Exit codes ($?) in Bash Scripting

Introduction

Exit codes tell us if a command worked or failed. They help scripts decide what to do next.

Check if a file was copied successfully before moving on.

Verify if a program ran without errors before starting another task.

Detect if a command failed and show a friendly message to the user.

Stop a script early if a critical step did not complete correctly.

Syntax

Bash Scripting

echo $?  # shows the exit code of the last command

Exit code 0 means success; any other number means an error.

Always check $? right after the command you want to test.

Examples

Lists files in /tmp and then prints the exit code. 0 means success.

Bash Scripting

ls /tmp
 echo $?

Tries to list a non-existing folder, then prints exit code. Non-zero means error.

Bash Scripting

ls /not_exist
 echo $?

Creates a folder and prints exit code to confirm success.

Bash Scripting

mkdir newfolder
 echo $?

Sample Program

This script creates a folder and checks if it worked using exit codes. Then it removes the folder and checks again.

Bash Scripting

#!/bin/bash

mkdir testfolder
if [ $? -eq 0 ]; then
  echo "Folder created successfully."
else
  echo "Failed to create folder."
fi

rm -r testfolder
if [ $? -eq 0 ]; then
  echo "Folder removed successfully."
else
  echo "Failed to remove folder."
fi

Important Notes

Always check the exit code immediately after the command you want to verify.

Use exit codes in scripts to handle errors gracefully and avoid surprises.

Common exit codes: 0 = success, 1 = general error, other numbers can mean specific errors.

Summary

Exit codes tell if a command worked or failed.

0 means success; other numbers mean errors.

Check $? right after a command to get its 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