Day 11 Task: Error Handling in Shell Scripting

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

3. Using trap 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

In a shell script, the exit status of a command indicates whether the command was successful or not. By convention:

• An exit status of 0 means the command was successful.

• Any non-zero exit status (usually 1) indicates that the command failed.

How to Check Exit Status

1. Using $?:

   • After a command is executed, you can check its exit status using the special variable $?. This variable holds the exit status of the last executed command.

#!/bin/bash

# Store the first argument as the directory path
directory_path=$1
# Create the directory
mkdir -p "$directory_path"
# Check if the directory was created successfully
if [ $? -eq 0 ]; then
  echo "Directory created successfully at: $directory_path"
else
  echo "Failed to create directory at: $directory_path"
  exit 1
fi

Output

ubuntu@ip-172-31-19-250:~/Scripts$ vim task11.sh 
ubuntu@ip-172-31-19-250:~/Scripts$ ./task11.sh /home/ubuntu/Vibhuti 
Directory created successfully at: /home/ubuntu/Vibhuti
ubuntu@ip-172-31-19-250:~/Scripts$

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

# Check if the user provided an argument (the directory path)
if [ -z "$1" ]; then
  echo "Usage: $0 <directory_path>"
  exit 1
fi

# Store the first argument as the directory path
directory_path=$1

# Create the directory
touch "$directory_path"

# Check if the directory was created successfully
if [ $? -eq 0 ]; then
  echo "File created successfully at: $directory_path"
else
  echo "Failed to create file at: $directory_path"
  exit 1
fi

Output:

ubuntu@ip-172-31-19-250:~/Scripts$ vim task11.sh 
ubuntu@ip-172-31-19-250:~/Scripts$ ./task11.sh /home/ubuntu/hello.txt
File created successfully at: /home/ubuntu/hello.txt

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)
function cleanup() {
        echo "Cleaning up....."
        rm -f ${tempfile}
        echo "Temporary file deleted" ${tempfile} 
} 

trap cleanup EXIT
echo "This is temporary file" > ${tempfile}
cat ${tempfile}
exit 1

ubuntu@ip-172-31-19-250:~/Scripts$ ./cleanup.sh 
This is temporary file
Cleaning up.....
Temporary file deleted /tmp/tmp.tHBEOuQgCO

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.

$cat hello.txt 2> error.log 
ubuntu@ip-172-31-19-250:~/Scripts$ cat error.log 
cat: hello.txt: No such file or directory

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

ubuntu@ip-172-31-19-250:~/Scripts$ ./custom.sh 
mkdir: cannot create directory ‘/tmp/mydir’: File exists
Error: Directory /tmp/mydir could not be created. Check if you have the necessary permissions.

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