Launching Background Processes and Detaching Jobs in Linux

Running programs in Linux often monopolizes your shell, preventing other tasks until the application exits. However, Linux offers flexible process control for executing commands in the background as well as fully detaching jobs from the terminal.

As a professional Linux system administrator, I utilize detached and background processes extensively for automating production systems without blocking interactive SSH sessions. Here‘s my in-depth guide on running Linux commands in the background and detaching jobs.

Why Run Commands in the Background?

There are several benefits for executing processes in the background or detaching jobs entirely in Linux:

• Reclaim shell access – Move long-running programs out of the foreground to issue additional commands without waiting for completion.
• Persist across sessions – Fully detached processes continue running even after terminal exit.
• Simplify process management – Background jobs help control and organize multiple programs.
• Improve performance – Overlapping CPU and memory usage with parallel execution.
• Install software – Run installers in the background while working on other tasks.
• Automate file transfers – Background copy/sync so you can continue editing the source files.

Let‘s explore the technical details…

How Linux Process Control Works

When launching an application like firefox in Linux, a parent process is created for the terminal or shell prompt you used. Firefox spawns a child process that runs independently but is linked to the parent.

By default, applications started in the terminal run as foreground processes with:

• Input connected to terminal keyboard
• Output printed to terminal display

If the parent shell exits, Linux sends a SIGTERM signal to kill associated child foreground processes.

Background Processes

Adding a trailing & to a command tells Linux to execute it as a background process instead:

$ firefox &

Now Firefox runs with:

• No direct terminal keyboard input
• Output redirected to a buffer instead of print display

The shell prompt returns immediately so you can continue working.

Background processes ignore terminal closure and keep running after shell exit, but may be impacted if Linux needs to free up RAM or CPU resources.

Detached Processes

Fully detached processes have no dependency or relationship to the parent shell process at all. They are not killed on terminal exit and do not receive terminal signals.

For example, a server daemon like nginx launched via systemd runs as an independent detached process. Detached daemons persist until the system shuts down or the process explicitly terminates.

Now we‘ll explore the techniques for executing processes in the background and detaching jobs.

Launch a Background Process

The most basic background command syntax appends a & character:

$ long_running_command &

For example, pinging a server in the background:

$ ping google.com &
[1] 21692

The returned PID confirms the process running with job number 1.

Verify with jobs:

$ jobs
[1]+ Running                 ping google.com &

Our shell prompt returns immediately, allowing more commands while ping continues executing.

Stopping Background Processes

Background processes continue until complete or forcibly stopped.

To halt a background job without killing it, use Ctrl + Z shortcut:

$ ping google.com &
<Ctrl+Z pressed>
[1]+ Stopped                 ping google.com

Check currently stopped jobs with jobs:

$ jobs
[1]+ Stopped                 ping google.com

Then resume stopped jobs either in the background with bg or foreground with fg:

$ bg %1
$ fg %1
<Ctrl+C to kill job>

Note the %1 refers to job 1 from jobs output.

Detach a Process from the Terminal

By default, background processes still terminate if the parent shell exits since they remain tied to the terminal.

We can fully detach jobs using disown so they persist beyond the session:

$ long_job &
[1] 23501
$ disown -h %1

disown detaches process 23501 from the shell. -h also removes it from the shell‘s job list.

Now the process will ignore any HUP signals on shell exit and continues running detached.

nohup – Ignore HUP Signals

The nohup command launches a process that detaches and ignores terminal HUP signals:

$ nohup iterative_script.sh &> /dev/null & 

This starts iterative_script.sh redirected to /dev/null to dispose output. The trailing & runs it in the background as well.

When our shell/terminal exits, nohup ensures iterative_script.sh keeps running rather than terminating.

By ignoring HUP, nohup provides a quick way to detach trivial scripts or one-off commands. But for more complex process trees, a containerized approach may be better.

