Background Jobs and nohup

Interactive shells track every command you launch as a 'job,' whether it's running in the foreground and holding your terminal or running in the background while you keep typing. Job control lets you juggle multiple tasks in a single terminal session, and understanding how jobs relate to the terminal's controlling process — and what happens to them when that terminal closes — is essential for running anything long-lived, from a data import script to a personal web server, without accidentally killing it the moment you log out.

Appending & to a command launches it in the background immediately, returning control of the shell to you while the command runs; the shell prints a job number and PID. The jobs builtin lists all jobs associated with the current shell session, showing their job number, state (Running, Stopped, Done), and command. fg %N brings job N to the foreground, and bg %N resumes a stopped job in the background. Pressing Ctrl+Z sends SIGTSTP to suspend the foreground job, after which bg can resume it without occupying the terminal. Job numbers (like %1) are shell-local shorthand for PIDs and only make sense within the shell session that created them.

When you close a terminal or SSH session, the shell typically sends SIGHUP to all jobs it started, and by default SIGHUP terminates a process. This is intentional: it stops orphaned background tasks from accumulating on shared systems. nohup works around this by explicitly telling the process to ignore SIGHUP, so it keeps running after the parent shell exits; it also redirects stdout/stderr to nohup.out if you don't redirect them yourself, since the original terminal (its stdout/stderr destination) will no longer exist. nohup alone doesn't background a process — you still need the trailing &. disown is a related but different shell builtin: it removes a job from the shell's job table so the shell won't send it SIGHUP at all, without changing the process's own signal handling.