Linux Run in the Background

⇢ 
Tech Linux

Environment: Ubuntu 22.04, but it should work in other Linux distributions.

Source: https://www.sobyte.net/post/2023-03/linux-services-in-the-background/

You run some commands, it may take a long time to execute, and you don’t want to wait for it. So what should you do in that scenario?

graph TD A(You don't want to wait
for a running command) --> B{Do you want to see output?} A --> C{Do you want the command to stay
running after the session ends?} C --> |Yes| E(screen) B --> |No| D(Ctrl-Z or &) B --> |Yes| F{Do you have requirements
for log cutting, running users?} F --> |No| G(nohup) F --> |Yes| H(supervisor or systemd)

Ctrl-Z or &

For example, you have to excute ping command:

ping -c 20 8.8.8.8

You want to send it to background, you can do the following:

$ ping -c 20 8.8.8.8 &
[1] 467176
# Or hit ctrl-z, the effect of the & symbol is similar to ctrl-z
$ ping -c 20 8.8.8.8 # Press ctrl+z to hang
^Z[1]  + 467680 suspended
# View the hung background processes
$ job -l
[1]  + 467680 suspended
# Bring the background task process back up and see that process is executing again
# fg %<number>
$ fg %1

screen

If you want the program to continue running after the session ends, consider using screen.


$ sudo apt install screen

# man screen
$ man screen

SCREEN(1)                                                                   General Commands Manual                                                                   SCREEN(1)

NAME
       screen - screen manager with VT100/ANSI terminal emulation

SYNOPSIS
       screen [ -options ] [ cmd [ args ] ]
       screen -r [[pid.]tty[.host]]
       screen -r sessionowner/[[pid.]tty[.host]]

...
# Create a ping session
$ screen -R <session-name>


# Exit the session, recreate another one and use the previous command
# you can see that the command is still running
$ screen -R <session-name>

nohup

screen is more suitable for executing some scripts than nohup, while nohup is suitable for running a long-time tasks.

$ nohup command > command.log 2>&1 &

# There are 3 types of input and output defined in Linux:
# 0: standard input
# 1: standard output
# 2: error output
#
# 2>&1: the error output 2 is redirected to the standard output &1
# combined with > to be output to the file log.

supervisor

nohup is still very good for performing simple background tasks, but if you have requirements for output log cutting, running users,… you need to consider daemon type programs such as supervisor and systemd.

supervisor is a lightweight process control system, used to run programs for long period of time, often used for web background programs, proxy programs, etc.

$ sudo apt install supervisor
$ sudo systemctl enable supervisor
$ sudo systemctl start supervisor

# Create supervisor file
$ sudo cat <<EOF >/etc/supervisor/conf.d/sync.conf
[program:syncthing]
directory=/home/apps/syncthing/syncthing-linux-amd64-v1.19.0
command=/home/apps/syncthing/syncthing-linux-amd64-v1.19.0/syncthing
autostart=true
autorestart=true
startsecs=10
stdout_logfile=/var/log/supervisor/%(program_name)s-stdout.log
stdout_logfile_maxbytes=5MB
stdout_capture_maxbytes=5MB
stdout_logfile_backups=5
stderr_logfile=/var/log/supervisor/%(program_name)s-stderr.log
stderr_logfile_maxbytes=5MB
stderr_capture_maxbytes=5MB
stderr_logfile_backups=5
user = apps
environment = HOME="/home/apps/syncthing/syncthing-linux-amd64-v1.19.0"
EOF

# Make the newly created service effective
$ sudo supervisorctl update
# update: means update the list of services and re-run the new/modified services
# status: check the running status of all services
# start/stop/restart: start/stop/restart the service

systemd

systemd is the system suite that comes with the most distributions to replace SysV init, providing reliable parallelism during the boot process and centralized management of processes, daemons, services and mount points. It is a relatively heavyweight system, and unless there is a clear need to use it, it is generally not considered a way to run small applications (or commands) in the background.

systemd common user unit configuration directory:

$HOME/.config/systemd/user
/usr/lib/systemd/user
$HOME/.local/share/systemd/user
/etc/systemd/user

# Create file
$ cat <<EOF >$HOME/.config/systemd/user/sync.service
[Unit]
Description=Sync Files
Documentation=https://syncthing.net
After=network.target

[Service]
Type=simple
Environment=HOME="/home/apps/syncthing/syncthing-linux-amd64-v1.19.0"
ExecStart=/home/apps/syncthing/syncthing-linux-amd64-v1.19.0/syncthing
# Logging requires systemd version > 240
StandardOutput=append:/home/apps/syncthing/syncthing-linux-amd64-v1.19.0/logs/standard-output.log
StandardError=append:/home/apps/syncthing/syncthing-linux-amd64-v1.19.0/logs/standard-error.log

[Install]
WantedBy=multi-user.target
EOF

$ systemctl start sync --user