Master Linux Background Tasks: nohup, &, ctrl+z, jobs, bg, fg, and screen

1. Problem Introduction

Developers often lose running programs when the SSH session is terminated due to network issues, closing the terminal, or pressing Ctrl+C, because the shell sends interrupt signals that cause processes to exit.

The two main signals are:

SIGINT – sent by Ctrl+C to actively terminate a program.

SIGHUP – sent when the terminal is closed or the network disconnects.

This article introduces several methods to run Linux tasks in the background and avoid these problems.

2. Using the & Symbol

Appending '&' to a command makes the started program ignore SIGINT, so Ctrl+C will not stop it, though closing the terminal or network disconnection will still terminate the process.

sh test.sh &

3. The nohup Command

'nohup' (no hang up) runs a command immune to SIGHUP, allowing it to continue after the SSH session ends. Ctrl+C can still stop the process. Typically, output is redirected to a file named nohup.out.

Basic syntax

nohup Command [Arg ...] [&]

Example

Run ./test.sh in the background, redirect stdout to out.log and stderr to err.log: nohup ./test.sh > out.log 2>err.log & File descriptor numbers:

0 – stdin

1 – stdout (default redirection command > out.log)

2 – stderr

Redirecting stderr to stdout:

nohup ./test.sh > out.log 2>&1 &

4. Job Control: ctrl+z, jobs, bg, fg

If a program was started without '&' or nohup, you can still manage it using job control commands.

4.1 ctrl+z

Suspends the foreground job and moves it to the background:

[1]+ Stopped ./test.sh

4.2 jobs

Lists current jobs and their statuses:

jobs
[1]+ Stopped ./test.sh
[2]+ Running ./test2.sh &

4.3 bg

Resumes a stopped job in the background (e.g., job 1):

bg 1
[1]+ ./test.sh &

4.4 fg

Brings a background job to the foreground (e.g., job 2):

fg 2
./test2.sh

5. The screen Utility

5.1 Overview

'screen' creates a virtual terminal session that persists after the SSH connection ends, allowing programs to keep running.

5.2 Installation

yum install screen

5.3 Usage

Create a new session: screen -S yourname List sessions: screen -ls Reattach to a session: screen -r yourname Detach a session: screen -d yourname (or screen -d -r yourname to end current and reattach)

Terminate a session:

screen -S pid-X quit