Understanding UNIX Job Control: A Comprehensive Guide

Table of Contents

• Introduction
• Background
• Signals
• Foreground and Background Jobs
• Job Control Commands
• Example
• Conclusion

Introduction

Job control is an essential aspect of UNIX operating systems. It enables users to manage and manipulate processes and jobs that are running on a system. Job control is especially useful for tasks that require multiple processes or for running processes in the background while performing other tasks.

Understanding job control can be challenging, but it is a critical skill for any UNIX user. In this article, we will explore the basics of job control and provide examples of how to manage processes and jobs on a UNIX system.Background

Before we dive into job control, it is essential to understand some background concepts.

Every UNIX process has a unique process identifier (PID). The shell is a process that runs on every UNIX system and has a PID. When a user runs a command on the shell, the shell creates a new process to execute that command. This new process inherits the PID of the shell and becomes a child process of the shell.

UNIX provides a set of signals that can be sent to processes. Signals are used to notify a process of various events, such as a request to terminate or a request to stop.

Signals

Signals are a core part of job control. There are numerous signals that can be sent to a process, but some are more commonly used than others. Here are some of the most commonly used signals:

• SIGINT – Interrupt signal. Sent when the user types Ctrl+C. This signal requests that the process terminate.
• SIGTERM – Termination signal. Sent when the system needs to terminate a process.
• SIGSTOP – Stop signal. Sent when the user types Ctrl+Z. This signal requests that the process stop.
• SIGCONT – Continue signal. Sent to a stopped process to resume execution.

Foreground and Background Jobs

UNIX provides two types of jobs: foreground and background jobs. A foreground job is a job that runs in the foreground and blocks the shell until it completes. A background job is a job that runs in the background and does not block the shell.

By default, all jobs run in the foreground. To run a job in the background, use the bg command followed by the command you want to run. For example:

$ bg sleep 10

This command runs the sleep command in the background for 10 seconds. The shell immediately returns to the prompt, and the sleep command runs in the background.

To bring a background job to the foreground, use the fg command followed by the job number. For example:

$ fg %1

This command brings the first background job to the foreground. The shell waits for the job to complete before returning to the prompt.

Job Control Commands

UNIX provides several commands for managing jobs and processes. Here are some of the most commonly used commands:

• jobs – Lists all jobs running in the current shell session.
• fg – Brings a job to the foreground.
• bg – Runs a job in the background.
• kill – Sends a signal to a process.
• ps – Lists all processes running on the system.

Example

Let’s walk through an example of how to use job control.

First, open a terminal and run the sleep command in the foreground:

$ sleep 10

This command runs the sleep command in the foreground for 10 seconds. The shell waits for the command to complete before returning to the prompt.

While the sleep command is running, press Ctrl+Z to stop the command:

^Z

The shell prints a message indicating that the process has been stopped:

[1]+  Stopped	sleep 10

To run the sleep command in the background, use the bg command:

$ bg %1

This command runs the sleep command in the background. The shell immediately returns to the prompt.

To list all jobs running in the current shell session, use the jobs command:

$ jobs

The shell prints a message indicating that the sleep command is running in the background:

[1]+  Running	sleep 10 &

To bring the sleep command back to the foreground, use the fg command:

$ fg %1

This command brings the sleep command back to the foreground. The shell waits for the command to complete before returning to the prompt.

Conclusion

Job control is an essential aspect of UNIX operating systems. Understanding job control and the commands that can be used to manage processes and jobs is critical for any UNIX user.

In this article, we explored the basics of job control, including foreground and background jobs, signals, and job control commands.