Exit codes ($?) in Bash Scripting - Mini Project: Build & Apply

Understanding Exit Codes ($?) in Bash Scripts

📖 Scenario: You are writing a simple bash script to run commands and check if they worked correctly. Exit codes help you know if a command succeeded or failed.In real life, this is like checking if a light bulb is working after you install it. If it works, you get a green light (exit code 0). If it doesn't, you get a red light (non-zero exit code).

🎯 Goal: Build a bash script that runs a command, saves its exit code using $?, and prints the exit code to understand if the command succeeded or failed.

📋 What You'll Learn

Create a variable to run a command

Save the exit code of the command using $?

Print the exit code to the terminal

💡 Why This Matters

🌍 Real World

Exit codes are used in scripts to detect errors and decide what to do next, like retrying a command or stopping the script.

💼 Career

Understanding exit codes is essential for writing reliable automation scripts and troubleshooting problems in system administration and DevOps roles.

1

Run a command and save its exit code

Write a line that runs the command ls /tmp and then save its exit code in a variable called exit_code using $?.

Bash Scripting

# Run the command ls /tmp and save its exit code in exit_code
# Your code here

Hint

Run the command first, then immediately save $? to exit_code.

2

Add a second command and save its exit code

Add a line that runs the command cat /not_exist_file and then save its exit code in a variable called exit_code using $?.

Bash Scripting

ls /tmp
exit_code=$?
# Run the command cat /not_exist_file and save its exit code in exit_code
# Your code here

Hint

Run the second command and save its exit code just like the first.

3

Print the exit code after the second command

Add a line to print the text Exit code is: followed by the value of the variable exit_code.

Bash Scripting

ls /tmp
exit_code=$?
cat /not_exist_file
exit_code=$?
# Print the exit code
# Your code here

Hint

Use echo to print the message and the variable.

4

Print exit code after the first command too

Add a line to print the text First command exit code: followed by the exit code of the first command. Save the first command's exit code in a variable called first_exit_code before running the second command.

Bash Scripting

ls /tmp
first_exit_code=$?
cat /not_exist_file
exit_code=$?
echo "First command exit code: $first_exit_code"
echo "Exit code is: $exit_code"  # Print the second command exit code
# Your code here

Hint

Save the first exit code before running the second command, then print it.

Practice

1. What does the exit code 0 usually mean in bash scripting?

easy

A. The command is still running

B. The command failed

C. The command was skipped

D. The command was successful

Solution

1. Step 1: Understand exit codes meaning

   In bash, exit code 0 means success, any other number means failure or error.

2. Step 2: Interpret exit code 0

   Since 0 means success, the command completed without errors.

3. Final Answer:

   The command was successful -> Option D

4. Quick Check:

   Exit code 0 = success [OK]

Hint: Exit code 0 means success, anything else means error [OK]

Common Mistakes:

• Thinking 0 means failure
• Confusing exit code with process ID
• Checking $? too late after command

2. Which of the following is the correct way to check the exit code of the last command in bash?

easy

A. echo $exit_code

B. echo $?

C. echo $lastcode

D. echo $status

Solution

1. Step 1: Recall special variable for exit code

   The special variable $? holds the exit code of the last command.

2. Step 2: Verify correct syntax

   Using echo $? prints the exit code correctly.

3. Final Answer:

   echo $? -> Option B

4. Quick Check:

   Use $? to get last exit code [OK]

Hint: Use $? immediately after command to get exit code [OK]

Common Mistakes:

• Using undefined variables like $exit_code
• Forgetting to check $? right after command
• Using $status which is not standard

3. What will be the output of this script?

ls /nonexistent_dir
 echo $?

medium

A. 2

B. 0

C. No output

D. ls: cannot access '/nonexistent_dir': No such file or directory

Solution

1. Step 1: Understand the command behavior

   The command ls /nonexistent_dir tries to list a folder that does not exist, so it fails.

2. Step 2: Check exit code after failure

   After failure, $? holds a non-zero exit code, 2 for ls missing directories.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Failed command exit code ≠ 0 [OK]

Hint: Failed commands return non-zero exit codes like 2 [OK]

Common Mistakes:

• Assuming output is 0 for failure
• Confusing error message with exit code
• Expecting no output from echo

4. Consider this script snippet:

mkdir test_dir
false
if [ $? -eq 0 ]; then
  echo "Created"
else
  echo "Failed"
fi

What is the problem if the script always prints "Failed" even when the directory is created?

medium

A. mkdir command does not set exit code

B. The syntax of if statement is wrong

C. The $? is checked too late after mkdir

D. echo command overwrites the exit code

Solution

1. Step 1: Understand exit code timing

   The exit code $? must be checked immediately after the command it relates to.

2. Step 2: Identify delay in checking exit code

   If any command runs between mkdir and checking $?, the exit code changes.

3. Final Answer:

   The $? is checked too late after mkdir -> Option C

4. Quick Check:

   Check $? immediately after command [OK]

Hint: Check $? right after command, no commands in between [OK]

Common Mistakes:

• Checking $? after other commands
• Assuming mkdir never fails
• Misunderstanding if syntax

5. You want to run two commands in a script: cmd1 and cmd2. You want to run cmd2 only if cmd1 succeeded (exit code 0). Which is the best way to do this?

hard

A. cmd1 && cmd2

B. cmd1 || cmd2

C. cmd1; if [ $status -eq 0 ]; then cmd2; fi

D. if [ cmd1 ]; then cmd2; fi

Solution

1. Step 1: Understand conditional execution operators

   The operator && runs the second command only if the first succeeds (exit code 0).

2. Step 2: Compare options for best practice

   cmd1 && cmd2 uses cmd1 && cmd2 which is concise and reliable for this purpose.

3. Final Answer:

   cmd1 && cmd2 -> Option A

4. Quick Check:

   Use && to run second command only if first succeeds [OK]

Hint: Use && to run next command only if previous succeeds [OK]

Common Mistakes:

• Using || which runs second if first fails
• Checking $? manually when && is simpler
• Using if with command but less concise