Exit codes ($?) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the special variable $? represent in bash scripting?

$? holds the exit code of the last command run. It tells if the command succeeded (0) or failed (non-zero).

beginner

What exit code usually means success in bash scripts?

An exit code of 0 means the command succeeded without errors.

beginner

How can you check the exit code of a command immediately after running it?

Right after running a command, type echo $? to see its exit code.

intermediate

Why is it important to check exit codes in scripts?

Checking exit codes helps scripts know if a command worked or failed, so they can act accordingly.

beginner

What does a non-zero exit code indicate?

A non-zero exit code means the command failed or had an error.

What does echo $? show in bash?

A Exit code of the last command

B The last command's output

C The current directory

D The process ID

$? stores the exit code of the last command run.

Which exit code means success?

A 0

B Any non-zero number

C 255

D 1

Exit code 0 means the command succeeded.

If a command fails, what is the usual exit code?

A 0

B 1 or other non-zero

C 100

D No exit code

Non-zero exit codes indicate failure or errors.

When should you check $? in a script?

A At the end of the script only

B Before running any command

C Right after running a command

D Only if the script crashes

You check $? immediately after a command to know its result.

What is a common use of exit codes in scripts?

A To open a new terminal window

B To change file permissions

C To print text on screen

D To decide next steps based on success or failure

Scripts use exit codes to decide what to do next.

Explain what $? means and how you use it in a bash script.

Think about how you know if a command worked or not.

Describe why checking exit codes is important when writing bash scripts.

Imagine your script needs to stop or try something else if a command fails.

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