Shebang line (#!/bin/bash) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this script with a shebang?

Consider this bash script saved as test.sh and made executable:

#!/bin/bash
echo "Hello from script!"

What will be the output when you run ./test.sh in a terminal?

Bash Scripting

#!/bin/bash
echo "Hello from script!"

A Hello from script!

B bash: ./test.sh: command not found

C Syntax error near unexpected token `echo'

D No output, script runs silently

💡 Hint

The shebang line tells the system which program to use to run the script.

The shebang #!/bin/bash tells the system to use bash to run the script. The echo command prints the text to the terminal.

💻 Command Output

intermediate

What happens if the shebang line is missing?

Consider this script saved as myscript.sh and made executable with chmod +x myscript.sh:

echo "Running script"

What will be the output when you run ./myscript.sh in a terminal?

Bash Scripting

echo "Running script"

A Nothing happens, script does not run

B bash: ./myscript.sh: cannot execute binary file: Exec format error

C Syntax error: missing shebang

D Running script

💡 Hint

Without a shebang, the system may not know how to run the script.

Without a shebang, the system uses the current shell to run the script, so the echo command runs and prints the text.

📝 Syntax

advanced

Identify the correct shebang line for bash scripts

Which of the following lines is the correct shebang to start a bash script?

A #!/bin/bash -c

B #!/bin/bash

C #!/bin/sh -x

D #!/usr/bin/python

💡 Hint

The shebang must point to the bash executable without extra arguments.

The correct shebang for bash is exactly #!/bin/bash. Options with python or extra flags are incorrect for a simple bash script.

🔧 Debug

advanced

Why does this script fail to run with ./script.sh?

You have this script:

#!/bin/bash
echo "Start"
exit 0

But when you run ./script.sh, you get bash: ./script.sh: Permission denied. What is the cause?

Bash Scripting

#!/bin/bash
echo "Start"
exit 0

A The echo command is misspelled

B The shebang line is incorrect

C The script file does not have execute permission

D The script has a syntax error

💡 Hint

Check the file permissions with ls -l.

Permission denied means the file is not executable. Use chmod +x script.sh to fix.

🚀 Application

expert

How to write a portable shebang line for bash scripts?

You want your bash script to run on different systems where bash might be in different locations. Which shebang line is best to ensure portability?

A #!/usr/bin/env bash

B #!/bin/bash

C #!/bin/sh

D #!/usr/local/bin/bash

💡 Hint

Use env to find bash in the user's PATH.

Using #!/usr/bin/env bash runs bash from the PATH, making the script portable across systems.

Practice

1. What is the main purpose of the shebang line #!/bin/bash in a script?

easy

A. It sets the script's file permissions to executable.

B. It comments out the first line of the script.

C. It tells the system to use the Bash shell to run the script.

D. It defines a variable named shebang.

Solution

1. Step 1: Understand the shebang line role

   The shebang line starts with #! and specifies the interpreter for the script.

2. Step 2: Identify the interpreter in the line

   /bin/bash means the script will run using the Bash shell.

3. Final Answer:

   It tells the system to use the Bash shell to run the script. -> Option C

4. Quick Check:

   Shebang = interpreter path [OK]

Hint: Shebang line always shows which shell runs the script [OK]

Common Mistakes:

• Thinking it is a comment line
• Confusing it with file permissions
• Assuming it defines variables

2. Which of the following is the correct way to write a shebang line for a Bash script?

easy

A. !#/bin/bash

B. #!/bin/sh

C. #!/bash/bin

D. #!/bin/bash

Solution

1. Step 1: Check the correct shebang syntax

   The shebang line must start with #! followed by the full path to the interpreter.

2. Step 2: Verify the path for Bash

   The correct path for Bash is /bin/bash, so the line is #!/bin/bash.

3. Final Answer:

   #!/bin/bash -> Option D

4. Quick Check:

   Shebang = #! + interpreter path [OK]

Hint: Shebang always starts with #! followed by interpreter path [OK]

Common Mistakes:

• Swapping the order of # and !
• Using wrong interpreter path
• Confusing /bin/sh with /bin/bash (different shells)

3. Given this script saved as test.sh with executable permission:

#!/bin/bash
echo "Hello, $USER!"

What will be the output when you run ./test.sh?

medium

A. Hello, $USER!

B. Hello, your_username!

C. Hello, root!

D. Syntax error

Solution

1. Step 1: Understand the shebang effect

   The shebang #!/bin/bash runs the script with Bash, which expands variables like $USER.

2. Step 2: Predict variable expansion

   $USER is replaced by the current user's username, so output is Hello, your_username!.

3. Final Answer:

   Hello, your_username! -> Option B

4. Quick Check:

   Variable expansion in Bash = username [OK]

Hint: Bash expands $USER to your username when run with shebang [OK]

Common Mistakes:

• Thinking $USER prints literally
• Assuming root user always runs script
• Expecting syntax error from shebang

4. You wrote a script starting with this line:

#! /bin/bash

But when you run it, you get: bad interpreter: No such file or directory. What is the likely problem?

medium

A. There is a space between #! and /bin/bash.

B. The script file is not executable.

C. The /bin/bash path does not exist on the system.

D. The script has Windows line endings.

Solution

1. Step 1: Check the shebang syntax

   The shebang must start exactly with #! with no space after it.

2. Step 2: Identify the error cause

   A space after #! causes the system to look for an invalid interpreter path, causing the error.

3. Final Answer:

   There is a space between #! and /bin/bash. -> Option A

4. Quick Check:

   No space after #! in shebang [OK]

Hint: No spaces allowed between #! and interpreter path [OK]

Common Mistakes:

• Ignoring spaces in shebang line
• Assuming file permission error
• Not checking interpreter path correctness

5. You want to write a script that runs with Bash but also works if run with sh script.sh (without executable permission). Which shebang line should you use to ensure it always runs with Bash when executed directly?

hard

A. #!/usr/bin/env bash

B. #!/bin/bash

C. #!/bin/sh

D. #!/usr/local/bin/bash

Solution

1. Step 1: Understand shebang with env

   #!/usr/bin/env bash uses the env command to find Bash in the user's PATH, making it portable.

2. Step 2: Compare with fixed paths

   /bin/bash may not exist on all systems, and /bin/sh is often not Bash. /usr/local/bin/bash is less common.

3. Step 3: Ensure script runs with Bash when executed directly

   Using #!/usr/bin/env bash ensures the script runs with Bash regardless of its location.

4. Final Answer:

   #!/usr/bin/env bash -> Option A

5. Quick Check:

   Use env to find Bash for portability [OK]

Hint: Use #!/usr/bin/env bash for portable Bash scripts [OK]

Common Mistakes:

• Using /bin/sh which may not be Bash
• Assuming /bin/bash exists everywhere
• Ignoring PATH differences on systems