String comparisons (=, !=, -z, -n) in Bash Scripting - Step-by-Step Execution

Concept Flow - String comparisons (=, !=, -z, -n)

Start

↓

Get two strings

↓

Check if strings are equal (=)

Yes→Print 'Equal'

No↓

Check if strings are not equal (!=)

Yes→Print 'Not Equal'

No↓

Check if string is empty (-z)

Yes→Print 'Empty String'

No↓

Check if string is not empty (-n)

Yes→Print 'Not Empty'

No↓

End

The flow checks string equality, inequality, emptiness, and non-emptiness step-by-step, printing results accordingly.

Execution Sample

Bash Scripting

str1="hello"
str2="world"
if [ "$str1" = "$str2" ]; then
  echo "Equal"
elif [ "$str1" != "$str2" ]; then
  echo "Not Equal"
fi

This script compares two strings and prints if they are equal or not.

Execution Table

Step	Condition Checked	Evaluation	Branch Taken	Output
1	[ "$str1" = "$str2" ]	"hello" = "world" → False	No
2	[ "$str1" != "$str2" ]	"hello" != "world" → True	Yes	Not Equal
3	End of conditions	-	-	-

💡 Condition for inequality is true, so 'Not Equal' is printed and script ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
str1	"hello"	"hello"	"hello"	"hello"
str2	"world"	"world"	"world"	"world"

Key Moments - 3 Insights

Why does the script print 'Not Equal' even though we only checked equality first?

Because the first condition [ "$str1" = "$str2" ] is false (see step 1 in execution_table), the script moves to the elif which checks inequality and finds it true (step 2), so it prints 'Not Equal'.

What does the -z test do and when would it be true?

The -z test checks if a string is empty. It returns true if the string length is zero. For example, if str1="", then [ -z "$str1" ] is true.

Why do we put variables in double quotes inside [ ]?

Double quotes prevent errors if the variable is empty or contains spaces. Without quotes, the test might fail or behave unexpectedly.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of the first condition check at step 1?

A False

B True

C Error

D Not evaluated

💡 Hint

Check the 'Evaluation' column in row 1 of execution_table.

At which step does the script print 'Not Equal'?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'Output' column in execution_table rows.

If str1 was empty (""), which test would return true?

A [ "$str1" = "$str2" ]

B [ -z "$str1" ]

C [ "$str1" != "$str2" ]

D [ -n "$str1" ]

💡 Hint

Refer to key_moments explanation about -z test.

Concept Snapshot

String comparisons in bash:
- Use [ "$a" = "$b" ] for equality
- Use [ "$a" != "$b" ] for inequality
- Use [ -z "$a" ] to check if string is empty
- Use [ -n "$a" ] to check if string is not empty
Always quote variables to avoid errors.

Full Transcript

This visual execution trace shows how bash scripts compare strings using =, !=, -z, and -n. The script starts by assigning two strings, then checks if they are equal. If not equal, it checks inequality and prints 'Not Equal'. The -z test checks if a string is empty, and -n checks if it is not empty. Variables are quoted to prevent errors. The execution table details each step's condition, evaluation, branch, and output. Key moments clarify common confusions like why the script prints 'Not Equal' and the purpose of quoting variables. The quiz tests understanding of condition results and string emptiness checks.