Run a Process in the Background: How to Background Process in Linux
Introduction
Running a process in the background is a common task in Linux, allowing you to execute commands without having them tied to your current terminal session. This can be particularly useful when you want to run long-running tasks or scripts that don’t require immediate attention. In this article, we will explore various methods to background process in Linux, including using the ampersand symbol, the nohup command, the bg command, the disown command, the screen command, the tmux command, the setsid command, the daemonize command, the at command, the cron command, and the systemd service. We will also cover techniques for redirecting output and logging the background processes.
Using the ampersand (&) symbol
The simplest way to run a process in the background is by appending the ampersand symbol (&) at the end of the command. For example, to run a command called “mycommand” in the background, you would enter:
mycommand &
This will start the process and immediately return control to the terminal, allowing you to continue using it while the process runs in the background. However, keep in mind that the process will still be attached to the current terminal session, and if you close the terminal, the process will be terminated.
Using the nohup command
The nohup command is another way to run a process in the background and ensure that it continues running even after you close the terminal. The syntax for using nohup is:
nohup command &
For example, to run a command called “mycommand” using nohup, you would enter:
nohup mycommand &
The nohup command redirects the standard output and standard error streams to a file called “nohup.out” by default. This file will contain any output or error messages generated by the process. You can also specify a different output file by using the redirection operator (>).
Using the bg command
The bg command is used to resume a suspended process in the background. When a process is suspended using the Ctrl+Z key combination, it is temporarily stopped, and control is returned to the terminal. To resume the process in the background, you can use the bg command. The syntax is:
bg
For example, if you have a suspended process with the job ID 1, you can resume it in the background by entering:
bg %1
This will resume the process and allow it to continue running in the background while you can continue using the terminal.
Using the disown command
The disown command is used to detach a process from the current terminal session, allowing it to continue running even after you close the terminal. This is useful when you have already started a process in the background using the ampersand symbol or the bg command. To use the disown command, follow these steps:
- Use the jobs command to list all the background processes and their job IDs.
- Identify the job ID of the process you want to disown.
- Enter the disown command followed by the job ID.
For example, if you have a background process with the job ID 1, you can disown it by entering:
disown %1
This will detach the process from the current terminal session, allowing it to continue running even after you close the terminal.
Using the screen command
The screen command is a powerful tool that allows you to create multiple terminal sessions within a single terminal window. It also enables you to run processes in the background and detach from them, similar to the functionality provided by the nohup and disown commands. To use the screen command, follow these steps:
- Start a new screen session by entering the command
screen. - Run the desired command or script within the screen session.
- Detach from the screen session by pressing Ctrl+A, followed by the d key.
The process will continue running in the background within the screen session, even if you close the terminal. To reattach to the screen session and view the output of the process, use the command screen -r.
Using the tmux command
Similar to the screen command, the tmux command allows you to create and manage multiple terminal sessions. To run a process in the background using tmux, follow these steps:
- Start a new tmux session by entering the command
tmux new-session -s session_name. - Run the desired command or script within the tmux session.
- Detach from the tmux session by pressing Ctrl+B, followed by the d key.
The process will continue running in the background within the tmux session, even if you close the terminal. To reattach to the tmux session and view the output of the process, use the command tmux attach-session -t session_name.
Using the setsid command
The setsid command is used to run a process in a new session, detached from the current terminal session. This ensures that the process continues running even after you close the terminal. The syntax for using setsid is:
setsid command
For example, to run a command called “mycommand” using setsid, you would enter:
setsid mycommand
The setsid command creates a new session and assigns the process to it. This allows the process to continue running independently, unaffected by the terminal session.
Using the daemonize command
The daemonize command is used to run a process as a daemon, which is a background process that runs independently of any terminal session. The syntax for using daemonize is:
daemonize command
For example, to run a command called “mycommand” as a daemon, you would enter:
daemonize mycommand
The daemonize command detaches the process from the current terminal session and ensures that it continues running even after you close the terminal. It also redirects the standard output and standard error streams to a log file, which can be specified using the -o and -e options.
Using the at command
The at command allows you to schedule a process to run in the background at a specific time. This can be useful when you want to run a command or script at a later time without having to wait for it to complete. To use the at command, follow these steps:
- Enter the command
at HH:MM, where HH:MM is the desired time in 24-hour format. - Enter the command or script you want to run in the background.
