Master Linux Background Tasks: From &, nohup to screen for Reliable Jobs

1. Problem Introduction

Programmers often lose running programs when the SSH session is interrupted by network loss, closing the terminal, or pressing Ctrl+C, because the shell sends interrupt signals that cause the processes to terminate.

The two main signals are:

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

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

2. Using the & Symbol

Appending & to a command runs it in the background and ignores SIGINT, but the process still exits if the terminal is closed or the network drops.

sh test.sh &

3. nohup Command

nohup

(no hang up) detaches a command from the terminal, ignoring SIGHUP so the process continues after SSH disconnects. Ctrl+C can still stop it. Combining nohup with & makes the job survive both terminal closure and Ctrl+C. By default output is written to 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 meanings:

0 – stdin

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

2 – stderr

Redirecting both stderr and stdout to the same file:

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

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

If a command was started without &, you can suspend it with Ctrl+Z, then manage it with job control commands.

4.1 ctrl+z

Suspends the foreground job and moves it to the background in a stopped state.

[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.

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

4.4 fg

Brings a background job to the foreground.

fg 2
./test2.sh

5. screen Command

5.1 Introduction

GNU Screen creates virtual terminals that persist after the SSH connection ends, allowing programs to keep running.

5.2 Installation

yum install screen

5.3 Usage

Create a session: screen -S yourname List sessions: screen -ls Resume a session: screen -r yourname Detach a session: screen -d yourname or screen -d -r yourname Kill a session:

screen -S pid-X quit