensuring that the terminal behaves normally.

5. Considerations and Best Practices

When working with background processes and managing their suspension, there are several considerations and best practices to keep in mind:

• Resource Management: We should be mindful of system resources when running background processes. Running too many background processes concurrently may lead to resource exhaustion and degradation of system performance.
• Terminal Responsiveness: It’s important that the terminal remains responsive even when background processes are running. We’ve learned that enabling the TOSTOP output mode helps prevent terminal hangs by allowing background processes to be suspended when writing output to the terminal.
• Process Monitoring: Regularly monitoring running processes, especially background processes, helps us identify and troubleshoot any issues promptly. Tools like ps, top, or specialized process monitoring utilities can provide valuable insights into process activity and resource utilization.

Adhering to these practices should ensure the smooth running of our systems.

6. Conclusion

In this article, we explored the TOSTOP output mode, its role in process suspension, and practical methods for managing suspended processes. In particular, we learned how to suspend the yes& command using the TOSTOP setting and how to efficiently manage suspended processes.