A Developer‘s Guide to Redirecting Nohup Output in Linux

As a developer, being able to run processes resiliently in the background is critical for automating tasks or providing always-available services. The nohup command is one of the simplest ways in Linux to launch background jobs that keep running even after exiting the shell or SSH session that triggered them.

By default, nohup redirects output to a nohup.out file. But as developers, we often need more customization and flexibility around logging. That‘s where output redirection comes in handy.

This comprehensive guide will cover:

• Common use cases for redirecting nohup output
• Detailed examples of managing background stdout/stderr streams
• Comparison to alternative solutions like systemd services
• Expert techniques for handling output and logging
• Best practices for using nohup in development environments

Whether you need to debug a tricky node process, isolate logs from cron jobs, or simply get more visibility into your background jobs, mastering nohup output redirection is an essential skill.

Why Redirect the Output of Nohup as a Developer

Before diving into the details, it‘s worth highlighting some common reasons for customizing where nohup saves output:

Log Rotation

Log files quickly grow very large. Redirecting to custom files makes it easier to set up log rotation to prevent clogged disks.

Log Classification

Sorting logs by source process avoids cluttering one general file. This helps pinpoint issues.

Resource Monitoring

By tying logs to specific applications, you can better correlate performance metrics like CPU usage.

Debugging

Redirecting to localized files prevents debugging confusion when running multiple background processes.

Deployment Traceability

Per-process logging improves tracing which command executions happened across multiple servers.

As over 87% of developers use nohup according to Stack Overflow surveys, tackling these scenarios by redirecting output is invaluable.

Core Concepts of Nohup I/O Streams

Before redirecting nohup‘s output, it helps to understand a bit about how Linux handles input and output streams conceptually.

There are three default streams at a low level:

Stream	Description
stdin	Input stream, file descriptor 0
stdout	Output stream, file descriptor 1
stderr	Error output stream, file descriptor 2

When you launch a process in Linux, you inherit these default streams – usually connected to the terminal.

The key idea behind nohup is that it detaches the process from the terminal so that closing SSH or exiting your shell session does not disrupt background processes using these streams.

By redirecting these stream file descriptors with > and 2> syntax, you get to control where the streams write data instead of just going to the terminal.

This sets the foundation for custom logging approaches.

Recommended Ways to Redirect Nohup Output

With the basics covered, let‘s explore some recommended patterns for actually implementing output redirection in various use cases.

We‘ll look at methods for controlling both standard out (stdout) and standard error (stderr) logging.

Combined stdout and stderr

The simplest approach is sending both stdout and stderr to the same file.

For example:

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

Breaking this down:

• > Redirects stdout to the log file
• 2>&1 makes stderr follow the same path as stdout
• Trailing & runs the process in the background

This keeps things clean by consolidating all output in one place.

Separate stdout and stderr Files

However, often developers want to isolate stdout and stderr logging.

For example, runtime output versus error logs:

nohup my_script.sh > my_logs/my_script.stdout 2> my_logs/my_script.stderr &

Now regular program output goes to my_script.stdout while errors get logged to my_script.stderr.

This splits things up to make debugging easier when needed.

Create Log Directory Tree by Process

Rather than dumping logs together into one directory, you can organize by background process:

nohup my_script.sh > my_logs/my_script/stdout.log 2> my_logs/my_script/stderr.log &

This keeps everything neatly packaged in one area on a per-process basis.

Rotate Log Files by Size

To manage growing log files, redirect output to self-rotating logs based on size.

For example using logrotate:

nohup my_script.sh > >(logger -t my_script -s 2>/dev/null) 2>&1 &

This pipes stdout/stderr into logger, which handles rotating output across 2 MB log files. Clean!

Mirror Output to Syslog

For centralized logging, take advantage of the system-wide syslog service:

nohup my_script.sh 2>&1 | tee -a /var/log/my_script.log | logger -t my_script

Here we take advantage of tee to split the streams – sending output to a local file as well as syslog. This gives both isolated and centralized log access.

As you can see, leveraging redirection syntax provides tons of options.

Alternative Approaches to Background Processes

While this guide focuses on redirecting nohup specifically, it‘s worth comparing to a couple alternative approaches to detached background processes – especially systemd and screen.

systemd Services

On modern Linux distributions, systemd has become the standard for spawning daemon processes. These involve configuration files with directives for handling output streams.

Some key advantages over nohup:

• More configuration options for logs with journalctl
• Easier to manage for complex applications
• Better process lifecycle management

However, nohup wins for quick ad-hoc background tasks because:

• No configuration needed
• Ideal for fire-and-forget processes
• Great for scripts executed over SSH

So consider systemd for formal production services, while nohup is perfect for personal utilities.

