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What is nohup (no hang-up)?
Nohup, short for “no hang-up,” is a command in UNIX systems that allows a process to continue running even after the user who started it logs out or terminates their session. When a user initiates a process in the terminal, the process becomes associated with the terminal. By default, when the user logs out or closes the terminal, all associated processes receive a hang-up signal (SIGHUP), which usually terminates them.
Nohup provides a way to prevent a process from receiving the hang-up signal, allowing it to continue running in the background even after the user has disconnected from the terminal. It achieves this by redirecting the standard input (stdin) and output (stdout) of the process, ensuring that it does not rely on the terminal’s direct input or output.
Basic concepts of process continuation
When a process is started on a UNIX system, it is typically associated with the terminal or shell from which it was launched. This means that the process inherits various attributes from the terminal, including the hang-up signal (SIGHUP) handling. When a user logs out or terminates their session in the terminal, the system sends the hang-up signal to all associated processes.
However, in certain cases, it is desirable to keep a process running even after the user’s session has ended. This is where nohup comes into play. By using nohup, the process becomes detached from the terminal and continues executing, unaffected by the hang-up signal.
Nohup achieves process continuation by performing the following steps:
1. Ignoring the SIGHUP signal: Nohup instructs the process to ignore the hang-up signal, ensuring that it doesn’t terminate when the user logs out.
2. Redirecting stdin and stdout: Nohup redirects the standard input and output of the process. By default, it redirects them to a file named nohup.out in the current directory, but this can be modified with command-line options.
3. Closing stderr: Nohup closes the standard error stream (stderr) of the process, preventing any error messages from being displayed in the terminal.
By default, when executing a command with nohup, it immediately returns the control to the terminal, freeing it for further work. However, the process continues running in the background until it finishes or encounters an error.
Benefits and usage of nohup
Nohup is particularly useful in scenarios where long-running processes need to be started and left unattended. For example, let’s say you need to run a complex data analysis script that might take several hours or even days to complete. By using nohup, you can initiate the process and disconnect from the terminal, confident that it will keep running until completion.
Here’s an example of how to use nohup:
“`
nohup python data_analysis.py > analysis.log &
“`
In this example, the Python script “data_analysis.py” is executed with nohup. The standard output is redirected to a file named “analysis.log,” which will contain any output generated by the script. The ampersand (&) at the end instructs the system to run the process in the background and return control to the terminal immediately.
Once the process is running with nohup, you can safely log out or close the terminal without interrupting the execution. The process will continue running until completion, and any output will be captured in the specified log file.
Nohup is a powerful tool for managing long-running processes on UNIX systems, providing a convenient way to detach them from the terminal and ensuring their continuation even after the user’s session ends.
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