Table of Contents for
Linux Shell Scripting Bootcamp

Version ebook / Retour

Cover image for bash Cookbook, 2nd Edition Linux Shell Scripting Bootcamp by James Kent Lewis Published by Packt Publishing, 2017
  1. Cover
  2. Table of Contents
  3. Linux Shell Scripting Bootcamp
  4. Linux Shell Scripting Bootcamp
  5. Credits
  6. About the Author
  7. Acknowledgement
  8. About the Reviewer
  9. www.PacktPub.com
  10. Customer Feedback
  11. Preface
  12. What you need for this book
  13. Who this book is for
  14. Conventions
  15. Reader feedback
  16. Customer support
  17. 1. Getting Started with Shell Scripting
  18. Demonstrating the use of scripts
  19. Summary
  20. 2. Working with Variables
  21. Validating parameters using conditional statements
  22. Comparison operators for strings
  23. Environment variables
  24. Summary
  25. 3. Using Loops and the sleep Command
  26. Screen manipulation
  27. Indenting your code
  28. Using the for statement
  29. Leaving a loop early
  30. The sleep command
  31. Watching a process
  32. Creating numbered backup files
  33. Summary
  34. 4. Creating and Calling Subroutines
  35. File redirection
  36. Command piping
  37. Subroutines
  38. Using parameters
  39. Making a current backup of your work
  40. Summary
  41. 5. Creating Interactive Scripts
  42. Summary
  43. 6. Automating Tasks with Scripts
  44. Summary
  45. 7. Working with Files
  46. Reading files
  47. Reading and writing files
  48. Reading and writing files interactively
  49. File checksums
  50. File encryption
  51. Summary
  52. 8. Working with wget and curl
  53. wget and recursion
  54. wget options
  55. curl
  56. Summary
  57. 9. Debugging Scripts
  58. Automatic backups
  59. More syntax errors
  60. Logic errors
  61. Using set to debug scripts
  62. Summary
  63. 10. Scripting Best Practices
  64. ssh and scp
  65. Find and use a good text editor
  66. Environment variables and aliases
  67. ssh prompt
  68. Testing an archive
  69. Progress indicator
  70. Creating new commands from a template
  71. Alerting the user
  72. Summary
  73. Index

Validating parameters using conditional statements

Variables can be tested and compared against other variables when using a variable as a number.

Here is a list of some of the operators that can be used:

Operator

Description

-eq

This stands for equal to

-ne

This stands for not equal to

-gt

This stands for greater than

-lt

This stands for less than

-ge

This stands for greater than or equal to

-le

This stands for less than or equal to

!

This stands for the negation operator

Let's take a look at this in our next example script:

Chapter 2 - Script 2

#!/bin/sh
#
# 6/13/2017
#
echo "script2"

# Numeric variables
a=100
b=100
c=200
d=300

echo a=$a b=$b c=$c d=$d     # display the values

# Conditional tests
if [ $a -eq $b ] ; then
 echo a equals b
fi

if [ $a -ne $b ] ; then
 echo a does not equal b
fi

if [ $a -gt $c ] ; then
 echo a is greater than c
fi

if [ $a -lt $c ] ; then
 echo a is less than c
fi

if [ $a -ge $d ] ; then
 echo a is greater than or equal to d
fi

if [ $a -le $d ] ; then
 echo a is less than or equal to d
fi

echo Showing the negation operator:
if [ ! $a -eq $b ] ; then
 echo Clause 1
else
 echo Clause 2
fi
echo "End of script2"

And the output is as follows:

Chapter 2 - Script 2

To help understand this chapter run the script on your system. Try changing the values of the variables to see how it affects the output.

We saw the negation operator in Chapter 1, Getting Started with Shell Scripting when we were looking at files. As a reminder, it negates the expression. You could also say it does the opposite of what the original statement means.

Consider the following example:

a=1
b=1
if [ $a -eq $b ] ; then
  echo Clause 1
else
  echo Clause 2
fi

When this script is run it will display Clause 1. Now consider this:

a=1
b=1
if [ ! $a -eq $b ] ; then    # negation
  echo Clause 1
else
  echo Clause 2
fi

Because of the negation operator it will now display Clause 2. Try it on your system.