Running Processes In The Background: Best Practices And Pitfalls
Launching Background Processes
A background process runs without blocking the shell, allowing the user to continue typing commands at the prompt while the process executes. The shell does not wait for background processes to finish before prompting for further input. Contrast this with foreground processes, which fully occupy the shell terminal – no new commands can be issued until the foreground process exits.
There are a few ways to launch a background process instead of running it in the default foreground mode:
- Append the
&operator to the command before hitting enter. For example:long_script.sh & - Use the
nohupcommand to ignore theHUPsignal sent when the parent shell exits. This allows the background process to continue running after user logout.
Using & Operator to Background a Process
The simplest way to run a process in the background is to append & to the command before hitting return. For example:
$ long_task & [1] 31364 $ ./app start & [2] 31365
This immediately returns the shell prompt while launching the process to run independently without blocking shell input. The shell prints the job ID (between square brackets) that can be used to monitor or control the background process at a later time.
nohup Command for Persisting Processes
By default, background processes are still children of the parent shell, so they will be terminated if user logs out or the shell is closed. To make a background process persist independently:
$ nohup long_process &
The nohup utility ignores the HUP (hangup) signal sent to child processes when the parent shell exits. Combined with running a process in the background, this allows it to continue running even after user logs out or switches terminals.
Monitoring Background Jobs
The shell provides built-in job control tools for listing active background processes, foregrounding them back to interactive shells, and more. Understanding these job control commands is key for monitoring and managing pipelines kicked off in the background.
jobs Command for Listing Background Processes
The most basic job control command is jobs, which prints currently running jobs launched from the current shell session:
$ sleep 1000 & [1] 31490 $ sleep 2000 & [2] 31491 $ jobs [1]- Running sleep 1000 & [2]+ Running sleep 2000 &
This provides the job ID, state (running, stopped, or terminated), and the originating command. The + indicates the default job that will be acted upon by other job control commands.
fg and bg for Foregrounding/Backgrounding Jobs
The fg and bg commands transition a background job back to the foreground or background respectively:
$ sleep 1000 & [1] 31520 $ fg %1 sleep 1000[1]+ Stopped(SIGTSTP) sleep 1000 $ bg %1 [1]+ sleep 1000 &
This illustrates pulling the sleep job to the foreground, stopping it interactively, then backgrounding it again using job ID syntax. Interactive input is possible while a job runs in the foreground.
Using ps for Finer Process Inspection
While jobs shows background processes spawned in the current shell session, the ps command provides deeper system-level process inspection across all sessions and users:
$ ps -ef | grep sleep saml 23492 31420 0 10:59 pts/0 00:00:00 sleep 1000 saml 23520 31420 0 10:59 pts/0 00:00:03 sleep 4000
This lets administrators query and manage processes by UNIX username, PID, runtime stats, and other attributes.
Best Practices
Properly managing background and foreground jobs is key to smooth system operation. Keep these best practices in mind:
- Check for available IO channels before backgrounding processes that intensively read/write from stdin/stdout.
- Use shell builtins, child process signaling, and graceful shutdown code to stop background jobs.
- Always redirect output streams to files for critical background pipelines to capture job output.
Avoiding Blocking Foreground Process IO
Background tasks that attempt frequent interactive terminal input/output can freeze by blocking the available stdin and stdout channels:
$ (while true; do echo "Foo"; sleep 1; done) & [1] 31560 Foo Foo Foo[1]+ Stopped ( while true; do echo Foo; sleep 1; done ) $
The foreground shell is blocked from input until the background job finishes or is forcibly stopped. Check stdin/stderr availability with bash -m before backgrounding IO-intensive processes.
Careful Use of Shell Builtins and Signal Handling
To terminate background jobs, avoid Kill signals. Instead, use POSIX shell builtins like kill %job_id that allow child processes to shutdown cleanly via signal handlers:
sleep_proc(){
trap "echo 'Sleep process shutting down...'; exit 0" SIGTERM
sleep 10000
}
sleep_proc &
kill %1
This defines a simple signal handler to catch SIGTERM shutdown requests from the shell parent process.
Example Code for Graceful Shutdown
Here is reference code for shutting down child processes gracefully when shell sessions exit:
trap "kill 0" EXIT backup_proc & compression_proc & wait # Parent shell waits until last background job exits
This trap handler sends standard SIGTERM to the process group on parent exit. The wait instruction blocks the shell until all entries in the job table terminate, preventing immediate forced termination.
Common Pitfalls
Take care to avoid these problematic scenarios when running background processes:
- Orphaned child processes persisting after parent session exits.
- Resource contention between foreground and computationally intensive background jobs.
- Failure to capture or log output streams from long-running background pipelines.
Orphaned Processes and Zombie Processes
“Orphaned” processes with expired parent sessions and “zombie” entries for terminated jobs clutter the process table and system resources:
$ ps -ef | grep zombie saml 9102 1 0 Jan05 ? 00:00:00 [mongodb] <defunct>
ThisOutputs “defunct” entries indicating dead parents. Routinely audit background process trees.
Resource Contention with Foreground Jobs
CPU or memory-intensive background processes can freeze responsive UX for foreground jobs sharing resources on the same system:
# running in background $ app build -t & # UI lag is noticeable $ app gui
Monitor system load and isolate resource groups with utilities like cgroups.
Forgetting to Save Output
Failure to capture stdout/stderr for background pipeline can lead to lost logs and monitoring data:
$ long_job & # output prints to terminal but is not saved!
Always redirect streams to files like long_job > /logs/long_job.log to retain output.