- Press Ctrl+D to save and exit.
The process will be scheduled to run in the background at the specified time. You can view the list of scheduled jobs using the atq command and remove a job using the atrm command followed by the job ID.
Using the cron command
The cron command allows you to schedule a process to run in the background at specific intervals, such as daily, weekly, or monthly. To use the cron command, follow these steps:
- Edit the cron table by entering the command
crontab -e. - Add a new entry specifying the schedule and the command or script to run.
- Save and exit the cron table.
The cron daemon will automatically execute the scheduled command or script in the background at the specified intervals. You can view the list of scheduled jobs using the crontab -l command and remove a job by editing the cron table again.
Using the systemd service
The systemd service allows you to run a process in the background as a service, which is managed by the systemd init system. To run a process as a systemd service, follow these steps:
- Create a service file in the /etc/systemd/system/ directory with a .service extension.
- Specify the service name, description, and the command or script to run in the ExecStart field.
- Save the service file and exit.
- Start the service using the command
systemctl start service_name.
The process will be started as a background service and managed by the systemd init system. You can view the status of the service using the command systemctl status service_name and stop the service using the command systemctl stop service_name.
Using the nohup command with redirection
The nohup command can also be used with redirection to redirect the output of a background process to a file. This can be useful for capturing the output or error messages generated by the process. The syntax for using nohup with redirection is:
nohup command > output.log 2>&1 &
For example, to run a command called “mycommand” in the background and redirect the output to a file called “output.log”, you would enter:
nohup mycommand > output.log 2>&1 &
The output.log file will contain any output or error messages generated by the process. The “2>&1” part of the command redirects the standard error stream to the same location as the standard output stream.
Using the setsid command with redirection
Similar to the nohup command, the setsid command can also be used with redirection to redirect the output of a background process to a file. The syntax for using setsid with redirection is:
setsid command > output.log 2>&1
For example, to run a command called “mycommand” in a new session and redirect the output to a file called “output.log”, you would enter:
setsid mycommand > output.log 2>&1
The output.log file will contain any output or error messages generated by the process. The “2>&1” part of the command redirects the standard error stream to the same location as the standard output stream.
Using the screen command with logging
The screen command can be used with logging to save the output of a background process to a file. This can be useful for capturing the output or error messages generated by the process. The syntax for using screen with logging is:
screen -L -Logfile output.log -dmS session_name command
For example, to run a command called “mycommand” in the background within a screen session and save the output to a file called “output.log”, you would enter:
screen -L -Logfile output.log -dmS session_name mycommand
The output.log file will contain any output or error messages generated by the process. The -L option enables logging, the -Logfile option specifies the output file, the -dmS options create a detached screen session with a specified name, and the command is the desired command or script to run.
Using the tmux command with logging
Similar to the screen command, the tmux command can also be used with logging to save the output of a background process to a file. The syntax for using tmux with logging is:
tmux new-session -d -s session_name 'command > output.log 2>&1'
For example, to run a command called “mycommand” in the background within a tmux session and save the output to a file called “output.log”, you would enter:
tmux new-session -d -s session_name 'mycommand > output.log 2>&1'
The output.log file will contain any output or error messages generated by the process. The -d option creates a detached session, the -s option specifies the session name, and the command is the desired command or script to run.
Conclusion
Running a process in the background in Linux can be achieved using various methods, each with its own advantages and use cases. Whether you prefer using the ampersand symbol, the nohup command, the bg command, the disown command, the screen command, the tmux command, the setsid command, the daemonize command, the at command, the cron command, or the systemd service, you have plenty of options to choose from. Additionally, techniques such as redirection and logging can further enhance your background processing experience. Experiment with these methods and find the one that best suits your needs.
FAQs
Q: Can I run multiple processes in the background simultaneously?
A: Yes, you can run multiple processes in the background simultaneously by using different methods such as the ampersand symbol, the screen command, or the tmux command. Each process will run independently and can be managed separately.
Q: How can I check the status of a background process?
A: You can check the status of a background process by using commands such as ps, top, or the job control built-in commands in your shell. These commands will provide information about the running processes, including their process IDs (PIDs) and status.
Q: How can I stop a background process?
A: To stop a background process, you can use the kill command followed by the process ID (PID) of the process. You can obtain the PID of a process by using commands such as ps or top. Alternatively, you can use the job control built-in commands in your shell to manage and stop background processes.
