Why Does nohup Keep a Process Running After the Terminal Closes?

When a command is started in a terminal and the terminal is closed, the operating system sends a SIGHUP (signal hang up) to the process, which by default terminates it. This behavior often surprises users who expect the process to continue running.

Background and Real‑World Example

The author recounts a personal experience configuring a Linux authentication client on a Netgear router. After SSHing into the router and starting the client, the network connection dropped as soon as the SSH session ended because the authentication program received SIGHUP and stopped.

Common Solutions

Append & to run the command in the background.

Use nohup to ignore SIGHUP: $ nohup python process.py & (outputs a job ID and creates nohup.out).

Alternative tools: screen, tmux, or setsid.

What nohup Does

The nohup utility invokes a command with its arguments and sets the SIGHUP signal to be ignored.

SIGHUP is sent to a process when its controlling terminal is closed on POSIX‑compatible systems (Unix, Linux, BSD, macOS). Ignoring this signal allows the process to keep running after the terminal disappears.

Understanding Signals

Signals are a form of inter‑process communication. A process can handle a signal in three ways: use the default action, install a custom handler, or ignore it (some signals cannot be ignored). For SIGHUP, the default action is termination.

The shell typically registers a handler that forwards SIGHUP to all its child processes before exiting. Programs like nohup explicitly set the handler to SIG_IGN so they survive.

Programming Example

#include <signal.h>
#include <unistd.h>
int main() {
    signal(SIGHUP, SIG_IGN);
    sleep(1000);
    return 0;
}

Compiling and running this program shows that it continues even after the terminal is closed.

Other Signal Uses

Beyond SIGHUP, other signals include:

SIGINT (Ctrl+C)

SIGKILL (cannot be ignored)

SIGTERM (graceful termination)

Custom signals SIGUSR1, SIGUSR2, SIGURG for application‑specific purposes.

For example, services like Apache or Nginx use SIGHUP to reload configuration without stopping.

Zombie Processes

If a child process exits but its parent does not call wait(), the kernel keeps a task_struct for the child, marking it as a zombie (state Z). Zombies appear in ps or top and cannot be killed until the parent reaps them.

Understanding SIGHUP and proper signal handling prevents unintended zombie processes and ensures background tasks run reliably.