1. Preface

• In the use of ssh When connecting to a remote server , We can manage the server remotely , Execute background command at remote terminal . however , If you log out before the command is executed , The execution of this command will be interrupted .
• Sometimes , We want to log out , But keep the program running . For example, when using remote server to train deep learning model , We don't want to stay connected to the server all the time
• We use nohup Command can solve this problem

2.nohup

2.1. Basic usage

[root@localhost ~]# nohup [ command ] &

• nohup yes no hangup Abbreviation , It means don't hang up , That is, the program does not exit . This command will Make the program ignore SIGHUP The signal , Ensure that the program can run normally .SIGHUP A signal is a signal sent to the process it is associated with when the terminal is terminated , When the process receives this signal, it will stop running . So if you don't want the process to be killed by this signal , You can ignore this signal . and nohup That's what the order does , Keep the process free shell The impact of closure .
  • meanwhile ,nohup After execution , Will produce files nohup.out, The result of executing a program stdout/stderr Redirect to this file . In the current directory , If the current directory is not writable , Then it is automatically saved to the user executing the command home Under the table of contents , for example root If so, keep it in /root/ Next .
• & Is to put the command in the background . Putting the program in the background means , We no longer interact with it . This command will Make the program ignore SIGINT The signal , We use it ctrl C, It will not affect the operation of the program .
  • & It doesn't redirect stdout/stderr, So if the program has any output or produces an error , Will be displayed directly on the terminal .
• [ command ] It is the command we input when we execute the program normally , such as ：python train.py

2.2. Example

nohup python train.py &

• perform train.py Program , Put it in the background , And ignore shell Close the given SIGUP Instructions
• python train.py Namely [ command ]

2.3. Redirect output

2.3.1. command ：command > /dev/null 2>&1 &

• > It's the output symbol , Where is the redirect
• 1 yes stdout standard output , The system default is 1. therefore > hello.txt Equate to 1 > hello.txt
• 2 yes stderr The standard error
• & Is equivalent to ,2>&1, Express ,stderr Redirect the output of to and stdout The same position

2.3.2. Program test.py

print('hello world')
p  #  Intentional error writing 

2.3.3. Carry out orders

• python test.py > hello.txt 2>&1

  • hello.txt The content in

    • You can see , The output and errors are relocated to hello.txt in

    hello world
    Traceback (most recent call last):
      File "/home/nsy/detection/yolov5-master/nsy_demo.py", line 2, in <module>
        p
    NameError: name 'p' is not defined
    

• python test.py > hello.txt

  • hello.txt The content in

    • You can see , Only the output is relocated to hello.txt in

    hello world
    

    • The error message is output at the terminal （ Default ）, Without being relocated

• We can use the relocation command with the suspend command

  nohup python test.py > hello.txt 2>&1 &

2.4. Terminate the process

nohup Combination of orders & Symbols enable processes to run in the background , Even if the terminal is turned off, it will not be affected . At this time , If you want to terminate the process , How to operate ?

• The simplest is to use kill command

  kill Process number

• How to find the corresponding process pid Well ？-> Use ps command

  ps aux | grep The program name . such as ps aux | grep train.py

  • After finding the process number , Use kill PID That's it