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Mastering Linux Shell Scripting

You're reading from   Mastering Linux Shell Scripting Master the complexities of Bash shell scripting and unlock the power of shell for your enterprise

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Product type Paperback
Published in Dec 2015
Publisher
ISBN-13 9781784396978
Length 198 pages
Edition 1st Edition
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Author (1):
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Andrew Mallett Andrew Mallett
Author Profile Icon Andrew Mallett
Andrew Mallett
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Table of Contents (16) Chapters Close

Preface 1. What and Why of Scripting with Bash FREE CHAPTER 2. Creating Interactive Scripts 3. Conditions Attached 4. Creating Code Snippets 5. Alternative Syntax 6. Iterating with Loops 7. Creating Building Blocks with Functions 8. Introducing sed 9. Automating Apache Virtual Hosts 10. Awk Fundamentals 11. Summarizing Logs with Awk 12. A Better lastlog with Awk 13. Using Perl as a Bash Scripting Alternative 14. Using Python as a Bash Scripting Alternative Index

Using the test shell builtin


It is probably time for us to pull over to the side of the scripting highway and look a little more at this command test. This is both a shell builtin and a file executable in its own right. Of course, we will have to hit the built-in command first, unless we specify the full path to the file.

When the test command is run without any expressions to evaluate, then the test will return false. So, if we run the test as shown in the following command:

$ test

The exit status will be 1, even though no error output is shown. The test command will always return either True or False or 0 or 1, respectively. The basic syntax of test is:

test EXPRESSION

Or, we can inverse the test command with:

test ! EXPRESSION

If we need to include multiple expressions, these can be AND or OR together using the -a and -o options, respectively:

test EXPRESSION -a EXPRESSION
test EXPRESSION -o EXPRESSION

We can also write in a shorthand version replacing the test with square brackets to surround...

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