[[ ]] extended test in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the [[ ]] syntax represent in bash scripting?

It is an extended test command used for conditional expressions, offering more features and safer syntax than the older [ ] test.

intermediate

How does [[ ]] differ from [ ] in bash?

The [[ ]] supports pattern matching with ==, allows && and || inside without escaping, and prevents word splitting and pathname expansion.

beginner

What is the result of this command: [[ "abc" == a* ]] && echo yes || echo no

It prints 'yes' because [[ ]] supports pattern matching, and 'abc' matches the pattern 'a*'.

intermediate

Can you use regex matching inside [[ ]]? If yes, how?

Yes, using the =~ operator, e.g., [[ $var =~ ^[0-9]+$ ]] tests if $var contains only digits.

intermediate

Why is [[ ]] considered safer than [ ] when testing variables?

Because [[ ]] does not perform word splitting or pathname expansion, it avoids errors when variables contain spaces or special characters.

Which of these is a valid use of [[ ]] in bash?

A [ $name = "John" ]

B [ $name == "John" ]

C [[ $name = "John" ]]

D [[ $name == "John" ]]

[[ $name == "John" ]] is valid and supports pattern matching. Single brackets require single = for string comparison.

What does this command print? [[ 5 -gt 3 && 2 -lt 4 ]] && echo yes || echo no

A Nothing

B yes

C Syntax error

D no

Inside [[ ]], you can use && without escaping. Both conditions are true, so it prints 'yes'.

Which operator is used inside [[ ]] for regex matching?

A ==

B -eq

C =~

D !=

The =~ operator is used for regex matching inside [[ ]].

What happens if a variable with spaces is tested using [ ] without quotes?

A It causes word splitting and errors

B It automatically quotes the variable

C It works fine

D It ignores spaces

Without quotes, [ ] splits words on spaces causing errors. [[ ]] avoids this problem.

Which is true about [[ ]] in bash?

A It is a shell keyword, not a command

B It is an external program

C It behaves exactly like [ ]

D It cannot be used with variables

[[ ]] is a shell keyword providing enhanced conditional testing.

Explain the advantages of using [[ ]] over [ ] in bash scripting.

Think about safety and extra features in condition tests.

Describe how to use regex matching inside [[ ]] and give an example.

Look for the operator that tests patterns, not just equality.