[[ ]] extended test in Bash Scripting - Step-by-Step Execution

Concept Flow - [[ ]] extended test

Start

↓

Evaluate [[ expression

↓

Is expression true?

No→Execute else or skip

Yes↓

Execute then block

↓

End

The [[ ]] extended test evaluates a condition. If true, it runs the then block; if false, it runs else or skips.

Execution Sample

Bash Scripting

if [[ -f /etc/passwd && -r /etc/passwd ]]; then
  echo "File exists and is readable"
else
  echo "File missing or not readable"
fi

Checks if /etc/passwd exists and is readable, then prints a message accordingly.

Execution Table

Step	Expression Evaluated	Result	Branch Taken	Output
1	-f /etc/passwd	true	Continue
2	-r /etc/passwd	true	Continue
3	-f /etc/passwd && -r /etc/passwd	true	Then block	File exists and is readable
4	End of if	N/A	Exit

💡 Both conditions true, so then block executed and if statement ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
-f /etc/passwd	N/A	true	true	true
-r /etc/passwd	N/A	N/A	true	true
Combined expression	N/A	N/A	N/A	true

Key Moments - 2 Insights

Why do we use [[ ]] instead of [ ] for extended tests?

Because [[ ]] supports more complex expressions like &&, ||, and pattern matching, as shown in steps 1-3 of the execution_table.

What happens if one condition in && is false?

The combined expression becomes false and the else block runs or the if is skipped, as the logical AND requires both true (see exit_note).

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of the expression '-r /etc/passwd' at step 2?

A error

B false

C true

D not evaluated

💡 Hint

Check the 'Result' column in row for step 2 in execution_table.

At which step does the script decide to execute the then block?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Branch Taken' column to see when 'Then block' is chosen.

If the file /etc/passwd was not readable, how would the combined expression result change?

A It would be true

B It would be false

C It would cause an error

D It would be skipped

💡 Hint

Recall that && requires both conditions true; see key_moments about false condition effect.

Concept Snapshot

[[ ]] extended test syntax:
if [[ condition ]]; then
  commands
fi

Supports complex tests with &&, ||, pattern matching.
Returns true or false to control flow.
Use for safer, more powerful condition checks.

Full Transcript

This example shows how the bash [[ ]] extended test works. The script checks if the file /etc/passwd exists and is readable using two conditions combined with && inside [[ ]]. Step 1 tests if the file exists (-f), which is true. Step 2 tests if the file is readable (-r), also true. Step 3 combines both with &&, resulting in true, so the then block runs and prints 'File exists and is readable'. If either condition was false, the else block would run or the if would skip. The [[ ]] syntax allows combining tests safely and clearly. This trace helps beginners see each step's result and understand how the condition controls the script flow.