How to Use the Command 'setsid' (with Examples)

The setsid command is a Unix utility used to run a program in a new session. This command detaches processes from the terminal, allowing the program to run independently from the terminal that started it. When effectively utilized, setsid can be instrumental in various system administration tasks, such as when creating daemons or running background processes.

By default, the created session is not controlled by the current terminal, ensuring that processes can continue to run even if the original terminal session is closed. Below are several practical use cases illustrating how setsid can be employed efficiently.

Run a Program in a New Session

Code:

setsid program

Motivation:

Using setsid to run a program in a new session is beneficial when you need the program to operate independently of your terminal. For instance, you might be starting a long-running server or a background script that must keep running even after logging out of an SSH session. Detaching such a program from your terminal session is essential to ensure it continues to execute in the background.

Explanation:

• setsid: The command that initiates a new session and makes the following program its leader. By detaching the process from the terminal, it ensures the program operates independently of terminal inputs or interruptions.
• program: Replace this with the actual command or script you wish to execute in the new session. This program will continue to run without terminal dependency.

Example Output:

When executed, setsid program will not provide direct output to the terminal, as it detaches the process. However, you can confirm that your program is running independently by using process monitoring commands like ps.

Run a Program in a New Session Discarding the Resulting Output and Error

Code:

setsid program > /dev/null 2>&1

Motivation:

Sometimes, you run processes that generate a lot of console output, which may not be necessary for post-processing or reviewing. In such cases, discarding the output ensures cleaner terminal access and reduces unnecessary logging. This approach is often adopted for running maintenance scripts, where the output does not need to be tracked.

Explanation:

• program: The script or command to be executed in a new session.
• > /dev/null: Redirects the standard output of the program to /dev/null, effectively discarding it.
• 2>&1: Redirects the standard error to standard output, which in turn is discarded by going into /dev/null. This ensures both error and standard output streams are ignored.

Example Output:

Running this command will generate no output on the terminal, as both standard and error outputs are directed to /dev/null. It’s effectively a “silent” run.

Run a Program Creating a New Process

Code:

setsid --fork program

Motivation:

When the goal is to ensure even greater separation from the calling process, creating a new process is a common approach. This use case is crucial in scenarios where complete detachment is required, such as when the parent process must terminate, and the child process (program) should continue to run.

Explanation:

• --fork: This option tells setsid to fork the process after creating a new session. Forking creates a new process, ensuring it’s even more detached from the parent process.
• program: The command or script to execute within the newly forked process.

Example Output:

Much like the first example, the result is not apparent directly on the terminal. To confirm the process is running as a new entity, process management tools like ps or top can be used.

Return the Exit Code of a Program as the Exit Code of setsid

Code:

setsid --wait program

Motivation:

This use case is beneficial when you need to monitor or act upon the exit code of a detached service. Running the program with --wait ensures that setsid exits with the same status as the invoked program, making it easier to integrate into scripts to handle success or errors explicitly.

Explanation:

• --wait: Instructs setsid to wait for the program to terminate and return its exit code. This is essential for cases where subsequent operations are conditional on the program’s success or failure.
• program: The designated application or script whose exit code needs to be monitored by setsid.

Example Output:

The terminal will not show output during execution but will return the program’s exit code after it completes. This can be checked using $? immediately after execution.

Run a Program in a New Session Setting the Current Terminal as the Controlling Terminal

Code:

setsid --ctty program

Motivation:

Occasionally, it’s vital for a newly created session to take control over the current terminal. This can be particularly helpful in specialized applications requiring direct terminal input or output, such as terminal-based user interfaces or specific debugging scenarios.

Explanation:

• --ctty: This flag sets the controlling terminal for the invoked program, linking it with the current terminal session.
• program: The application or command being executed, needing direct terminal interaction.

Example Output:

The program will run with access to the current terminal, allowing it to output directly and accept inputs just as a regular terminal-linked process would.

Conclusion:

The setsid command is a versatile tool within Unix-like systems that allows users to run programs independently from a terminal. By leveraging various options and redirections, setsid aids in effectively managing background processes, controlling output, and ensuring processes continue independent of their parent terminal session. Each use case underscores a different aspect of process management, from output suppression to process forking and session control.