Exit codes ($?) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the exit code of the last command.

Bash Scripting

echo "Hello"; echo [1]

Drag options to blanks, or click blank then click option'

A$?

B$$

C$!

D$#

Attempts:

3 left

💡 Hint

Common Mistakes

Using $$ which gives the process ID instead of the exit code.

Using $! which gives the PID of the last background command.

Using $# which gives the number of positional parameters.

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

2fill in blank

medium

Complete the code to check if the last command succeeded (exit code 0).

Bash Scripting

some_command
if [ [1] -eq 0 ]; then echo "Success"; fi

Drag options to blanks, or click blank then click option'

A$$

B$?

C$!

D$#

Attempts:

3 left

💡 Hint

Common Mistakes

Using $$ which is the process ID, not the exit code.

Using $! which is the PID of the last background command.

Using $# which is the number of arguments.

The variable $? holds the exit code of the last command, which is 0 on success.

3fill in blank

hard

Fix the error in the code to correctly capture the exit code of a command.

Bash Scripting

ls /nonexistent_directory
exit_code=[1]
echo "Exit code: $exit_code"

Drag options to blanks, or click blank then click option'

A$?

B$#

C$!

D$$

Attempts:

3 left

💡 Hint

Common Mistakes

Using $$ which is the process ID, not the exit code.

Using $! which is the PID of the last background command.

Using $# which is the number of arguments.

The exit code of the last command is stored in $?. Assigning it to a variable captures it for later use.

4fill in blank

hard

Fill both blanks to create a script that runs a command and prints if it failed.

Bash Scripting

command_to_run
if [ [1] -ne 0 ]; then
  echo "Command failed with exit code [2]"
fi

Drag options to blanks, or click blank then click option'

A$?

B$$

D$#

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables for the exit code in condition and echo.

Using $$ or $# which are not exit codes.

Use $? to check the exit code and also to print it. Both blanks need the exit code variable.

5fill in blank

hard

Fill all three blanks to save the exit code, check it, and print a message.

Bash Scripting

some_command
exit_code=[1]
if [ [2] -eq 0 ]; then
  echo "Command succeeded with code [3]"
fi

Drag options to blanks, or click blank then click option'

A$?

Bexit_code

D$$

Attempts:

3 left

💡 Hint

Common Mistakes

Using $? directly in the if after assigning it to a variable.

Using $$ which is the process ID.

Assign $? to exit_code, then check exit_code and print it. This saves and reuses the exit code.

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