📘 Variable Declaration

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🎯 What You Will Learn

1. Understand how to declare, assign, and reference variables in Bash.
2. Learn the difference between local and global variables.
3. Practice naming conventions, valid characters, and scope rules.
4. Prevent variable overwrites and unexpected behavior in scripts.
5. Use variables effectively in WordPress automation (backups, updates, configs).
6. Apply best practices for readability and portability in VPS environments.

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1. 5W + 1H Framework

Element	Description
What	A variable is a named memory reference used to store data (string, number, path, etc.) in Bash.
Why	Variables allow reusability, configurability, and clean code — essential for automation scripts.
Who	System administrators, Bash developers, and WordPress engineers automating VPS workflows.
Where	In any Bash shell or script file (.sh), often inside /usr/local/bin or /home/scripts/.
When	When storing credentials, directory paths, or script configuration data.
How	Use var=value to define and $var or ${var} to reference it.

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2. Prerequisites

• Completed Module 2.5 (Comments).
• Familiarity with basic Bash syntax and file execution.
• Working knowledge of environment variables like $HOME or $PATH.

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3. Core Syntax & Concept

No	Syntax Formula	Type	Syntax Example	Description	Output Behavior
1	name=value	Basic Declaration	site="WordPress"	Assigns string or number; no spaces around =.	Creates variable in current shell.
2	$name	Reference Variable	echo $site	Retrieves variable content.	Prints variable’s value.
3	${name}	Braced Reference	echo "Welcome to ${site}!"	Safer form for concatenation or complex names.	Expands variable correctly.
4	readonly name=value	Constant Declaration	readonly author="Donny"	Prevents further modification.	Immutable variable.
5	unset name	Delete Variable	unset site	Removes variable from memory.	Variable no longer exists.
6	declare -i name=value	Integer Variable	declare -i count=5	Stores arithmetic-only value.	Treated as number.
7	declare -r name=value	Read-Only Variable	declare -r wp_path="/var/www/html"	Makes variable unchangeable.	Permanent for session.
8	local name=value	Local Variable (in function)	local status="active"	Restricts variable scope inside function.	Exists only within function.
9	export name=value	Environment Variable	export PATH=$PATH:/usr/local/bin	Makes variable accessible to child processes.	Available system-wide.
10	name=$(command)	Command Substitution	today=$(date +%F)	Assigns command output to variable.	Executes and stores output.
11	declare -p name	Inspect Variable	declare -p site	Displays variable type and value.	Prints definition.
12	typeset name=value	Alternate Declaration (POSIX)	typeset version="1.2"	Similar to declare, legacy compatible.	Sets variable accordingly.

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4. Demonstration Example

Input Script:

#!/bin/bash
site="WordPress"
path="/var/www/html"
today=$(date +%F)

echo "Site: $site"
echo "Path: $path"
echo "Backup Date: $today"

Expected Output:

Site: WordPress
Path: /var/www/html
Backup Date: 2025-10-09

Explanation: Variables are declared using name=value with no spaces. Values are accessed using $name or ${name}.

5. Use Cases (WordPress-VPS Focused)

5.1 (Ref: #1) – Basic Declaration for Site Metadata

Input:

site="metaxenith.com"
echo "Deploying for $site..."

Expected Output:

Deploying for metaxenith.com...

Explanation: Simple variable assignment for flexible automation scripts. Use Case: For domain-specific deployment scripts.

5.2 (Ref: #3) – Using Braced Variables for Safe Expansion

Input:

project="wpstrategist"
echo "Project directory: /home/${project}/public_html/"

Expected Output:

Project directory: /home/wpstrategist/public_html/

Explanation: Braces prevent ambiguity when concatenating. Use Case: Used when combining paths or filenames dynamically.

5.3 (Ref: #4) – Creating Immutable Script Variables

Input:

readonly author="Donny"
echo "Author: $author"
author="John"   # Attempt to change

Expected Output:

Author: Donny
bash: author: readonly variable

Explanation: Prevents accidental overwrite of critical metadata. Use Case: Protect script-level constants like version or author.

5.4 (Ref: #8) – Local Variables in Functions

Input:

function wp_backup() {
  local folder="/home/wpbackup"
  echo "Backing up to $folder..."
}
wp_backup
echo "Outside: $folder"

Expected Output:

Backing up to /home/wpbackup...
Outside:

Explanation: Local variable exists only inside function. Use Case: Isolates variable scope in modular WP scripts.

