📘 Exit Status Codes - General Explain

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🎯 What You Will Learn

1. Understand the meaning and purpose of exit status codes in Bash.
2. Learn how to use the special variable $? to check command results.
3. Identify standard system exit codes (0–255) and their meanings.
4. Implement success/failure logic using exit codes for automation reliability.
5. Apply exit handling in WordPress backup, update, and monitoring scripts.
6. Use exit manually in scripts to signal custom outcomes.

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1. 5W + 1H Framework

Element	Description
What	Exit status codes are numeric values returned by commands to indicate success or failure.
Why	They allow scripts to make decisions, retry, or stop based on whether a command succeeded or failed.
Who	Used by Bash, system daemons, and developers automating Linux/WordPress workflows.
Where	Every executed command (interactive or script) produces an exit code.
When	After each command or script execution, accessed immediately using $?.
How	0 means success, non-zero values (1–255) mean failure or specific errors.

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2. Prerequisites

• Basic understanding of running Bash commands and scripts.
• Knowledge of command chaining (&&, ||) from Module 2.2.
• Familiarity with terminal navigation and permissions.

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3. Core Concept

Every command executed in Linux returns an exit status to indicate whether it succeeded or failed. You can view the result using the $? variable immediately after execution.

ls /var/www/html
echo $?

Expected Output (if directory exists):

0

Expected Output (if directory doesn’t exist):

2

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4. Standard Exit Codes Reference

Code	Meaning	Description
0	Success	Command executed successfully.
1	General Error	Catch-all for general failures.
2	Misuse of Shell Builtins	Syntax or incorrect command usage.
126	Command Invoked Cannot Execute	Permission or execution problem.
127	Command Not Found	Missing or invalid command.
128	Invalid Exit Argument	Exit called with invalid number.
130	Script Terminated (Ctrl+C)	Interrupted manually.
137	Killed (SIGKILL)	Terminated by system or OOM (Out Of Memory).
255	Out of Range / Fatal Error	Typically custom fatal errors.

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5. Checking Exit Codes in Practice

5.1 Successful Command

echo "Hello, world!"
echo $?

Expected Output:

Hello, world!
0

5.2 Failed Command

ls /nonexistent/path
echo $?

Expected Output:

ls: cannot access '/nonexistent/path': No such file or directory
2

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6. Using Exit Codes in Conditional Logic

wp plugin update --all --path=/var/www/html
if [ $? -eq 0 ]; then
  echo "✅ Plugins updated successfully."
else
  echo "❌ Plugin update failed."
fi

Expected Output (success):

✅ Plugins updated successfully.

Expected Output (failure):

❌ Plugin update failed.

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7. Short Form Using Chaining

Instead of if, you can use chaining (&&, ||):

wp core update --path=/var/www/html && echo "Core update success" || echo "Core update failed"

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8. Manual Exit in Scripts

You can explicitly exit a script with a custom code:

#!/bin/bash
tar -czf /home/backup/wp.tar.gz /var/www/html || { echo "Backup failed"; exit 1; }
echo "Backup completed successfully"
exit 0

Explanation:

• exit 1 — signals failure to parent process (cron, CI, etc.).
• exit 0 — signals success. Expected Behavior: If backup fails, script stops immediately with exit code 1. Check script result:

echo $?

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9. Integrating with Cron and Monitoring Systems

Monitoring tools and cron jobs rely on exit codes:

• 0 → no alert
• 1 or higher → alert, retry, or log error Example cron entry:

0 2 * * * /home/user/scripts/wp_backup.sh || echo "Backup failed on $(hostname)" >> /var/log/alerts.log

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10. Multi-Step Workflow Example

#!/bin/bash
echo "Starting WordPress maintenance..."

wp plugin update --all --path=/var/www/html
code=$?

if [ $code -eq 0 ]; then
  echo "Plugins updated successfully."
else
  echo "Plugin update failed with code: $code"
  exit $code
fi

wp theme update --all --path=/var/www/html
echo "Maintenance completed successfully."
exit 0

Expected Output (success):

Starting WordPress maintenance...
Success: Updated all plugins.
Plugins updated successfully.
Maintenance completed successfully.

If plugin update fails:

Starting WordPress maintenance...
Error: Connection timeout.
Plugin update failed with code: 1

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11. Best Practices

Practice	Description
Always check $? after critical commands	Ensures error handling logic runs correctly.
Use meaningful exit codes (0, 1, 2, 255)	Helps monitoring tools identify type of failure.
Exit early on failure using set -e	Stops script immediately when a command fails.
Log both command and exit code	Simplifies debugging and audit trails.
Use chaining for concise scripts	Replace verbose if checks with && and `

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12. Static vs Dynamic Framing

Framing	Description	Behavior
Static	Hardcoded return codes	Simple but less flexible
Dynamic	Contextual based on $?	Adapts to runtime results, ideal for automation

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13. Troubleshooting Matrix

Issue	Symptom	Cause	Solution
$? always 0	Variable checked too late	A new command executed before reading $?	Immediately check after command.
Script keeps running after failure	No exit or set -e used	Missing termination control	Add set -e or manual exit codes.
“Command not found”	Exit code 127	Missing binary or typo	Verify with which command.
“Permission denied”	Exit code 126	Script lacks execute permission	chmod +x script.sh.

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14. Quick Lab – Error-Aware Backup Script

Script: safe_backup.sh

#!/bin/bash
set -e
echo "Starting backup..."
tar -czf /home/backup/wp_backup_$(date +%F).tar.gz /var/www/html || exit 2
echo "Backup created successfully."
exit 0

Run:

chmod +x safe_backup.sh
./safe_backup.sh
echo $?

Expected Output (success):

Starting backup...
Backup created successfully.
0

Expected Output (failure):

tar: /var/www/html: No such file or directory
2

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15. Cheat Sheet

Task	Command / Example
Check last command status	echo $?
Success code	0
General failure	1
Command not found	127
Stop script on error	set -e
Custom exit code	exit 5
Combine check + message	`cmd && echo "OK"
Log exit code	echo $? >> /var/log/result.log

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16. Mini-Quiz

#	Question	Answer
1	What does exit code 0 represent?	Success
2	How do you check the last command’s exit code?	echo $?
3	What does exit code 127 mean?	Command not found
4	How can you stop a script when a command fails?	Use set -e or exit after error.
5	Why use exit codes in cron jobs?	To detect failures and trigger alerts or retries.

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Would you like me to continue next with Module 2.4 – Conditional Execution (if, elif, else) to connect this module’s exit logic into full control flow scripting?