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Learning Linux Shell Scripting
Learning Linux Shell Scripting

Learning Linux Shell Scripting: Leverage the power of shell scripts to solve real-world problems , Second Edition

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Learning Linux Shell Scripting

Drilling Deep into Process Management, Job Control, and Automation

In the last chapter, we introduced ourselves to the Bash shell environment in Linux. You learned basic commands and wrote your first shell script as well.

You also learned about process management and job control. This information will be very useful for system administrators in automation and in terms of solving many problems.

In this chapter, we will cover the following topics:

  • Monitoring processes with ps
  • Job management–working with fg, bg, jobs, and kill
  • Exploring at and crontab

Introducing process basics

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.
  1. The boot loader program then copies the kernel in the memory and, after basic checks, it calls a kernel function start_kernel().
  2. The kernel then initializes the OS and creates the first process called init.
  3. You can check the presence of this process with the following command:
    $ ps -ef  
  1. Every process in the OS has one numerical identification associated with it. It is called a process ID. The process...

Monitoring processes using ps

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...

Process management

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...

Process monitoring tools – top, iostat, and vmstat

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...

Understanding "at"

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...

Understanding crontab

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...

Summary

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.

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Key benefits

  • •Learn to impeccably build shell scripts and develop advanced applications
  • •Create smart solutions by writing and debugging scripts
  • •A step-by-step tutorial to automate routine tasks by developing scripts

Description

Linux is the most powerful and universally adopted OS. Shell is a program that gives the user direct interaction with the operating system. Scripts are collections of commands that are stored in a file. The shell reads this file and acts on commands as if they were typed on the keyboard. Learning Linux Shell Scripting covers Bash, GNU Bourne Again Shell, preparing you to work in the exciting world of Linux shell scripting. CentOS is a popular rpm-based stable and secured Linux distribution. Therefore, we have used CentOS distribution instead of Ubuntu distribution. Linux Shell Scripting is independent of Linux distributions, but we have covered both types of distros. We start with an introduction to the Shell environment and basic commands used. Next, we explore process management in Linux OS, real-world essentials such as debugging and perform Shell arithmetic fluently. You'll then take a step ahead and learn new and advanced topics in Shell scripting, such as decision making, starting up a system, and customizing a Linux environment. You will also learn about grep, stream editor, and AWK, which are very powerful text filters and editors. Finally, you'll get to grips with taking backup, using other language scripts in Shell Scripts as well as automating database administration tasks for MySQL and Oracle. By the end of this book, you will be able to confidently use your own shell scripts in the real world.

Who is this book for?

Learning Linux Shell Scripting is ideal for those who are proficient at working with Linux and want to learn about shell scripting to improve their efficiency and practical skills.

What you will learn

  • •Familiarize yourself with the various text filtering tools available in Linux
  • •Understand expressions and variables and how to use them practically
  • •Automate decision-making and save a lot of time and effort of revisiting code
  • •Get to grips with advanced functionality such as using traps, dialogs to develop screens & Database administration such as MySQL or Oracle
  • •Start up a system and customize a Linux system
  • •Taking backup of local or remote data or important files.
  • •Use existing other language scripts such as Python, Perl & Ruby in Shell Scripts
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Publication date : May 21, 2018
Length: 332 pages
Edition : 2nd
Language : English
ISBN-13 : 9781788993197
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Table of Contents

16 Chapters
Getting Started and Working with Shell Scripting Chevron down icon Chevron up icon
Drilling Deep into Process Management, Job Control, and Automation Chevron down icon Chevron up icon
Using Text Processing and Filters in Your Scripts Chevron down icon Chevron up icon
Working with Commands Chevron down icon Chevron up icon
Exploring Expressions and Variables Chevron down icon Chevron up icon
Neat Tricks with Shell Scripting Chevron down icon Chevron up icon
Performing Arithmetic Operations in Shell Scripts Chevron down icon Chevron up icon
Automating Decision-Making in Scripts Chevron down icon Chevron up icon
Automating Repetitive Tasks Chevron down icon Chevron up icon
Working with Functions Chevron down icon Chevron up icon
Using Advanced Functionality in Scripts Chevron down icon Chevron up icon
System Startup and Customizing a Linux System Chevron down icon Chevron up icon
Pattern Matching and Regular Expressions with sed and awk Chevron down icon Chevron up icon
Taking Backup and Embedding Other Languages in Shell Scripts Chevron down icon Chevron up icon
Database Administration Using Shell Scripts Chevron down icon Chevron up icon
Other Books You May Enjoy Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Half star icon Empty star icon 3.4
(5 Ratings)
5 star 60%
4 star 0%
3 star 0%
2 star 0%
1 star 40%
Tapan kumar Nov 01, 2018
Full star icon Full star icon Full star icon Full star icon Full star icon 5
well written
Amazon Verified review Amazon
mohamed azarudeen Jul 21, 2019
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Easy to learn and understand
Amazon Verified review Amazon
James Oct 16, 2019
Full star icon Full star icon Full star icon Full star icon Full star icon 5
well laid out reference book with plenty of useful examples
Amazon Verified review Amazon
flohoe May 29, 2020
Full star icon Empty star icon Empty star icon Empty star icon Empty star icon 1
This is by far the worst book I read from Packt. Well I just started, won't finish. Was there ever some editing?! Not only about the content (more or less copy paste from everywhere a bit, no real thread), but there are just wrong scripts sometimes, text/screenshots are missing. Poor quality, don't buy
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Kristjan Stefansson Oct 04, 2018
Full star icon Empty star icon Empty star icon Empty star icon Empty star icon 1
This is by far the worst computer and programming book I've ever read, I am still waiting for my mind to come back to former state after all the confusion. Don't buy it. It's a waste of time because it's badly written and confusing and nothing in it you can't find on the web.
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