Module 2.2 – Command Chaining

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🎯 What You Will Learn

1. Understand what command chaining means in Bash scripting.
2. Learn how to use the chaining operators ;, &&, and || effectively.
3. Differentiate between sequential, conditional, and logical chaining.
4. Combine commands efficiently to automate multi-step WordPress server tasks.
5. Apply chaining with exit statuses for advanced scripting control.
6. Avoid common pitfalls such as unintended command execution or missing error checks.

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1. 5W + 1H Framework

Element	Description
What	Command chaining lets you link multiple commands on a single line, controlling how the next command executes based on the success or failure of the previous one.
Why	Reduces script size, improves readability, and adds conditional flow without full if statements.
Who	Useful for DevOps engineers, sysadmins, or WordPress managers automating shell operations.
Where	Within Bash scripts or directly in terminal sessions.
When	When you need to run tasks sequentially or stop execution upon failure.
How	Use chaining operators like ;, &&, and `

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2. Prerequisites

• Familiarity with Bash syntax and execution permissions (chmod +x).
• Basic knowledge of exit status codes (0 = success, non-zero = failure).
• Understanding of WordPress server tasks and automation.

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3. Core Structure: Chaining Operators Overview

Operator	Name	Execution Behavior	Example	Description
;	Sequential	Executes commands one after another, regardless of success or failure.	cmd1; cmd2	Always runs both commands.
&&	AND (Conditional)	Executes the next command only if the previous one succeeded (exit code 0).	cmd1 && cmd2	Runs cmd2 only if cmd1 succeeded.
`		`	OR (Alternative)	Executes the next command only if the previous one failed (non-zero exit code).

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4. Basic Syntax and Output

4.1 Using ; – Sequential Execution

echo "Updating system..."; apt update; echo "Done."

Expected Output:

Updating system...
Hit:1 http://archive.ubuntu.com/ubuntu ...
Done.

Even if one command fails, Bash continues executing the rest.

4.2 Using && – Conditional Success

mkdir /home/backup && echo "Backup directory created successfully."

Expected Output:

Backup directory created successfully.

If the directory already exists (error), the echo command will not run.

4.3 Using || – Conditional Failure

mkdir /home/backup || echo "Backup directory already exists."

Expected Output:

Backup directory already exists.

If mkdir fails, the echo command is executed as a fallback.

5. Combining Operators

You can chain multiple operators in one line.

systemctl restart lsws && echo "Server restarted successfully" || echo "Server restart failed"

Logic Explanation:

1. If restart succeeds → print “Server restarted successfully.”
2. If restart fails → print “Server restart failed.” Expected Output (success case):

Server restarted successfully

Expected Output (failure case):

Server restart failed

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6. Practical Examples for WordPress VPS

Use Case	Command Example	Description
Backup and Clean	tar -czf /home/backup/wp.tar.gz /var/www/html && rm -rf /tmp/cache	Compress WordPress directory, then clear cache if backup succeeded.
Update & Restart OLS	wp plugin update --all && systemctl reload lsws	Updates all plugins; reloads web server only if successful.
Database Export Fallback	`wp db export /home/backup/db.sql
Sequential Tasks	apt update; apt upgrade -y; systemctl restart lsws	Run three maintenance commands in sequence regardless of success.
Conditional Chain	`[ -f /etc/lsws/httpd_config.conf ] && echo "Config exists"

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7. Combining with Subshells and Grouping

You can combine grouping ({} or ()) with chaining for cleaner logic:

{ wp plugin update --all && wp theme update --all; } || echo "Update failed"

Expected Output (if success):

Success: Updated all plugins.
Success: Updated all themes.

If one fails:

Update failed

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8. Using Exit Codes for Control Flow

You can check the exit code of the previous command using $?.

wp db export /home/backup/db.sql
if [ $? -eq 0 ]; then
  echo "Database exported successfully."
else
  echo "Database export failed."
fi

Short equivalent using chaining:

wp db export /home/backup/db.sql && echo "Success" || echo "Fail"

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9. Static vs Dynamic Framing

Type	Description	Behavior
Static (Sequential)	Uses ; — always runs all commands.	Ideal for linear workflows.
Dynamic (Conditional)	Uses && and `

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10. Troubleshooting Matrix

Issue	Symptom	Cause	Solution
Unexpected command execution	Both success and failure messages printed	Wrong operator combination	Use && and `
Command stops too early	Chain breaks on error	&& stops when first fails	Use ; for sequential execution
Missing spaces	Syntax error near unexpected token	No space between operators	Always use spaces: cmd1 && cmd2
Cron chain fails	Works manually but not in cron	PATH differences	Use full command paths (e.g., /usr/bin/wp)

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11. Quick Lab – WordPress Maintenance Chain

Script: wp_maintenance.sh

#!/bin/bash
echo "Starting maintenance..."
wp plugin update --all --path=/var/www/html && \
wp theme update --all --path=/var/www/html && \
wp core update --path=/var/www/html && \
echo "All updates completed successfully" || \
echo "One or more updates failed"

Run:

chmod +x wp_maintenance.sh
./wp_maintenance.sh

Expected Output (success):

Success: Updated all plugins.
Success: Updated all themes.
Success: WordPress updated successfully.
All updates completed successfully

Expected Output (failure):

Error: Connection timeout.
One or more updates failed

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12. Cheat Sheet

Operator	Description	Example
;	Run sequentially regardless of result	cmd1; cmd2
&&	Run next only if previous succeeded	cmd1 && cmd2
`		`
Combine && + `		`
Test command exit code	$?	echo $?

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13. Mini-Quiz

#	Question	Answer
1	Which operator runs commands sequentially regardless of success?	;
2	Which operator runs the next command only on success?	&&
3	Which operator runs the next command only on failure?	`
4	What does $? represent?	The exit status of the previous command.
5	Why prefer && over ; in automation?	It ensures dependent tasks only run if prior ones succeed.

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