Shell Scripting Error Handling Basics

Learning Objectives

Understanding how to handle errors in shell scripts is crucial for creating robust and reliable scripts. Today, you'll learn how to use various techniques to handle errors effectively in your bash scripts.

Topics to Cover

Understanding Exit Status: Every command returns an exit status (0 for success and non-zero for failure). Learn how to check and use exit statuses.
Using if Statements for Error Checking: Learn how to use if statements to handle errors.
Using trap for Cleanup: Understand how to use the trap command to handle unexpected errors and perform cleanup.
Redirecting Errors: Learn how to redirect errors to a file or /dev/null.
Creating Custom Error Messages: Understand how to create meaningful error messages for debugging and user information.

Tasks

Task 1: Checking Exit Status

Write a script that attempts to create a directory and checks if the command was successful. If not, print an error message.

#!/bin/bash
mkdir /tmp/mydir
if [ $? -ne 0 ]; then
  echo "Failed to create directory /tmp/mydir"
fi

Task 2: Using `if` Statements for Error Checking

Modify the script from Task 1 to include more commands (e.g., creating a file inside the directory) and use if statements to handle errors at each step.

#!/bin/bash
dir="/tmp/mydir"
file="$dir/myfile.txt"

mkdir "$dir"
if [ $? -ne 0 ]; then
  echo "Error: Failed to create directory $dir"
  exit 1
fi

touch "$file"
if [ $? -ne 0 ]; then
  echo "Error: Failed to create file $file"
  exit 1
fi

echo "Success: Directory and file created successfully."

Task 3: Using `trap` for Cleanup

Write a script that creates a temporary file and sets a trap to delete the file if the script exits unexpectedly.

#!/bin/bash
tempfile=$(mktemp)
trap "rm -f $tempfile" EXIT

echo "This is a temporary file." > "$tempfile"
cat "$tempfile"

# Simulate an error
exit 1

Task 4: Redirecting Errors

Write a script that tries to read a non-existent file and redirects the error message to a file called error.log.

#!/bin/bash
cat non_existent_file.txt 2> error.log

Task 5: Creating Custom Error Messages

Modify one of the previous scripts to include custom error messages that provide more context about what went wrong.

#!/bin/bash
dir="/tmp/mydir"
mkdir "$dir"
if [ $? -ne 0 ]; then
  echo "Error: Directory $dir could not be created. Check if you have the necessary permissions."
fi

Additional Best Practices

Using set -e: Automatically exit the script if any command fails.
```
  set -e
```
Using set -u: Treat unset variables as errors.
```
  set -u
```

Using || for inline error handling:

  mkdir /tmp/mydir || echo "Failed to create directory"

By mastering these techniques, you'll make your shell scripts more robust and fault-tolerant. 🚀

Day 11: Error Handling in Shell Scripting