nohup Background Process

What it is

nohup is a command-line utility that allows you to run commands or scripts that will continue to run even after you log out of your terminal session. It’s particularly useful for long-running background tasks.

Installation

nohup is a standard Unix utility and is available on virtually all Linux and macOS systems by default. No installation is typically required.

Core Concepts

• HUP Signal: When a terminal session ends, it sends a SIGHUP (hangup) signal to all processes associated with that session. By default, most processes will terminate upon receiving SIGHUP.
• nohup’s Behavior: nohup intercepts the SIGHUP signal, preventing the command it’s running from terminating. It also redirects standard output and standard error to a file (typically nohup.out) if they are not already redirected.

Commands / Usage

Running a Command in the Background and Detaching from Terminal

nohup ./my_long_script.sh &

Starts my_long_script.sh in the background, making it immune to terminal hangup signals. Output is appended to nohup.out.

nohup python3 my_server.py &

Runs a Python web server in the background, ensuring it keeps running after logout.

nohup find / -name "*.log" > /tmp/found_logs.txt &

Runs a find command, redirects its standard output to /tmp/found_logs.txt, and detaches it from the terminal. Standard error will still go to nohup.out.

Redirecting Output and Error

nohup ./my_script.sh > output.log 2>&1 &

Runs my_script.sh in the background, redirecting both standard output and standard error to output.log.

nohup ./my_script.sh > /dev/null 2>&1 &

Runs my_script.sh in the background and discards all its output (stdout and stderr). Useful for processes you don’t need to monitor via logs.

nohup ./my_script.sh & echo "Process started"

Starts my_script.sh in the background and immediately prints "Process started" to your current terminal.

Specifying an Output File

nohup -p my_process.pid ./my_script.sh &

This is not a standard nohup option. nohup itself doesn’t have a -p flag to specify a PID file. The common pattern is to redirect output manually.

Correction: nohup does not have a flag to specify the output file directly. It defaults to nohup.out. You must use shell redirection (>, 2>&1) to control output file location.

Example with a Command String

nohup sh -c 'echo "Starting process"; sleep 60; echo "Done"' &

Runs a short shell command string in the background, detached from the terminal.

Common Patterns

Running a long-running server process

nohup gunicorn --bind 0.0.0.0:8000 myapp:app > app.log 2>&1 &

Starts a Gunicorn web server, logs all output to app.log, and detaches it.

Running a data processing job

nohup python3 data_processor.py --input data.csv --output results.json &

Executes a Python data processing script in the background.

Monitoring nohup.out

nohup ./my_script.sh &
tail -f nohup.out

Starts a script and then continuously monitors its output file.

Finding and killing a nohup process

First, find the process ID (PID):

pgrep -f "my_long_script.sh"

Then, kill it:

kill <PID>

Or force kill:

kill -9 <PID>

Gotchas

• Default Output File: If you don’t redirect stdout and stderr, nohup will create (or append to) a file named nohup.out in the current working directory where you ran the nohup command. If this directory is not writable, the command will fail.
• SIGHUP Behavior: nohup prevents the command itself from receiving SIGHUP. However, other signals might still affect it.
• No Interactive Input: Commands run with nohup cannot read from standard input interactively. If your command requires user input, it will likely hang or fail.
• Shell Redirection is Key: To effectively manage where output goes, you must use shell redirection (>, 2>&1). nohup doesn’t offer built-in flags for this.
• Backgrounding (&): nohup alone doesn’t put the process in the background. You almost always need to combine it with the shell’s background operator (&).