test command and [ ] syntax in Bash Scripting - Step-by-Step Execution

Concept Flow - test command and [ ] syntax

Start

↓

Evaluate condition inside test or [

↓

Condition True?

No→Return 1 (False)

Yes↓

Return 0 (True)

↓

Use return code for if/else or other logic

↓

End

The test command or [ ] evaluates a condition and returns 0 if true, 1 if false, which scripts use to decide next steps.

Execution Sample

Bash Scripting

if [ 5 -gt 3 ]; then
  echo "Yes"
else
  echo "No"
fi

This script checks if 5 is greater than 3 and prints 'Yes' if true, otherwise 'No'.

Execution Table

Step	Condition Evaluated	Result	Return Code	Branch Taken	Output
1	5 -gt 3	True	0	Then branch	Yes
2	End of if	-	-	-	-

💡 Condition 5 -gt 3 is true, so return code 0 leads to 'then' branch execution.

Variable Tracker

Variable	Start	After Step 1	Final
Return Code ($?)	-	0	0
Output	-	Yes	Yes

Key Moments - 3 Insights

Why do we use spaces around [ and ] in the condition?

Because [ and ] are commands in bash, they need spaces to separate them from the condition; see execution_table step 1 where condition is evaluated correctly only with spaces.

What does the return code 0 or 1 mean in test or [ ]?

Return code 0 means condition is true, 1 means false; scripts use this to decide which branch to run, as shown in execution_table step 1.

Can we use test and [ ] interchangeably?

Yes, test and [ ] do the same thing; [ ] requires spaces and a closing ], test does not; both return codes behave the same.

Visual Quiz - 3 Questions

Look at the execution_table step 1, what is the return code when condition '5 -gt 3' is true?

A 5

B 1

C 0

D -1

💡 Hint

Check the 'Return Code' column in execution_table row 1.

At which step does the script decide to print 'Yes'?

A Step 1

B Step 2

C Before Step 1

D No step prints 'Yes'

💡 Hint

Look at the 'Branch Taken' and 'Output' columns in execution_table row 1.

If the condition was '[ 2 -eq 3 ]', how would the return code change?

A Return code would be 0

B Return code would be 1

C Return code would be 2

D Return code would be -1

💡 Hint

Recall that return code 0 means true, 1 means false; 2 -eq 3 is false.

Concept Snapshot

test and [ ] check conditions in bash.
Return code 0 means true, 1 means false.
Spaces are required around [ and ]
Used in if statements to control flow.
Example: if [ 5 -gt 3 ]; then echo Yes; fi

Full Transcript

The test command or [ ] syntax in bash is used to check conditions. When you write a condition inside [ ], you must have spaces around the brackets because they are commands. The condition inside is evaluated, and if true, the command returns 0; if false, it returns 1. Scripts use this return code to decide which branch of code to run, like in an if statement. For example, if [ 5 -gt 3 ]; then echo Yes; else echo No; fi will print Yes because 5 is greater than 3. Remember, 0 means true, 1 means false. You can use test or [ ] interchangeably, but [ ] needs spaces and a closing bracket. This simple mechanism helps scripts make decisions based on conditions.

Practice

1. What does the [ -f filename ] test check in a bash script?

easy

A. If filename is a directory

B. If filename is empty

C. If filename is executable

D. If filename exists and is a regular file

Solution

1. Step 1: Understand the -f test

   The -f option in [ ] checks if the file exists and is a regular file (not a directory or special file).

2. Step 2: Compare with other options

   -d checks for directory, -x for executable, and -s for non-empty file, so they don't match.

3. Final Answer:

   If filename exists and is a regular file -> Option D

4. Quick Check:

   -f means regular file check [OK]

Hint: Remember: -f means regular file exists [OK]

Common Mistakes:

• Confusing -f with -d for directories
• Forgetting spaces inside [ ]
• Using filename without quotes causing errors

2. Which of the following is the correct syntax to test if variable var is equal to string hello in bash?

easy

A. [ $var = hello ]

B. [ $var == hello ]

C. [ "$var" = "hello" ]

D. [ "$var" == hello ]

Solution

1. Step 1: Check quoting rules

   Variables should be quoted to avoid errors if empty or contain spaces, so "$var" is correct.

2. Step 2: Check operator correctness

   Single equals = is POSIX compliant for string comparison inside [ ]. Double equals == works in [[ ]] but not always in [ ].

3. Final Answer:

   [ "$var" = "hello" ] -> Option C

4. Quick Check:

   Quote variables and use = inside [ ] [OK]

Hint: Always quote variables and use = inside [ ] [OK]

Common Mistakes:

• Not quoting variables causing syntax errors
• Using == inside [ ] which may fail
• Missing spaces around brackets

3. What will be the output of this script snippet?

var=""
if [ -z "$var" ]; then
  echo "Empty"
else
  echo "Not Empty"
fi

medium

A. Empty

B. Not Empty

C. Syntax Error

D. No output

Solution

1. Step 1: Understand the -z test

   -z returns true if the string length is zero (empty string).

2. Step 2: Check variable value

   Variable var is set to empty string "", so [ -z "$var" ] is true and the if branch runs.

3. Final Answer:

   Empty -> Option A

4. Quick Check:

   -z tests empty string [OK]

Hint: Use -z to check empty strings [OK]

Common Mistakes:

• Not quoting variable causing errors
• Confusing -z with -n
• Expecting else branch to run

4. Identify the error in this bash condition:

if [ $num -gt 10 ]; then
  echo "Greater"
fi

medium

A. Missing spaces inside [ ]

B. Variable $num should be quoted

C. Wrong operator for numeric comparison

D. Missing then keyword

Solution

1. Step 1: Check variable usage in test

   When using variables in [ ], they should be quoted to avoid errors if empty or unset.

2. Step 2: Analyze the given code

   The code uses $num unquoted. If num is empty, the test becomes invalid syntax.

3. Final Answer:

   Variable $num should be quoted -> Option B

4. Quick Check:

   Always quote variables in [ ] to avoid syntax errors [OK]

Hint: Quote variables inside [ ] to prevent errors [OK]

Common Mistakes:

• Not quoting variables causing syntax errors
• Confusing numeric and string operators
• Forgetting spaces around brackets

5. You want to check if a file data.txt exists and is not empty before processing it in a script. Which condition correctly does this?

hard

A. [ -f data.txt ] && [ -s data.txt ]

B. [ -f data.txt -o -s data.txt ]

C. [ -f data.txt && -s data.txt ]

D. [ -f data.txt ] || [ -s data.txt ]

Solution

1. Step 1: Understand the tests

   -f checks if file exists and is regular file; -s checks if file size is greater than zero (not empty).

2. Step 2: Choose correct logical operator

   To require both conditions, use logical AND. Inside a single [ ], use -a; or chain separate tests with &&. Using && directly inside [ ] causes syntax error.

3. Step 3: Analyze options

   The option with && inside single [ ] has syntax error. Chained tests with && are clear and reliable. Options using -o or || apply OR logic, incorrect.

4. Final Answer:

   [ -f data.txt ] && [ -s data.txt ] -> Option A

5. Quick Check:

   Use && to combine separate [ ] tests [OK]

Hint: Use separate [ ] with && for AND conditions [OK]

Common Mistakes:

• Using -o or || which means OR, not AND
• Using && inside a single [ ] (syntax error)
• Not checking file size with -s