DevOps

Objectives🎯

Handling errors in shell scripts is crucial for creating reliable scripts. Today, we will learn effective error-handling techniques in 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.


  ubuntu@ip-172-31-45-42:~$ mkdir day11
  ubuntu@ip-172-31-45-42:~$ cd day11/
  ubuntu@ip-172-31-45-42:~/day11$ pwd
  /home/ubuntu/day11
  ubuntu@ip-172-31-45-42:~/day11$ vim create_directory.sh
  ubuntu@ip-172-31-45-42:~/day11$ cat create_directory.sh
  ubuntu@ip-172-31-45-42:~/day11$ ./create_directory.sh
  -bash: ./create_directory.sh: Permission denied
  ubuntu@ip-172-31-45-42:~/day11$ chmod +x create_directory.sh
  ubuntu@ip-172-31-45-42:~/day11$ ./create_directory.sh

  #!/bin/bash
  #Create directory
  mkdir /home/ubuntu/day11/directory1

  #Check the status
  if [ $? -ne 0 ]; then
          echo "Fail to create directory /home/ubuntu/day11/"
  else
          echo "Directory created successfully.!!!! "

  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.

  ubuntu@ip-172-31-45-42:~/day11$ vim create_directory2.sh
  ubuntu@ip-172-31-45-42:~/day11$ cat create_directory2.sh
  ubuntu@ip-172-31-45-42:~/day11$ chmod +x create_directory2.sh
  ubuntu@ip-172-31-45-42:~/day11$ ./create_directory2.sh

   #!/bin/bash
  #create a directory
  mkdir /home/ubuntu/day11/directory3

  if [ $? -ne 0 ]; then
     echo "Error: Fail to create directory /home/ubuntu/day11"
     exit 1
  fi

  #create file inside the directory
  touch /home/ubuntu/day11/directory3/task2.txt

  if [ $? -ne 0 ]; then
     echo "Error: Fail to create file /home/ubuntu/day11/directory3/task2.txt"
     exit 1
  fi

  echo "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.

  ubuntu@ip-172-31-45-42:~/day11$ ls
  create_directory.sh  create_directory2.sh  directory3  trap.sh
  ubuntu@ip-172-31-45-42:~/day11$ cat trap.sh
  ubuntu@ip-172-31-45-42:~/day11$ chmod +x trap.sh
  ubuntu@ip-172-31-45-42:~/day11$ ./trap.sh

  #!/bin/bash
  #Create a temp file
  temp_file="/home/ubuntu/day11/temp_file_$$.txt"
  touch "$temp_file"
  echo "Temporary file created at $temp_file"

  #set up a trap to delete the temporary file
  trap 'echo "Deleting temporary file"; rm -f "$temp_file"' EXIT

  #Timer
  echo "Script is running...."
  sleep 10

  #Normal exit
  echo "Script completed successfully.....!"

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.

  ubuntu@ip-172-31-45-42:~/day11$ ls
  create_directory.sh  create_directory2.sh  directory3  trap.sh
  ubuntu@ip-172-31-45-42:~/day11$ vim reditect_errors.sh
  ubuntu@ip-172-31-45-42:~/day11$ cat reditect_errors.sh
  ubuntu@ip-172-31-45-42:~/day11$ chmod +x reditect_errors.sh
  ubuntu@ip-172-31-45-42:~/day11$ ./reditect_errors.sh

  #!/bin/bash
  #

  #Read a non-exitent file and redirect the error
  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.

  ubuntu@ip-172-31-45-42:~/day11$ vim custom_error_msg.sh
  ubuntu@ip-172-31-45-42:~/day11$ cat custom_error_msg.sh
  ubuntu@ip-172-31-45-42:~/day11$ chmod +x custom_error_msg.sh
  ubuntu@ip-172-31-45-42:~/day11$ ./custom_error_msg.sh

  #!/bin/bash
  #create a directory
  mkdir /home/ubuntu/day11/directory5

  if [ $? -ne 0 ]; then
     echo "Error: Directory /home/ubuntu/day11 could not be create.Please check t                                                                         he permissions.!"
     exit 1
  fi

  #create file inside the directory
  touch /home/ubuntu/day11/directory5/task5.txt

  if [ $? -ne 0 ]; then
     echo "Error: File /home/ubuntu/day11/directory5/task5.txt could not be creat                                                                         ed. please check directory is created or not "
     exit 1
  fi

  echo "Directory and File created Successfully"

Conclusion💻🚀

Errors handling and ensuring proper cleanup in scripts is crucial for robust and reliable script execution. Here’s a summary of the tasks and the key points:

Checking Exit Status: Use $? to check the exit status of commands. If a command fails (i.e., returns a non-zero status), you can handle the error accordingly.
Using if Statements for Error Checking: Incorporate if statements to check the success of each command step-by-step. This helps in pinpointing exactly where a script might fail.
Using trap for Cleanup: Set up a trap to clean up resources (like temporary files) if the script exits unexpectedly. This ensures that resources are not left in an inconsistent state.
Redirecting Errors: Redirect error messages to a file using 2> to capture and log errors for further analysis.
Creating Custom Error Messages: Enhance error handling by providing custom error messages. This gives more context about what went wrong, making debugging easier.

Effective Error Handling Techniques in Shell Scripting – Day 11