Error Handling in Shell Scripting

Error handling is crucial in shell scripting to ensure smooth execution and better debugging.

👉 https://youtu.be/YGX9pTRSUls?si=wPFoAUsfai8q-0WS

Below are various ways to handle errors effectively:

1. Exit Status ($?):

=> Every command in Linux returns an exit status (0 for success, non-zero for failure).

=> You can check the exit status using $?.

Example:


#!/bin/bash
mkdir /root/testdir  # Trying to create a directory in a restricted location
echo "Exit Status: $?"  # Check the exit status (0 = success, non-zero = failure)

💡 Best Practice: Use exit status checks to handle errors.

Before handling the error:

Explanation

mkdir returns an error because /root/testdir already exists.
The exit status ($?) of mkdir is 1, indicating failure.
The script correctly prints Exit Status: 1, confirming that mkdir failed.

After handling the error:


#!/bin/bash
mkdir /root/testdir
if [[ $? -ne 0 ]]; then
    echo "Error: Failed to create directory!"
    exit 1  # Exit script with error
fi

2. Using set -e (Exit on Error):

set -e makes the script exit immediately if any command fails.


#!/bin/bash
set -e  # Exit on error
echo "Error handling with set -e command"
mkdir /root/testdir  # This will cause the script to exit if it fails
echo "This will not execute if mkdir fails!"

⚠️ Be cautious: This might terminate the script abruptly if you don’t handle errors properly.

3.1. Using trap for Error Handling:

trap allows custom error handling before the script exits.

✅ trap helps in error handling and cleanup.
✅ trap ‘command’ ERR runs when a command fails.
✅ Always test trap carefully to avoid unexpected behaviors.


#!/bin/bash
trap 'echo "An error occurred! Exiting..."; exit 1' ERR
mkdir /root/testdir  # If this fails, the trap executes
echo "This will not run if an error occurs!"

3.2. Custom cleanup using trap:

✅ Use trap ‘command’ ERR to run cleanup on script exit

✅ Use trap ‘command’ EXIT to run cleanup on script exit (even if successful).

Example 1: A script which exits when a command fails


#!/bin/bash
trap 'echo "Cleaning up..."; rm -rf /tmp/tempfile; exit 1' ERR

touch /tmp/tempfile  # Temporary file
mkdir /root/testdir  # Fails, triggering the trap

1. The trap command sets up a handler for errors (ERR).

2. If any command fails, the script:

Prints “Cleaning up…”.
Prints “Cleaning up…”.
Removes the temporary file (/tmp/tempfile).

What Happens If /root/testdir Does Not Exist?

mkdir /root/testdir succeeds.
Since there is no error, the trap is not triggered.
The script exits normally.

Example 2: A script which exits normally


#!/bin/bash
trap 'echo "Cleaning up..."; rm -rf /tmp/tempfile; exit 1' ERR

touch /tmp/tempfile  # Temporary file
echo "Hello World!"

✅ Ensure Cleanup Even on Manual Script Exit:


#!/bin/bash
trap 'echo "Cleaning up..."; rm -rf /tmp/tempfile' ERR EXIT

touch /tmp/tempfile
echo "Hello World!"

Now, cleanup happens even if the script exits normally (EXIT).
Prevents leftover temp files.

4. Using || (OR) for Error Handling

The || operator allows fallback commands if the first one fails.


#!/bin/bash
mkdir /root/testdir || echo "Error: Cannot create directory!"

5. Using && (AND) to Ensure Success Before Execution

The && operator only runs the second command if the first succeeds.

✅ When /tmp/mydir does not exist (Success case):


#!/bin/bash
mkdir /tmp/mydir && echo "Directory created successfully!"

❌ When /tmp/mydir already exists (Failure case):


#!/bin/bash

mkdir /tmp/mydir && echo "Directory created successfully!" || echo "Error: Failed to create directory!"

📌 Explanation:

If mkdir succeeds → “Directory created successfully!” prints.
If mkdir fails → “Error: Failed to create directory!” prints.

6. Using if Condition for Error Handling

Using if to check command success/failure.


#!/bin/bash
if mkdir /root/testdir; then
    echo "Directory created successfully!"
else
    echo "Error: Cannot create directory!"
    exit 1
fi

7. Redirecting Error Messages

Redirect errors to a file for logging.


#!/bin/bash
mkdir /root/testdir 2>> error.log
echo "Check error.log for details!"

If the directory is created successfully, nothing happens (no error is logged).
If the directory already exists or the user does not have permission, an error message is logged.
2>> redirects only errors (stderr) to the error.log file.
If mkdir fails, the error message is appended (>>) to error.log.

8. Handling Errors in Loops

Break or continue on errors in loops.


#!/bin/bash
for dir in /root/testdir /tmp/testdir; do
    mkdir "$dir" || { echo "Failed to create $dir, skipping..."; continue; }
    echo "$dir created!"
done

🔍 How to Break the Loop When mkdir Fails:


#!/bin/bash
for dir in /root/testdir /tmp/testdir; do
    mkdir "$dir" || { echo "Failed to create $dir, skipping..."; break; }
    echo "$dir created!"
done

9. Debugging with set -x

set -x prints each command before execution for debugging.


#!/bin/bash
set -x  # Enable debugging
mkdir /root/testdir
echo "Debugging enabled!"

Disable debugging with set +x:


set +x  # Disable debugging

10. Using exit to Terminate on Errors

Use exit with an error code (non-zero) to terminate script execution.


#!/bin/bash
mkdir /root/testdir || { echo "Error: Cannot create directory!"; exit 1; }

✔ If mkdir succeeds, nothing happens (unless you add a success message).
✔ If mkdir fails, it prints "Error: Cannot create directory!” and exits.
✔ Use && echo “Success!” for better visibility.

📌 Best Practices for Shell Script Error Handling:

✅ Always check exit codes ($?)

✅ Use set -e for critical scripts

✅ Use trap to handle unexpected failures

✅ Log errors using 2>> logfile.log

✅ Use || to execute fallback commands

✅ Use set -x for debugging complex scripts

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