Nohup: Ignoring Input and Appending Output to nohup.out

Imagine running a crucial process on your Unix-like system, only to be met with the message “nohup: ignoring input and appending output to ‘nohup.out’.” What does this mean, and how does it impact your command execution? Let’s delve into the intricacies of the nohup command and unravel the significance behind this common notification, ensuring you have a comprehensive understanding of its implications.

Understanding the ‘nohup: ignoring input and appending output to ‘nohup.out’

The message “nohup: ignoring input and appending output to ‘nohup.out’” is a common occurrence when using the nohup command in Unix-like systems. Let’s break down what this means:

nohup Command Explanation:
- The nohup command is used to run a process or command in the background, even after you log out of the terminal.
- It ensures that the process continues running even if the terminal session is closed.
Message Interpretation:
- The message “nohup: ignoring input and appending output to ‘nohup.out’” is not an error; it’s normal behavior.
- It informs you that once the process starts, its input and output are detached from your console.
- Specifically:
  - If standard input (stdin) is a terminal, it redirects it from /dev/null.
  - If standard output (stdout) is a terminal, it appends output to a file called 'nohup.out' (or $HOME/nohup.out if the former is not possible).
  - If standard error (stderr) is a terminal, it redirects it to standard output.
Silencing the Message:
- To avoid seeing this message, you can redirect stderr to /dev/null:
```
nohup mycommand 2> /dev/null
```
  This way, the message won’t clutter your console output.
Viewing Output:
- If you want to see the output of the executed command later, you can follow these steps:
  - Execute the command:
```
$ tail -f nohup.out
```
  - Note that the nohup command creates a file named 'nohup.out' in the same location where you executed the nohup command.

Remember, this behavior is by design, and the message is simply informing you of the process’s behavior. If you need to monitor the output, use the tail

: Stack Overflow
: Stack Overflow
: Unix Stack Exchange

Understanding Input and Output Handling with nohup Command

The nohup command is a handy utility in Unix-like systems that allows you to run a process in the background even after you log out of your terminal session. When using nohup, there are a few important points to consider regarding standard input behavior:

Output Handling:
- By default, nohup writes the standard output and standard error of the command to a file called nohup.out in the current directory.
- However, if you’ve redirected the output elsewhere (such as to /dev/null), nohup won’t create the nohup.out file.
- For example, to suppress both standard output and standard error, you can use:
```
nohup command >/dev/null 2>&1
```
  Alternatively, in most shells, you can abbreviate this as:
```
nohup command >&/dev/null
```
Input Handling:
- On Linux, running a job with nohup automatically closes its input as well.
- However, on other systems (such as BSD and macOS), this behavior doesn’t apply. In those cases, when running a process in the background, you might want to manually close the input.
- Closing input doesn’t affect the creation of nohup.out, but it prevents another issue: if a background process tries to read from an open standard input, it will pause, waiting for you to bring it back to the foreground and type something.
- For a completely detached process, use:
```
nohup command /dev/null 2>&1 &
```
- To disassociate the background process from your shell’s process group (so it won’t receive signals from the shell), you can run:
```
disown
```
  (Note: This works in bash, ksh, or zsh.)

Remember that these details ensure smooth execution of background processes with nohup

Understanding and Customizing nohup Command Behavior

Let’s delve into the world of nohup, a handy Linux command that allows us to start processes and keep them running even after we close the terminal or lose our connection. However, there are a few nuances to be aware of:

Default Behavior:
- By default, when you use nohup, it redirects both output and error messages to a file named nohup.out.
- This can be useful for logging, but sometimes it’s not ideal, especially if your script generates a lot of unhelpful output.
Example Process:
- Imagine we have a simple shell script called simple.sh. It does two things:
  - Prints the current date and time every three seconds.
  - Attempts to list a specific file called missing.txt.
- Let’s run it:
```
$ cat simple.sh
#!/bin/bash
while true; do
    sleep 3
    date +"The current date and time is %F %T"
    ls missing.txt
done
```
- When we execute ./simple.sh, it prints the date and time and tries to list missing.txt.
Redirecting Output and Errors:
- To avoid the default behavior, we can redirect the standard streams:
```
$ ./simple.sh 1>simple.out 2>simple.err
```
  - Here, 1> redirects stdout (standard output) to simple.out.
  - 2> redirects stderr (standard error) to simple.err.
- Now, simple.out contains the date and time output, and simple.err contains the error from the ls command.
Running in the Background:
- To prevent a long-running process from tying up the terminal, run it in the background:
```
$ ./simple.sh 1>simple.out 2>simple.err &
```
  - The process ID (PID) is shown (e.g., 5281).
  - Verify with ps -o pid,cmd | grep 5281.
Customizing Output Location:
- If you want to redirect output and errors to custom files:
```
$ nohup [command] > /path/to/output/file.txt
```
  - For example:
```
$ nohup bash -c 'date && cal' > output.txt
```
  - This way, you can avoid the default nohup.out and tailor your log files.

