📘 Environment Variables

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🎯 What You Will Learn

1. Understand what environment variables are and how they differ from normal variables.
2. Learn how they are created, exported, listed, modified, and persisted.
3. Explore system and user-level variable configuration files (.bashrc, /etc/environment).
4. Pass environment data across subshells, cron jobs, and WP-CLI scripts.
5. Apply environment variable management to WordPress VPS automation.
6. Safely debug and verify variable states during runtime.

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1. 5W + 1H Framework

Element	Description
What	Environment variables are key–value pairs defining configuration and behavior for user sessions and processes.
Why	They allow flexible and reusable configuration across scripts, shells, and applications without hardcoding values.
Who	System administrators, Bash developers, and WordPress VPS engineers maintaining multiple environments.
Where	Stored in profile files like /etc/environment, /etc/profile, ~/.bashrc, and ~/.profile.
When	When controlling system paths, app configurations, or running WordPress scripts under different environments.
How	Use export, env, or printenv to define and view environment variables; modify profile files for persistence.

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2. Prerequisites

• Completed Module 3.1 (Variable Declaration).
• Understanding of bashrc, profile files, and Linux user sessions.
• Access to a VPS environment with root privileges.

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3. Core Syntax & Concept (Expanded and Factual)

No	Syntax Formula	Command / Type	Syntax Example	Description	Behavior / Output
1	export NAME=value	Create Environment Variable	export WP_ENV=production	Creates a new environment variable available to subshells and child processes.	Variable persists within current session.
2	NAME=value	Shell Variable (non-exported)	WP_ENV=staging	Defines variable only for current shell.	Not inherited by child shells.
3	echo $NAME	Reference Variable	echo $PATH	Prints the value of an environment variable.	Displays variable’s content.
4	env	List All Environment Variables	env	Lists all exported environment variables.	Prints full environment list.
5	printenv	View Environment Variable	printenv HOME	Shows value of a specific environment variable.	Displays only selected variable.
6	set	List Shell + Environment Variables	`set	less`	Displays both shell and environment variables.
7	export -p	List All Exported Variables	`export -p	grep WP_`	Lists all variables currently exported to environment.
8	declare -x NAME=value	Alternate Export Syntax	declare -x PHP_VER=8.3	Equivalent to export. Preferred for clarity in scripts.	Creates global variable.
9	unset NAME	Remove Variable	unset WP_ENV	Deletes an environment or shell variable.	Variable no longer exists.
10	export NAME=$(command)	Dynamic Assignment	export TODAY=$(date +%F)	Assigns command output to environment variable.	Command executes; result stored.
11	readonly NAME=value	Read-Only Variable	readonly SHELL_TYPE=bash	Prevents modification or unsetting.	Immutable within session.
12	export -n NAME	Unexport Variable	export -n WP_ENV	Removes export property, keeps it local.	Variable no longer global.
13	env -i command	Run Command in Empty Environment	env -i bash	Executes command with a clean, minimal environment.	Isolated environment.
14	env VAR=value command	Temporary Environment Variable	env WP_ENV=dev wp plugin list	Sets variable only for duration of command.	Reverts after command ends.
15	`printenv	grep keyword`	Filtered Listing	`printenv	grep PHP`
16	source ~/.bashrc	Reload Environment	source ~/.bashrc	Reloads updated user environment variables.	Applies new settings immediately.
17	. ~/.profile	POSIX Reload Method	. ~/.profile	Equivalent to source, ensures compatibility.	Loads profile variables.
18	/etc/environment	System-Wide Persistent Vars	export BACKUP_DIR=/home/wpbackup	File used for persistent variables across all users.	Loaded on login.
19	`printenv	sort`	Inspect Organized Output	`printenv	sort
20	`env	grep -E 'PATH	USER'`	Combined Filter Example	`env
21	echo $0	Identify Shell	echo $0	Displays current shell name (bash, sh, zsh).	Useful when debugging variable scope.
22	export PATH=$PATH:/new/path	Extend Environment Variable	export PATH=$PATH:/usr/local/bin	Adds directories to $PATH.	Appends without overwriting.
23	printenv SHELL	Display Current Shell Path	printenv SHELL	Shows location of current shell binary.	Example: /bin/bash.
24	envsubst	Substitute Environment Variables in Files	envsubst < input.conf > output.conf	Replaces variables with their values.	Dynamic templating for scripts.
25	`grep -E 'export	declare' ~/.bashrc`	Inspect User Exports	grep export ~/.bashrc	Checks what’s defined permanently in config.
26	cat /proc/$$/environ	Inspect Raw Environment	`cat /proc/$$/environ	tr '\0' '\n'`	Displays environment vars at process level.

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4. Demonstration Example

Input Script:

#!/bin/bash
export WP_ENV="production"
export BACKUP_DIR="/home/wpbackup"
export TODAY=$(date +%F)
export PHP_MEMORY_LIMIT="512M"

echo "Environment: $WP_ENV"
echo "Backup directory: $BACKUP_DIR"
echo "Today's date: $TODAY"
echo "PHP Memory Limit: $PHP_MEMORY_LIMIT"

Expected Output:

Environment: production
Backup directory: /home/wpbackup
Today's date: 2025-10-09
PHP Memory Limit: 512M

Explanation: All exported variables are accessible globally, including subshells and child processes like WP-CLI or cron tasks.

