What default signal does htop send on kill?

When managing processes in Linux using the htop interactive viewer, pressing the F9 key (or k) opens the signal menu to terminate a process. By default, htop highlights and sends SIGTERM (signal 15) to the selected process. This article explains why SIGTERM is the default choice, how it operates, and how it compares to the more aggressive SIGKILL alternative.

Understanding SIGTERM: The Default Choice

In htop, when you select a process and initiate the kill command, the utility defaults to SIGTERM (Signal number 15). This is a deliberate design choice aligned with standard Linux system administration best practices.

SIGTERM acts as a polite request for termination. When a program receives this signal:

• It is given the opportunity to catch the signal and handle it gracefully.
• It can save its current state, close open file descriptors, and release network connections.
• It can finish ongoing critical operations to prevent data corruption before exiting.

How to Send a Signal in htop

Navigating the htop interface to manage process signals is straightforward:

1. Launch htop in your terminal.
2. Use the arrow keys to highlight the target process.
3. Press F9 or k to open the “Send signal” menu on the left side of the screen.
4. Notice that 15 SIGTERM is automatically selected by default.
5. Press Enter to send SIGTERM, or use the arrow keys to select a different signal.

SIGTERM vs. SIGKILL

While SIGTERM is the default, users frequently navigate to SIGKILL (signal 9) when a process becomes unresponsive. The differences between these two primary termination signals are vital for system stability.

Because SIGKILL cannot be caught or ignored by the application, the Linux kernel terminates the process immediately. This leaves no time for cleanup, which can occasionally result in orphaned child processes or corrupted files. Consequently, htop defaults to SIGTERM to ensure system integrity, leaving SIGKILL as a secondary option for uncooperative software.