GNU Screen

GNU screen is another alternative for persistent terminal sessions in the background. This works by multiplexing a single physical terminal into virtual sessions rather than fully detaching processes.

Benefits include:

• Keeps processes bound to terminal for ncurses UIs
• Avoid having to reconnect to interrupt jobs
• Sessions persist even if network drops

Downsides are:

• Additional utility to install/manage
• Extra complexity for straight background execution

For developers needing an interactive environment nohup cannot provide, screen is fantastic. But for running batch jobs, nohup‘s simplicity generally works better.

Expert Patterns for Advanced Use Cases

While the basics of redirecting stdout and stderr cover many cases, nohup logging can become more complex in some situations like multiple chained processes, internal process logging, or output handling in container environments.

Let‘s take a look at some expert-level patterns for addressing more intricate logging scenarios when using nohup.

Prefixing/Tagging Logs

When combining outputs from multiple commands, tagging logs allows differentiating their source.

For example:

nohup sh -c ‘{ 
    echo [cmd1] Running command 1...; 
    command1; 
    echo [cmd2] Running command 2...;
    command2;
}‘ > combined.log 2>&1 &

The log output will now be clearly marked coming from either command 1 or 2 accordingly.

Centralized Logging via Remote Syslog

For teams running infrastructure across disparate systems, funneling all nohup logs into a centralized rsyslog server is essential:

nohup my_script.sh 2>&1 | tee -a /var/log/my_script.log | logger -n 192.168.1.100 -P 514 -t my_script

Here we take advantage of the standard syslog protocol over TCP to ship logs from source machines to the aggregator at 192.168.1.100 for correlation.

Handling Docker Container Logs

Running detached processes with nohup inside Docker containers introduces interesting logging challenges.

Rather than redirecting output locally within containers, send logs directly to Docker‘s logging driver configured on the daemon using special redirects:

nohup my_script.sh > >(/usr/bin/logger -t my_script) 2>&1

Now stdout and stderr will bypass container storage and route through Docker logging.

Buffer Logs Before Writing

For applications generating high-frequency writes, buffering before async file handling reduces IO load:

nohup my_script.sh | stdbuf -oL awk ‘{line=$0; print line; system(""tee -a log_file <<< "line"}‘;}‘

This buffers stdout line-by-line before asynchronously appending to the log file as the buffer fills up.

Multithreading Log Writing

Similarly, dispatching write operations across threads helps control bottlenecks:

nohup my_script.sh 2>&1 | stdbuf -oL python3 log_threaded.py  

Where log_threaded.py implements a producer/consumer queue to enable parallel writes.

There are many more exotic logging patterns like these possible once you become comfortable manipulating stdin/stdout streams!

Best Practices for Developers Using Nohup

While redirecting output is the primary focus, following some general best practices helps improve the entire experience using nohup.

Prefix Batch Commands

Rather than just launching a process directly with nohup, wrap it in a shell script beginning with commands like:

#!/bin/bash
exec > ./logs/nohup.out 2>&1
date
echo "Command executed: $0 $*"  
# Then run actual command...

This adds vital context on when the job ran and what exact invocation triggered it.

Track Process Origin Users

Tag logs with the invoking Linux user via:

whoami >> ./logs/nohup.out

This audits what account launched background processes.

Handle Cleanup on Exit

Use trap to register cleanup functions that run on termination:

trap ‘rm -f $TEMP_FILE; echo "Exiting..." >> log.out‘ EXIT INT TERM

This deletes temp files and logs shutdown events.

Following best practices like these for all nohup usage improves reliability and understanding of your background environments.

Conclusion and Recommendations

Being able to redirect the output of nohup commands allows developers finely-grained control over logging and process visibility.

Here are some key recommendations based on everything covered:

Leverage separate stdout and stderr – Differentiate output types into isolated logs organized by process and stream.

Consider syslog integration – Centralized logging often easier to manage long-term than local files.

Add log identifier tags – Help disambiguate sources in combined streams.

Follow file size log rotation policy – Prevents storage issues with unbounded growth.

Handle containers properly – Route logs through container runtime driver rather than within containers.

Buffer writes for high-volume output – Reduce logging overhead and increase performance.

Mind security and best practices – Tag logs with key metadata like invoking user.

While nohup‘s out-of-the-box logging works, explicitly configuring redirection unlocks many advantages. Mastering the various methods for customizing stdout and stderr described here will save developers hours of frustration debugging background processes.

Redirecting output is an easy way to tame unruly detached process logging!