A running instance of a program is called a process. A program stored in the hard disk or pen drive is not a process. When that stored program starts executing, then we say that process has been created and is running.

Let's very briefly understand the Linux operating system boot-up sequence:

1. In PCs, initially, the BIOS chip initializes system hardware, such as PCI bus, and display device drivers.
2. Then the BIOS executes the boot loader program.

3. The boot loader program then copies the kernel in the memory and, after basic checks, it calls a kernel function start_kernel().
4. The kernel then initializes the OS and creates the first process called init.
5. You can check the presence of this process with the following command:

    $ ps -ef  

6. Every process in the OS has one numerical identification associated with it. It is called a process ID. The process...

We have used the ps command in the introduction. Let's learn more about it:

• To list the processes associated with our current Bash shell Terminal, enter the following command:

    $ ps  

• To list processes, along with the parent process ID associated with the current Terminal, enter the following command:

    $ ps -f

• We can see the process ID in the PID column and the parent process ID, in the PPID column in the preceding output.

• To list processes with the parent process ID along with the process state, enter the following command:

    $ ps -lf

• In the preceding output, the column with S (state) shows the current state of a process, such as R for running and S for suspended state.

• To list all the processes running in the operating system, including the system processes, enter the following command:

    $ ps -ef 

• The process names...

Since we have understood the command to check processes, we will learn more about managing different processes.

• In a Bash shell, when we enter any command or start any program, it starts running in the foreground. In such a situation, we cannot run more than one command in the foreground. We need to create many Terminal windows for starting many processes. If we need to start many processes or programs from the same Terminal, then we will need to start them as background processes.
• If we want to start a process in the background, then we need to append the command in the Bash shell by &.
• If I want to start my Hello program as the background process, then the command would be as follows:

    $ Hello &

• If we terminate any command by &, then it starts running as the
  background process.

For example, we will issue a simple sleep command, which creates...

We can view the native performance of various processes in an OS by using the following tools:

• To view a dynamic real-time view of the running processes in an OS, use the following command:

    $ top

An explanation of the top command generated output is as follows:

The $top command displays a lot of information about the running system.

The first line of the display is shown as follows:

The description of fields in the first line is as follows:

• Current time
• System uptime
• Number of users logged in
• Load average of 5, 10, and 15 minutes, respectively

The second line is shown as follows:

This line shows the summary of tasks or processes. It shows the total number of all the processes, which includes the total number of running, sleeping, stopped, and zombie processes. The third line is shown as follows:

This line shows...

Many a time, we need to schedule a task for a future time, say in the evening at 8 p.m. on a specific day. We can use the at command in such a situation.

Sometimes, we need to repeat the same task at a specific time, periodically, every day, or every month. In such situations, we can use the crontab command.

Let's learn more about the use of the at command. To use the at command, the syntax is as follows:

    $ at time date

The following are examples of the at command:

• The Ctrl + D command will save the at job. The task will be executed at 11.15 A.M. This command will log messages to the log.txt file at 11.15 a.m.:

    $ at 11.15 AM
    at >  echo "Hello World" > $HOME/log.txt
    at >  Control + D

• The following command will send an email on March 31, 2015, at 10 A.M.:

    $ at 10am mar 31 2015
    at> echo "taxes...

If we need to run a specific task repetitively, then the solution is to use crontab. The syntax of the command is as follows:

    $ crontab -e

This will open a new editor. The following diagram is the syntax to add tasks. The fields to use for repeating tasks at a particular time are explained here:

Finally, to save the jobs, use the following:

    Press Esc then type :wq
    

The preceding operations will save the job and quit crontab.

The following are a few examples of the crontab command:

• Use the following command to run a script every hour at the fifth minute, every day:

    5 * * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1

• Use the following command to run 5 minutes after midnight every day:

    5 0 * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1

• Use the following command to run at 2.15 p.m. on the first of...

In this chapter, we studied basic process management. You learned about the ps command. Using commands such as jobs, fg, bg, kill, and pkill, we studied job management. Later on, you learned about the top, iostat, and vmstat process monitoring tools.

In the next chapter, you will learn about standard input/output, various meta–characters, and text filters used in shell scripting.