Using nohup in Linux

The nohup command in Linux is a powerful tool for running processes that need to continue executing even after you log out of the terminal or close the shell. Let’s explore how to use it and some common examples:

Running a Process with nohup:
- To run a command using nohup without any arguments, follow this syntax:
```
nohup [command]
```
- The shell ignores the output and appends it to the nohup.out file.
- For instance, if you run a simple Hello World script called example.sh, the output will be captured in nohup.out:
```
nohup bash example.sh
```
- You can verify the contents of the file using:
```
cat nohup.out
```
Running a Process in the Background with nohup:
- Running a Linux process in the background frees up the terminal you’re working in.
- To run a process in the background with nohup, add the & symbol at the end of the command:
```
nohup [command] &
```
- For example, to run the example.sh script in the background:
```
nohup bash example.sh &
```
- The output will display the shell job ID and process ID (e.g., 1 7366).
- To bring the command to the foreground, type:
```
fg
```
Running Multiple Processes in the Background with nohup:
- You can run multiple commands simultaneously using nohup.
- Use the following format:
```
nohup bash -c '[command1] && [command2]'
```
- Replace [command1] and [command2] with your desired commands, separated by &&.
- For example, to show the date/time and the calendar of the current month:
```
nohup bash -c 'date && cal'
```
- The output is directed to nohup.out, so you can verify it using:
```
cat nohup.out
```

Remember that nohup is different from daemons. While daemons continuously run in the background, nohup is used for processes that take a long time but don’t continue running once done

Best Practices for Using nohup Command

When using the nohup command for running background processes, here are some best practices to keep in mind:

Check the Output File:
- Always inspect the nohup.out file for any output or errors generated by your command.
- If you prefer a different output file, you can use the > operator to specify an alternative file name.
Running a Process with nohup:
- To execute a command using nohup without any arguments, follow this syntax:
```
nohup [command]
```
- The shell will disregard the output and append it to the nohup.out file.
- For example, if you run a simple “Hello World” script named example.sh, the output will be stored in nohup.out:
```
nohup bash example.sh
```
  Verify the contents of the file with:
```
cat nohup.out
```
Running a Process in the Background with nohup:
- Running a process in the background frees up your terminal.
- To achieve this, add the & symbol at the end of the command:
```
nohup [command] &
```
- For instance, to run the example.sh script in the background:
```
nohup bash example.sh &
```
- The output will display the shell job ID and process ID (e.g., [1] 7366).
- To bring the command back to the foreground, type:
```
fg
```
Running Multiple Processes in the Background with nohup:
- Use the following syntax to run multiple commands simultaneously:
```
nohup bash -c '[command1] && [command2]'
```
- Replace [command1] and [command2] with your desired commands.
- Separate multiple commands with &&.
- For example, to display the date/time and the calendar of the current month:
```
nohup bash -c 'date && cal'
```
- The output will be directed to nohup.out.

Remember that while nohup is useful for long-running processes, it’s not the same as daemons. Daemons continuously run in the background, whereas nohup is suitable for processes that complete their tasks and then terminate.

In conclusion, the message “nohup: ignoring input and appending output to ‘nohup.out'” is a regular occurrence when utilizing the nohup command in Unix-like systems. Understanding that this message signifies the detachment of input and output from your console is essential for effective command execution. By redirecting stderr to /dev/null or customizing output locations, you can manage and monitor your processes efficiently.

Remember, the design of nohup to append output to ‘nohup.out’ streamlines background process handling, offering a seamless experience even after you log out of your terminal. Embrace the power of nohup and leverage its capabilities for smooth and uninterrupted task execution.

Aug 27, 2023
Roderick Webb
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