5.5 (Ref: #9) – Exporting Environment Variables

Input:

export WP_ENV="production"
echo "Current WP environment: $WP_ENV"

Expected Output:

Current WP environment: production

Explanation: export makes variable available to child processes or subshells. Use Case: Pass environment mode (staging, production, etc.) to WP-CLI tasks.

5.6 (Ref: #10) – Storing Command Output

Input:

backup_date=$(date +%F)
echo "Backup filename: site_$backup_date.tar.gz"

Expected Output:

Backup filename: site_2025-10-09.tar.gz

Explanation: Uses command substitution to insert dynamic values. Use Case: Automate file naming for backups and logs.

5.7 (Ref: #6) – Integer Variable Example

Input:

declare -i count=5
count+=10
echo "Total count: $count"

Expected Output:

Total count: 15

Explanation: declare -i ensures arithmetic behavior. Use Case: Counting sites, users, or backup iterations.

5.8 (Ref: #5) – Unset Variables for Cleanup

Input:

tmp_file="/home/tmp/test.txt"
echo "Temp file: $tmp_file"
unset tmp_file
echo "After unset: $tmp_file"

Expected Output:

Temp file: /home/tmp/test.txt
After unset:

Explanation: Releases variable to avoid memory or data leaks. Use Case: For cleaning temporary paths in automation jobs.

6. Best Practices

Practice	Description
Avoid spaces around =	Bash treats it as syntax error.
Always quote variables ("$var")	Prevents issues with spaces in values.
Use uppercase for constants	Improves readability (e.g., BACKUP_DIR).
Prefix local vars inside functions	Prevents global overrides.
Use braces for concatenation (${var}_suffix)	Avoid misinterpretation.
Use descriptive names	e.g., wp_backup_path, db_name, env_mode.

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7. Common Mistakes & Fixes

Mistake	Wrong Example	Correct Example	Explanation
Space around =	site = "WordPress"	site="WordPress"	Spaces invalidate assignment.
Forgetting $ when referencing	echo site	echo $site	$ required to retrieve value.
Overwriting constants	readonly version="1.0" then version="2.0"	Don’t modify readonly vars.	Read-only variable is immutable.
Using undefined variable	echo $notset	Define before use.	Returns blank or error if unset.

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8. Quick Lab – WordPress Environment Config

Script: wp_env_config.sh

#!/bin/bash
# -----------------------------------------
# Script Name: wp_env_config.sh
# Purpose: Manage WordPress environment variables
# -----------------------------------------

SITE_NAME="wpstrategist.com"
WP_PATH="/var/www/html"
ENV_MODE="production"
TODAY=$(date +%F)

echo "🔧 Configuring $SITE_NAME..."
echo "Path: $WP_PATH"
echo "Mode: $ENV_MODE"
echo "Date: $TODAY"

Expected Output:

🔧 Configuring wpstrategist.com...
Path: /var/www/html
Mode: production
Date: 2025-10-09

Explanation: Demonstrates practical use of multiple variables for WP automation scripts.

9. Troubleshooting Matrix

Issue	Symptom	Cause	Solution
“command not found” error	Variable unexpanded	Forgot $ prefix	Always use $name or ${name}.
Empty variable	Output blank	Variable unset or misspelled	Check with declare -p name.
Permission denied	Cannot change variable	Declared readonly	Remove readonly or restart shell.
Variable lost after script ends	Value disappears	Not exported	Use export var=value for persistence.

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10. Static vs Dynamic Framing

Framing	Description	Behavior
Static	Hardcoded assignments (e.g., site="wpstrategist")	Reliable but not adaptable.
Dynamic	Variables based on input or commands	More flexible and automated.

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11. Cheat Sheet

Task	Syntax / Example
Declare variable	var=value
Access variable	$var
Read-only variable	readonly var=value
Remove variable	unset var
Export variable	export var=value
Local variable (function)	local var=value
Command substitution	var=$(command)
Inspect variable	declare -p var

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12. Mini-Quiz

#	Question	Answer
1	What’s the correct syntax to assign a value in Bash?	name=value
2	How do you print the value of a variable?	echo $name
3	What happens if you use spaces around =?	Syntax error.
4	How do you make a variable read-only?	readonly name=value
5	Which command removes a variable from memory?	unset name

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