5. Use Cases (WordPress-VPS Focused)

5.1 Persistent Server-Wide Environment Setup

sudo nano /etc/environment
# Add:
WP_PATH="/var/www/html"
BACKUP_DIR="/home/wpbackup"
PHP_MEMORY_LIMIT="512M"

Use Case: Keeps WP configuration and backup directories consistent across all users and reboots.

5.2 Temporary Environment Variables for WP Commands

env WP_ENV=staging wp plugin update --all --allow-root

Expected Output:

Success: Updated 10 of 10 plugins.

Use Case: Quickly test or perform actions under different WP environments without permanent change.

5.3 Dynamic Assignment for Logging

export LOG_DATE=$(date +%F_%H-%M)
echo "Backup started at $LOG_DATE"

Expected Output:

Backup started at 2025-10-09_19-45

Use Case: Automates time-stamped log or backup filenames.

5.4 Clean Session Testing

env -i bash
echo $PATH

Expected Output:

(blank)

Use Case: Test minimal environment for troubleshooting WP scripts without interference from global settings.

5.5 Using envsubst for Config Templates

export DB_NAME=wp_main
export DB_USER=wp_user
export DB_PASS=securepass
cat <<EOF > config_template.conf
database=${DB_NAME}
user=${DB_USER}
password=${DB_PASS}
EOF

envsubst < config_template.conf > wpconfig_ready.conf
cat wpconfig_ready.conf

Expected Output:

database=wp_main
user=wp_user
password=securepass

Use Case: Generate dynamic configuration files using environment variable substitution.

6. Best Practices

Practice	Description
Always quote variables containing spaces	e.g., export PATH="/usr/local/bin:/usr/bin".
Avoid redefining core system vars like $HOME or $SHELL.
For permanent variables, use ~/.bashrc or /etc/environment.
Prefix application-specific vars (WP_, DB_) for clarity.
Use envsubst or template files for injecting env values safely.
Re-source session after edits with source ~/.bashrc.
Keep sensitive values (passwords) restricted with correct file permissions.

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7. Common Mistakes & Fixes

Mistake	Wrong Example	Correct Example	Explanation
Space around =	export WP_ENV = production	export WP_ENV=production	Bash treats space as syntax error.
Missing export	WP_ENV=staging	export WP_ENV=staging	Variable not inherited by subshells.
Overwriting $PATH	export PATH=/usr/local/bin	export PATH=$PATH:/usr/local/bin	Must append, not overwrite.
Unreloaded session	source ~/.bashrc missing	source ~/.bashrc	Reload needed after file edits.
Exposing secrets	echo $DB_PASS in logs	Store secrets in .env with chmod 600	Avoid leaking sensitive data.

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8. Quick Lab – Global WP Environment Setup

Script: wp_env_global.sh

#!/bin/bash
# --------------------------------------------
# Script Name: wp_env_global.sh
# Purpose: Define and verify environment variables
# --------------------------------------------

export WP_ENV="production"
export WP_PATH="/var/www/html"
export BACKUP_DIR="/home/wpbackup"
export PHP_VER="8.3"
export TODAY=$(date +%F)

echo "🌐 Environment: $WP_ENV"
echo "📁 WordPress Path: $WP_PATH"
echo "💾 Backup Directory: $BACKUP_DIR"
echo "⚙️ PHP Version: $PHP_VER"
echo "📅 Today: $TODAY"

Expected Output:

🌐 Environment: production
📁 WordPress Path: /var/www/html
💾 Backup Directory: /home/wpbackup
⚙️ PHP Version: 8.3
📅 Today: 2025-10-09

Explanation: All variables are globally accessible, simplifying other script integrations like WP-CLI updates or backup automations.

9. Troubleshooting Matrix

Issue	Symptom	Cause	Solution
Variable not found	echo $VAR returns blank	Variable unset or unexported	Run export VAR=value.
Not persistent after reboot	Reset after login	Defined only in current session	Add to .bashrc or /etc/environment.
Command not working	PATH lost	Overwrote $PATH	Restore default or append instead.
Wrong env variable value in cron	Different shell context	Cron uses /bin/sh	Define variables inside cron script or use absolute path.
Exposed credentials	Visible via printenv	Sensitive vars unprotected	Move to .env file and secure permissions.

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10. Static vs Dynamic Framing

Framing	Description	Behavior
Static	Predefined, persistent variables in config files	Stable, ideal for constants like $WP_PATH.
Dynamic	Runtime variables from commands or scripts	Auto-updates each run (e.g., timestamps, runtime state).

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11. Cheat Sheet

Task	Command Example
Create variable	export NAME=value
Print variable	echo $NAME
Remove variable	unset NAME
List all env vars	env
Show one var	printenv NAME
Temporary variable	env NAME=value command
Reload session	source ~/.bashrc
Persistent vars	Add to /etc/environment
Substitute vars in file	envsubst < in > out
Append to PATH	export PATH=$PATH:/usr/local/bin

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12. Mini-Quiz

#	Question	Answer
1	Which command lists all environment variables?	env
2	How to make a variable available to subshells?	Use export.
3	Where are persistent environment variables stored?	In ~/.bashrc or /etc/environment.
4	What command reloads your shell environment?	source ~/.bashrc
5	How to assign a dynamic value to an env variable?	export VAR=$(command)

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