How to use the nohup command in Linux

What is nohup

nohup is a command in Linux and Unix systems. Its function is to allow the process to continue running in the background when the terminal exits. Its full name is "no hang up", which means "no hang up". The nohup command allows you to continue running commands after exiting the terminal or closing the SSH connection.

nohup syntax rules

The basic syntax of the nohup command is as follows:

nohup COMMAND [ARGS ...] [> output-file 2> error-file] &

The meaning of the parameters is as follows:

• COMMAND: A command or script that needs to be run in the background.

• ARGS: Parameters of the command or script.

• > output-file: Output is redirected to the specified file.

• 2> error-file: The error message is redirected to the specified file.

• &: Run the command in the background.

The execution process of the nohup command is divided into the following steps:

• The nohup command changes the standard input, standard output and standard error of the current shell All output is redirected to the /dev/null device to avoid being interrupted by a signal that closes the terminal.

• The nohup command puts the process into the background for execution and outputs the PID of the process to the terminal.

• The process starts executing and redirects standard output and standard error output to the specified file.

• Users can exit the terminal or close the terminal window, and the process will still run in the background.

nohup usage method

Using the nohup command is very simple, just follow the basic syntax above. Here are some usage examples of the nohup command:

Run command in the background

To run a command in the background, just enter the following command on the command line:

nohup COMMAND &

For example , run a Bash script in the background:

nohup bash test.sh &

Redirect standard output to a file

nohup bash test.sh > stdout.txt &

Redirect standard error output to a file

nohup bash test.sh 2> stderr.txt &

Redirect both standard output and standard error output Redirect to a file

1. Redirect to the same file

nohup bash test.sh > output.txt 2>&1 &

2. Redirect to a different file

nohup bash test.sh > stdout.txt 2> stderr.txt &

3. A more complex example, redirection criteria Input (stdin):

nohup ./myprogram > foo.out 2> foo.err < /dev/null &

There is an extra </dev/null, which means to redirect the standard input to /dev/null to ensure The program does not read any data from standard input.

This is to solve a practical problem: SSH sessions often refuse to log out (or hang) because they are unwilling to lose the data interacting with the background job(s). When encountering this problem, you can use the above command to solve it by redirecting three times.

nohup background process management

Use the jobs command to view the list of tasks running in the background in the current shell, including task numbers, status and commands.

For example, we execute a sleep 1000 command in the background and use the jobs command to view:

$ jobs
[1]+  Running                 nohup sleep 1000 &

Among them, the number in square brackets represents the task The number, plus or minus sign indicates the priority of the task, Running indicates that the task is running in the background. In addition, there are other possible states, including Stopped (stopped), Done (completed), etc.

We can also use the fg command to move a background task to the foreground to continue running. For example:

$ fg %1

This command will move the task with task number 1 to the foreground. , continue execution. If you want to pause or resume the task, you can use the Ctrl-Z keys to send the SIGTSTP signal in the current shell.

$ fg %1
nohup sleep 1000

^Z
[1]+  Stopped                 nohup sleep 1000

If you want to resume operation at this time, you can use the bg command:

$ bg %1
[1]+ nohup sleep 1000 &

If you want to kill the task, you can use kill Command:

$ kill %1
[1]+  Terminated              nohup sleep 1000

If you want to kill all background tasks, but find it troublesome to do them one by one, you can use the disown command to solve the problem:

$ disown -a

This command can Kill all background tasks, but there will be no prompt. You can confirm with the jobs command.

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