Table of Contents for
Learning Linux Shell Scripting

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition Learning Linux Shell Scripting by Ganesh Sanjiv Naik Published by Packt Publishing, 2015
  1. Cover
  2. Table of Contents
  3. Learning Linux Shell Scripting
  4. Learning Linux Shell Scripting
  5. Credits
  6. About the Author
  7. Acknowledgments
  8. About the Reviewers
  9. www.PacktPub.com
  10. Preface
  11. What you need for this book
  12. Who this book is for
  13. Conventions
  14. Reader feedback
  15. Customer support
  16. 1. Getting Started and Working with Shell Scripting
  17. Tasks done by shell
  18. Working in shell
  19. Learning basic Linux commands
  20. Our first script – Hello World
  21. Compiler and interpreter – difference in process
  22. When not to use scripts
  23. Various directories
  24. Working more effectively with shell – basic commands
  25. Working with permissions
  26. Summary
  27. 2. Drilling Deep into Process Management, Job Control, and Automation
  28. Monitoring processes using ps
  29. Process management
  30. Process monitoring tools – top, iostat, and vmstat
  31. Understanding "at"
  32. Understanding "crontab"
  33. Summary
  34. 3. Using Text Processing and Filters in Your Scripts
  35. IO redirection
  36. Pattern matching with the vi editor
  37. Pattern searching using grep
  38. Summary
  39. 4. Working with Commands
  40. Command substitution
  41. Command separators
  42. Logical operators
  43. Pipes
  44. Summary
  45. 5. Exploring Expressions and Variables
  46. Working with environment variables
  47. Working with read-only variables
  48. Working with command line arguments (special variables, set and shift, getopt)
  49. Understanding getopts
  50. Understanding default parameters
  51. Working with arrays
  52. Summary
  53. 6. Neat Tricks with Shell Scripting
  54. The here document and the << operator
  55. The here string and the <<< operator
  56. File handling
  57. Debugging
  58. Summary
  59. 7. Performing Arithmetic Operations in Shell Scripts
  60. Using the let command for arithmetic
  61. Using the expr command for arithmetic
  62. Binary, octal, and hex arithmetic operations
  63. A floating-point arithmetic
  64. Summary
  65. 8. Automating Decision Making in Scripts
  66. Understanding the test command
  67. Conditional constructs – if else
  68. Switching case
  69. Implementing simple menus with select
  70. Looping with the for command
  71. Exiting from the current loop iteration with the continue command
  72. Exiting from a loop with a break
  73. Working with the do while loop
  74. Using until
  75. Piping the output of a loop to a Linux command
  76. Running loops in the background
  77. The IFS and loops
  78. Summary
  79. 9. Working with Functions
  80. Passing arguments or parameters to functions
  81. Sharing the data by many functions
  82. Declaring local variables in functions
  83. Returning information from functions
  84. Running functions in the background
  85. Creating a library of functions
  86. Summary
  87. 10. Using Advanced Functionality in Scripts
  88. Using the trap command
  89. Ignoring signals
  90. Using traps in function
  91. Running scripts or processes even if the user logs out
  92. Creating dialog boxes with the dialog utility
  93. Summary
  94. 11. System Startup and Customizing a Linux System
  95. User initialization scripts
  96. Summary
  97. 12. Pattern Matching and Regular Expressions with sed and awk
  98. sed – noninteractive stream editor
  99. Using awk
  100. Summary
  101. Index

User initialization scripts

Till now, we have seen different scripts, these scripts initialize the operating system, prior to the login of any user. Once the basic operating system in initialized, the user login process starts. This process is explained in the following topics.

Systemwide settings scripts

In the /etc/ folder, the following files are related to the user level initialization:

  • /etc/profile: Few distributions will have additional folder /etc/profile.d/. All the scripts from the profile.d folder will be executed.
  • /etc/bash.bashrc

The preceding scripts are called by all the users, including root and normal users. Initially, the /etc/profile script will be called. This script creates system-wide environment settings. Few distributions will have the /etc/profile.d/ folder. SuSE Linux has additional /etc/profile.local script. The scripts in this folder will also be called. Then, the /etc/bash.bachrc script will be executed.

User level settings – default files

Scripts in the /etc/ folder will be called for all the users. Particular user-specific initialization scripts are located in the HOME folder of each user. These are as follows:

  • $HOME/.bash_profile: This contains user-specific bash environment default settings. This script is called during the login process.
  • $HOME/.bash_login: This contains the second user environment initialization script called during login process.
  • $HOME/.profile: If present, this script internally calls the .bashrc script file.
  • $HOME/.bashrc: This is an interactive shell or terminal initialization script.

All the preceding script's names start with dot. These are hidden files. We will need to give the ls -a command to view these files.

  • Non - login shells.

Whenever we create a new shell terminal, such as, if we pressed the Ctrl + Alt + T key combination or we start a terminal from the applications tab then the terminal which is created is called the interactive shell terminal. We use this terminal to interact with the operating system. This is not the login shell, which is created during the boot-up process. But this interactive shell terminal gives us the CLI prompt for entering the command to execute.

Whenever we create an interactive bash terminal, Shell scripts from /etc/profile and similar are not called, only the ~/.bashrc script is called every time we create a new interactive shell terminal. If we want any environment customization for every newly created interactive shell terminal, we need to customize the .bashrc script from the home folder of the user.

If you check the content of $HOME/.bashrc, you will observe the following:

  • The .bashrc script is the setting prompt
  • It initializes the environmental variables, HISTCONTROL, HISTSIZE, and HISTFILESIZE
  • It customizes the output of the less command
  • It creates various alias commands such as grep, fgrep, egrep, ll, la, l, and similar

If we customize .bashrc such as adding new alias commands or declaring a new function or environment variables, then we should execute .bashrc to take its effect. The following are the two ways to run the .bashrc script so that the environment of the current shell will also be updated as per the customization done in the .bashrc script:

  • $ source .bashrc
  • $ . bashrc

In these two techniques, the child shell is not created but the new function is. Environment and similar variables will become a part of the current shell environment.

Every user's home folder has one more script called .bash_logout. This script is called or executed when the user exits from the login shell.

If the system user is an embedded system developer, who is interested in adding or modifying the device's driver-related command, then he or she will have to make changes in the /etc/rc*.d folder scripts, or they may have to modify the /etc/rc.local script.

If the administrator wants to modify the environment for all the users, then they will have to modify the /etc/profile and /etc/bash_bashrc scripts.

If we want to customize the environment related to a particular user, then the scripts located in the user's home folder, such as $HOME/.profile, $HOME/bash_profile, and $HOME/bash_login scripts, should be modified.

If the user wants to customize only the interactive shell terminal environment, then they will have to customize the $HOME/.bashrc script.

If you are working in system administration, then I would suggest you learn about the /etc/fstab file and it's editing. This file is used for configuring mount points and how file systems are mounted.