Move A Running Process To Screen

Here’s a quick guide on moving a running Linux process to the screen terminal multiplexer.

When working on a remote Linux server, tasks like viewing log files or routine maintenance are quick. However, some processes, such as large log file analysis, web scraping, or database migrations, can take hours.

To maximize efficiency, I start these long-running tasks in the afternoon and monitor them from home. This lets me begin tasks at the office and continue them remotely without staying late. The key tool for this is a terminal multiplexer, which allows me to create, detach, and reattach Linux sessions seamlessly between different locations.

For quick tasks, I don’t use screen since the session only lasts a few minutes. However, sometimes tasks take longer than expected, or I forget to use b. In these cases, I have three options:

1. Stay late in the office for an unknown duration.Stop the process,

2. losing unsaved work and potentially wasting hours.

3. Move the running process to a new screen session. I’ll describe this solution next.

Move The Running Process To Another Screen Shell:

To move a running Linux process to a screen session, you’ll need two programs: screen and reptyr. Screen manages terminal sessions, and reptyr transfers running processes between sessions. Here’s a brief guide:

1. Suspend the process.
2. Resume the process in the background.
3. Disown the process.
4. Launch a screen session.
5. Find the PID of the process.
6. Use reptyr to take over the process.

Let’s break down each step.

Suspend the Process:

The first step is to suspend the running process by pressing Ctrl+Z. This sends the TSTP (Terminal Stop) signal, temporarily pausing the process. The operating system stops scheduling CPU time for the process, effectively putting it on hold.

Move the Process to the Background:

Resume the suspended process in the background by typing bg. This sends the SIGCONT (Continue) signal, allowing the process to run in the background without occupying your terminal.

Disown the Process:

Run disown %1 to remove the process from the shell’s job table. This detaches the process from your terminal, allowing it to run independently and be controlled by another session later.

Start a Screen Session:

This step is straightforward. Simply run the screen command to start a new screen session. This session will soon take over the process, allowing you to manage it within the screen environment.

Identify the Process ID (PID):

Next, you need to find the Process ID (PID) of the process you want to take over. The recommended method is to use the pgrep command. For example, if your process is named program, you can run the following command to retrieve its PID:

Take Over The Process With reptyr:

Finally, use reptyr to take over the process. If pgrep gave you a PID of 1234, you can execute the following command:

This command attaches the reptyr utility to the process identified by PID 1234, allowing you to interact with it within the current session.

Quick Reference Guide: Moving A Linux Process To Screen:

1. Suspend the Process: Press Ctrl+Z to suspend the process. 

2. Resume in Background: Use bg to resume the process in the background. 

3. Disown the Process: Run disown %1 to detach the process from the current shell. 

4. Launch a Screen Session: Start a new screen session with screen. 

5. Find the PID: Use pgrep to find the PID of the process (e.g., pgrep myprogram). 

6. Use reptyr: Execute reptyr PID (e.g., reptyr 1234) to take over the process. 

Conclusion:

Follow these steps to efficiently manage and transfer long-running processes using screen and reptyr. This method enables you to start tasks from one location and monitor or control them from another, ensuring flexibility and continuity in your workflow.

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