Containers vs Background Processes

Modern Linux administrators often utilize containerization platforms like Docker rather than raw background processes:

• Easier process tracking – Containers have unique names and metadata for managing groups of processes. A background PID becomes ambiguous quickly.
• Improved reliability – Containers guarantee resources and enforce quotas, limiting impact on other host processes. Background processes have no constraints.
• Consistent deployment – Container images provide pre-configured application environments vs local shell setup.
• Portability – Containers run identically on any Linux host without dependency conflicts.

With Docker, we can run the equivalent of a detached background process:

$ docker run -d --name my_process busybox sleep 1000000
d297562de6c1f3e4149e9300d1fa8a23b595d40fa36384dc9db76c5cfc15d05a

This launches a container named my_process that will sleep in the background for 1 million seconds, detached from our terminal. The container persists across sessions – we can track its status and logs or reconnect to manage the processes running inside it.

Containers facilitate running detached applications in production Linux environments. But raw background and detached processes still serve purposes on client desktops or temporary automation.

tmux – Detach Terminal Sessions

tmux is a terminal multiplexer allowing multiple console programs to run within detached sessions. Think of it like a terminal emulator with background process and job control superpowers.

Common uses:

• Launch remote headless programs that persist after SSH disconnection
• Manage long running shell scripts without blocking terminal
• Reattach and observe the output of background processes

For example, run a process detached in the a tmux session:

$ tmux new-session -d ‘tail -f /var/log/syslog‘

This starts a background tail session tracing /var/log/syslog.

Later we can reattach to view the logged output:

$ tmux attach 

Aug 22 22:01:01 linux4 nohup: redirecting stderr to stdout
Aug 22 22:01:06 linux4 systemd: Started Session c2 of user root.
Aug 22 22:01:39 linux4 systemd-logind: New session 1 of user jsmith.

When finished inspecting, Ctrl+B then D detaches again without interrupting tail.

tmux allows you to manage persistent processes across terminal sessions. The screen tool offers similar functionality for process continuity.

Orphan Processes

When a parent shell exits unexpectedly, leftover child processes often become orphaned.

Shells treat background jobs differently on exit:

• Jobs started with & are sent SIGHUP signal and terminated
• Jobs disowned with disown persist but become orphaned

Orphan processes continue running though they lack a parent shell monitoring them. This can cause unintended resource usage without visibility.

Always detach intended long-running tasks explicitly using nohup or containers rather than background jobs. Then gracefully halt detached processes when finished.

Potential Issues

While background processes help workflows, beware risks from excessive detached jobs:

• Orphan process accumulation
• System performance degradation
• Log file overconsumption
• Difficult debugging
• Security policy violations

Set up monitoring like log rotation, disk quotas, and system limits to control risks. Fully utilize containers where possible since they enforce isolated resources.

Also design automation scripts carefully to chain background steps cleanly. Always trap signals to handle unexpected termination gracefully.

Here is an example Bash script executing interdependent jobs in the background:

#!/bin/bash

# Start long running process
long_job &
LONG_JOB_PID=$!

# Wait for completion before next step
wait $LONG_JOB_PID 

# Ensure next job runs even if wait fails
trap ‘start_dependent_job‘ ERR   

function start_dependent_job() {
  dependent_job &
  DEP_JOB_PID=$! 

  wait $DEP_JOB_PID
  trap - ERR
}  

This launches long_job, captures its PID, waits to complete, then starts dependent_job. Trapping ERR signals lets us continue process chains on failure.

Carefully constructed scripts keep background pipeline sequences tidy.

Conclusion

Running Linux commands in the foreground blocks shell access until completion. Background and detached processes allow:

• Parallel execution of multiple commands
• Long-running tasks to persist beyond session
• Automation sequences without blocking terminals

But beware downsides like accumulating orphan processes or resource overload. Containers provide isolation preferable for distributed applications.

Learning Linux process control techniques levels up shell scripting and system automation capabilities. Certification study guides like RHCSA help explain POSIX process concepts useful for administering background jobs.

With robust monitoring and orchestration, background processes unlock efficient Linux usage. Let me know if you have any other best practices to share!