stop

kill [-s SIGNAL | -SIGNAL_NAME | -SIGNAL_NUMBER] PID...

For stopping a process:
kill -STOP PID...
kill -s STOP PID...
kill -19 PID...

PID...
    One or more Process IDs (PIDs) of the processes to which the signal will be sent. You can find PIDs using commands like ps or top.

-s SIGNAL
    Specifies the signal to send by name or number. For stopping a process, use STOP or its number 19.

-STOP
    A shorthand option to specify the SIGSTOP signal. This is equivalent to -s STOP.

-19
    A shorthand option to specify the SIGSTOP signal by its numeric value. This is equivalent to -s 19.

kill is a fundamental Unix/Linux command used to send signals to processes. When used with the STOP signal (signal number 19), it instructs the operating system to pause a running process without terminating it. This is distinct from other signals like SIGTERM or SIGKILL, which are used to request or force process termination. A process stopped using SIGSTOP will remain in a suspended state, consuming system resources but not executing any instructions, until it receives a SIGCONT signal (signal number 18), which resumes its execution.

SIGSTOP is primarily used for job control, allowing users to temporarily halt a background process, inspect its state, or make system adjustments before resuming it. Unlike most other signals, SIGSTOP cannot be caught, ignored, or blocked by the target process, making it a reliable way for the kernel to enforce a halt. It is often employed in debugging scenarios or when managing long-running background tasks.

Permissions: To stop a process, you must either be its owner or have root privileges. Insufficient permissions will result in an 'Operation not permitted' error.

Unblockable Signal: SIGSTOP is a unique signal in that it cannot be caught, ignored, or blocked by the target process. This ensures that the operating system can always halt a process, regardless of its state or programming.

Terminal Job Control: While kill -STOP can be used for any process, the SIGTSTP signal (usually triggered by Ctrl+Z) is sent to the foreground process group of a terminal to stop it. This is part of shell job control and works similarly to SIGSTOP but is initiated differently.

After a process has been stopped using SIGSTOP, it can be resumed by sending it the SIGCONT signal (signal number 18). This is typically done using the command:
kill -CONT PID... or kill -s CONT PID.... The process will then continue execution from where it left off.

It's crucial to understand that SIGSTOP does not terminate a process. It merely pauses it. To terminate a process, other signals like SIGTERM (default for kill, allows graceful shutdown) or SIGKILL (forces immediate termination) are used. A stopped process consumes memory and other resources but no CPU cycles.

The kill command and the concept of signals have been fundamental to Unix-like operating systems since their early development. Signals, including SIGSTOP, were introduced as a form of inter-process communication (IPC) and control. SIGSTOP specifically became a critical component of job control, allowing users to manage multiple tasks within a single terminal session by suspending and resuming processes. Its reliability (being unblockable) has ensured its consistent role in system administration and debugging over decades of Unix and Linux evolution.

kill(1), killall(1), pkill(1), ps(1), signal(7), jobs(1), fg(1), bg(1)