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This article mainly introduces the relevant knowledge of "how to start the daemon process of Linux". The editor shows you the operation process through an actual case. The operation method is simple, fast and practical. I hope this article "how to start the daemon process of Linux" can help you solve the problem.

Linux Daemon (daemon) is a special process that runs in the background. It is independent of the control terminal and periodically performs certain tasks or waits for certain events to be processed. It does not require user input to run and provide a service, either to the entire system or to a user program.

I. the origin of the problem

After the Web app is written, the next thing to do is to start it and keep it running in the background, which is not easy. For example, here is the simplest Node app, server.js, with only six lines.

Var http = require ('http'); http.createServer (function (req, res) {res.writeHead (200,{' Content-Type': 'text/plain'}); res.end (' Hello World');}) .subscription (5000)

You start it at the command line.

$node server.js

Everything seems to be fine, and everyone can happily access port 5000; however, once you exit the command line window, the application exits and cannot be accessed, so how can you make it a daemon of the system, a service, and run there all the time?

II. Foreground tasks and background tasks

The script started above is called a foreground task (foreground job). It will monopolize the command line window, only after running or manually aborting, can execute other commands, become the first step of the daemon is to change it to a "background task" (background job).

$node server.js &

As long as you add a symbol & to the end of the command, the started process becomes a "background task". If you want to change a running foreground task to a background task, press ctrl + z and then execute the bg command (let the most recently paused background task continue).

"background tasks" have two characteristics.

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1. Inherits the standard output (stdout) and standard error (stderr) of the current session (conversation). As a result, all output from background tasks is still displayed synchronously on the command line.

two。 The standard input (stdin) of the current session is no longer inherited. You can't enter instructions into this task. If it tries to read standard input, it pauses execution (halt). As you can see, there is only one essential difference between "background task" and "foreground task": whether or not to inherit standard input. Therefore, while performing background tasks, the user can also enter other commands.

3. SIGHUP signal

When it becomes a "background task", does a process become a daemon? In other words, will the "background task" continue to be executed after the user quits session? The Linux system is designed in this way.

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1. Users are ready to exit session 2. The system sends SIGHUP signals to the session 3.session sends SIGHUP signals to all child processes 4. After receiving the SIGHUP signal, the child process automatically exits

The above process explains why the "foreground task" exits with the exit of session: because it receives a SIGHUP signal. So, will the "background task" also receive a SIGHUP signal? This is determined by the huponexit parameter of Shell.

$shopt | grep huponexit

Execute the above command and you will see the value of the huponexit parameter. On most Linux systems, this parameter is off by default (off). Therefore, when session exits, the SIGHUP signal is not sent to the "background task". So, generally speaking, "background tasks" do not exit with session.

4. Disown command

It is not safe to start daemons through background tasks, because some systems may have huponexit parameters turned on (on). A safer way is to use the disown command. It removes the specified task from the background tasks list (the result returned by the jobs command). Session will certainly not send a SIGHUP signal to a "background task" as long as it is not on this list.

$node server.js & $disown

After executing the above command, the server.js process is removed from the background tasks list. You can execute the jobs command to verify that there will be no such process in the output.

The usage of disown is as follows.

# remove the most recent background task in progress $disown# remove all ongoing background tasks $disown-r # remove all background tasks $disown-a # do not move out of background tasks, but let them not receive SIGHUP signals $disown-h # according to jobId, remove the specified background task $disown% 2$ disown-h% 2, standard Icando

After using the disown command, there is one more problem. That is, after exiting the session, if the background process interacts with the standard Imax O, it will still die. Take the above script as an example, now add a line.

Var http = require ('http'); http.createServer (function (req, res) {console.log (' server starts...'); / / join this line res.writeHead (200,{ 'Content-Type':' text/plain'}); res.end ('Hello World');}) .subscription (5000)

Start the script above, and then execute the disown command.

