In linux, each running program will create at least one process in memory, and each process will be assigned a process ID, we can use the ID of the process to do the corresponding operation of a running program.

Prepare the Necessary Shell Script

Let’s create a shell script program that doesn’t exit itself to do the demonstration:

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echo '#!/bin/bash' > endless_job.sh
echo 'while true; do continue; done' >> endless_job.sh
cat endless_job.sh

We’re not going to talk too much about the shell script right now. The only thing we need to know is that this script doesn’t exit itself after it runs.

Execute Scripts in the Background

Let’s execute two scripts one at a time and let them run constantly in the background:

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bash ./endless_job.sh &
bash ./endless_job.sh &

List All Programs Running in the Background

We can use the jobs command to see which programs are running in the background.

Some common parameters of the jobs command are:

Let’s take a look at the output of the jobs command without specifying parameters:

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jobs

List With Their Process IDs

List the jobs and the process ID for each program with the -l option:

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jobs -l

List the Process IDs Only

List the jobs and the process ID for each program with the -l option:

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jobs -p

See How the Program is Running in the Background

We can see how the two scripts work through the top command:

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top

Through the screenshot, we know that there are three programs currently running, two of which are the two scripts we just ran, and the other is the top command itself.

We also found that both scripts have a common problem, which is that they consume CPU very much during their run-time.

Terminate Running Processes

We can use the kill command to end the life cycle of a running process with some associated signal values.

The kill command sends a TERM(15) signal to a process by default:

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kill process_id [more process IDs]    # This is equivalent to both kill -TERM and kill -15

Some common signal options are:

The kill command has three running results, success, failure, and partial success:

Forced Termination of a Running Process

But sometimes situations are special, and some processes cannot be terminated by the default signal.

Then we need to use the KILL(9) signal to terminate the process:

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kill -KILL process_id [more process IDs]    # This is equivalent kill -9

Unforced Termination of a Running Process

Usually, we would only use the TERM(15) signal to terminate a running process:

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kill process_id [more process IDs]

Because the code for this script is relatively simple, so the two processes running based on this script can be terminated using the kill command’s default signal:

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kill 9476 9475
jobs

From the screenshot, we can see that both processes have been killed.

References 7.2 Job Control Builtins, 24 Process control, 24.1 kill: Send a signal to processes, 2.5 Signal specifications, KILL(1)

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