#90DaysOfDevops | Day 11

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

1. 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.

2. Usingif Statements for Error Checking: Learn how to use if statements to handle errors.

3. Usingtrap for Cleanup: Understand how to use the trap command to handle unexpected errors and perform cleanup.

4. Redirecting Errors: Learn how to redirect errors to a file or /dev/null.

5. 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

ubuntu@ip-172-31-44-248:~/scripts$ bash ./exitstatus.sh
mkdir: cannot create directory '/tmp/mydir': File exists
Failed to create directory /tmp/mydir
ubuntu@ip-172-31-44-248:~/scripts$

Task 2: Usingif 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

mkdir /tmp/mydir1

if [ $? -ne 0 ]; then
  echo "Failed to create directory /tmp/mydir"
else
  echo "Directory Created Successfully."        
fi

ubuntu@ip-172-31-44-248:~/scripts$ bash ./exitstatus1.sh
Directory Created Successfully.
ubuntu@ip-172-31-44-248:~/scripts$ bash ./exitstatus1.sh
mkdir: cannot create directory '/tmp/mydir1': File exists
Failed to create directory /tmp/mydir

Task 3: Usingtrap 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

ubuntu@ip-172-31-44-248:~/scripts$ bash ./exitstatus2.sh 
This is a temporary file.

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.

In general, here are some common redirection operators in bash:

• >: Redirects standard output (file descriptor 1).

• 2>: Redirects standard error (file descriptor 2).

• &>: Redirects both standard output and standard error to the same file.

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

ubuntu@ip-172-31-44-248:~/scripts$ bash ./exitstatus3.sh

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