$node server.js & $disown

Then, you exit session, access port 5000, you will find that the connection is not available. This is because the standard I session,disown O for background tasks inherits from the current command does not change this. Once the "background task" reads and writes the standard Imando O, it will be found that it no longer exists, so the execution will be terminated with an error. In order to solve this problem, it is necessary to redirect the standard Imando O of "background task".

$node server.js > stdout.txt 2 > stderr.txt disown

If the above is carried out in this way, there will basically be no problem.

VI. Nohup command

There is a more convenient command than disown, which is nohup.

$nohup node server.js &

The nohup command does three things to the server.js process.

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Prevent SIGHUP signals from being sent to this process. Turn off standard input. The process can no longer receive any input, even if it is running in the foreground. Redirect standard output and standard error to the file nohup.out.

That is, the nohup command actually separates the child process from its session; note that the nohup command does not automatically turn the process into a "background task", so the & symbol must be added.

7. Screen command and Tmux command

Another idea is to use terminal multiplexer (terminal multiplexer: managing multiple session on the same terminal), typically Screen commands and Tmux commands.

They can create another session in the current session. In this way, once the current session ends, it does not affect other session. And if you log in again later, you can also connect to the session you created earlier. The usage of Screen is as follows.

# create a new session$ screen$ node server.js and then press ctrl + An and ctrl + D to return to the original session and log out from there. The next time you log in, cut back. $screen-r if you create more than one background session, you need to specify names for them. $screen-S name# switches back to the specified session$ screen-r name$ screen-r pid_number# lists all session$ screen-ls. If you want to stop a session, you can switch back to it, and then press ctrl + c and ctrl + d.

Tmux is more and more powerful than Screen, and its basic usage is as follows.

$tmux$ node server.js# returns the original session$ tmux detach. In addition to tmux detach, another way is to press Ctrl + B and d, or you can return to the original session. # the next time you log in, the service session$ tmux attach is running at the backend. If you create multiple session, you need to specify a name for each session. # create a new session$ tmux new-s session_name# and switch to the specified session$ tmux attach-t session_name# to list all session$ tmux list-sessions# to exit the current session, and return to the previous session$ tmux detach# to kill the specified session$ tmux kill-session-t session- name VIII and Node tools

For Node applications, without the above methods, there are some special tools for launching: forever,nodemon and pm2. The simple function of forever is to ensure that the application will restart automatically when the process exits.

# start $forever server.js# as a foreground task start $forever start app.js# stop service process $forever stop Id# restart service process $forever restart Id# monitor changes in files in the current directory, restart $forever-w server.js#-m parameter specify maximum number of restarts $forever-m 5 server.js# list all processes $forever list

Nodemon is generally used only during development, and its greatest strength lies in the watch function, which automatically restarts the process as soon as the file changes.

# default to monitor changes in files in the current directory $nodemon server.js# monitors changes in specified files $nodemon-- watch app-- watch libs server.js

Pm2 is the most powerful, collecting logs and monitoring in real time in addition to restarting the process.

# start the application $pm2 start app.js# specify how many processes start at the same time (determined by the number of CPU cores), form a cluster $pm2 start app.js-I max# list all tasks $pm2 list# stop specified tasks $pm2 stop "restart specified tasks $pm2 restart" delete specified tasks $pm2 delete "save all current tasks" You can later restore $pm2 save# list statistics for each process $pm2 monit# view all logs $pm2 logs# export data $pm2 dump# restart all processes $pm2 kill$ pm2 resurect# start the web interface http://localhost:9615$ pm2 web 9, Systemd

In addition to specialized tools, the Linux system has its own daemon management tool, Systemd. It is a part of the operating system, directly interacts with the kernel, excellent performance, extremely powerful. We can give the program to Systemd, let the system unified management, and become a real system service.

That's all for "how to start the daemon for Linux". Thank you for reading. If you want to know more about the industry, you can follow the industry information channel. The editor will update different knowledge points for you every day.

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