test command and [ ] syntax in Bash Scripting

Introduction

The test command and [ ] syntax help check conditions in scripts to decide what to do next.

Check if a file exists before reading it.

Compare two numbers to see which is bigger.

See if a variable is empty or not.

Decide what to do based on user input.

Verify if a directory is present before creating files.

Syntax

Bash Scripting

test CONDITION
[ CONDITION ]

Both test and [ ] do the same thing: check a condition.

Make sure to put spaces around [ and ] when using [ ].

Examples

Check if a file named filename.txt exists.

Bash Scripting

test -f filename.txt
if [ -f filename.txt ]; then echo "File exists"; fi

Check if variable var equals 'hello'.

Bash Scripting

if [ "$var" = "hello" ]; then echo "Greeting found"; fi

Check if number num is greater than 10.

Bash Scripting

if test $num -gt 10; then echo "Number $num is greater than 10"; fi

Sample Program

This script checks if a file named myfile.txt exists and prints a message. Then it checks if the number 5 is less than 10 and prints the result.

Bash Scripting

#!/bin/bash

filename="myfile.txt"

if [ -e "$filename" ]; then
  echo "File '$filename' exists."
else
  echo "File '$filename' does not exist."
fi

num=5
if test $num -lt 10; then
  echo "Number $num is less than 10."
else
  echo "Number $num is 10 or more."
fi

Important Notes

Always put spaces after [ and before ] when using [ ] syntax.

Use quotes around variables to avoid errors if they are empty or have spaces.

test and [ ] are interchangeable, but [ ] is more common in scripts.

Summary

test and [ ] check conditions in bash scripts.

Use them to make decisions like if-else.

Remember spaces and quotes to avoid mistakes.

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