Env_vars

Chapter 26: Environment Variables in Bash

Overview

Environment variables are named values that affect how processes run. They are crucial for configuration, secrets management, and controlling program behavior in DevOps environments.

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Understanding Environment Variables

What are Environment Variables?

Environment variables are key-value pairs that:

• Configure system and application behavior
• Store user preferences
• Hold sensitive information (credentials, tokens)
• Enable process communication

┌────────────────────────────────────────────────────────────────┐
│                   Environment Variable Flow                     │
├────────────────────────────────────────────────────────────────┤
│                                                                │
│   Shell Startup                                                │
│   ────────────                                                │
│                                                                │
│   /etc/profile      ──┐                                        │
│   ~/.bash_profile   ──┼──► Environment Variables               │
│   ~/.bashrc         ──┘                                        │
│                                                                │
│        │                                                        │
│        ▼                                                        │
│   Process gets copy                                            │
│        │                                                        │
│        ▼                                                        │
│   Child processes inherit                                       │
│                                                                │
│   ┌───────┐     ┌───────┐     ┌───────┐                       │
│   │ bash  │────►│ nginx │────►│ app   │                       │
│   └───────┘     └───────┘     └───────┘                       │
│                                                                │
│   Each gets its own copy                                       │
│                                                                │
└────────────────────────────────────────────────────────────────┘

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Viewing Environment Variables

printenv

# Print all environment variables
printenv

# Print specific variable
printenv HOME
printenv PATH
printenv USER

# Print with grep
printenv | grep -i path

env

# List all environment variables
env

# Run command with modified env
env VAR=value command

# Remove variable
env -u VAR command

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Common System Variables

Essential Variables

# User information
echo $USER        # Current username
echo $HOME        # Home directory
echo $UID         # User ID
echo $GID         # Group ID

# Shell information
echo $SHELL       # Default shell
echo $PWD         # Current directory
echo $OLDPWD      # Previous directory

# System information
echo $HOSTNAME    # Host name
echo $HOSTTYPE    # Machine type
echo $ARCH        # Architecture

# Path
echo $PATH        # Executable search path
echo $PATH        # Separate with:
                  # export PATH="/new/path:$PATH"

# Terminal
echo $TERM        # Terminal type
echo $LINES       # Terminal lines
echo $COLUMNS     # Terminal columns

Language and Locale

echo $LANG        # Default language
echo $LC_ALL      # Locale override
echo $LANGUAGE    # Language priority
echo $TZ          # Timezone

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Setting Variables

Temporary Variables

# Set variable (current shell only)
MY_VAR="value"

# Export for child processes
export MY_VAR="value"

# Combined
export MY_VAR="value"

# Or equivalently
declare -x MY_VAR="value"

Persistent Variables

~/.bashrc

# Add to ~/.bashrc
export EDITOR=vim
export VISUAL=vim
export PATH="/home/user/bin:$PATH"
export MY_VARIABLE="value"

# Reload
source ~/.bashrc

~/.bash_profile (or ~/.profile)

# Add to ~/.bash_profile
export PATH="$HOME/bin:$PATH"
export EDITOR=vim

# For system-wide, edit:
# /etc/environment
# /etc/profile
# /etc/profile.d/*.sh

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Variable Scope

Shell vs Environment

# Shell variable (not exported)
MY_VAR="shell_only"
echo $MY_VAR

# Export to environment
export MY_VAR
printenv MY_VAR   # Now visible

Inheritance

# Parent exports variable
export MY_VAR="parent_value"

# Child inherits
bash -c 'echo $MY_VAR'

# Child cannot affect parent
bash -c 'export MY_VAR="child_value"'
echo $MY_VAR  # Still parent_value

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Special Variable Operations

unset - Remove Variable

# Remove variable
unset MY_VAR

# Remove exported variable
unset -v MY_VAR

# Remove function
unset -f my_function

readonly - Make Immutable

# Read-only variable
readonly MY_CONST="value"
MY_CONST="new"  # Error

# Read-only function
readonly -f my_function

declare - Variable Declaration

# Declare variable
declare -x MY_VAR="value"  # Exported
declare +x MY_VAR          # Remove export
declare -r MY_VAR="value"  # Read-only
declare -i MY_VAR=5        # Integer
declare -a MY_ARRAY=(a b c)  # Array
declare -A MY_ASSOC=( [key]=value )  # Associative

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Advanced Topics

Parameter Expansion in Variables

# Default value
echo ${VAR:-default}   # Use default if unset/null

# Assign default
echo ${VAR:=default}   # Set if unset/null

# Use alternate value if set
echo ${VAR:+alternate} # Use alternate if set

# Error if unset
echo ${VAR:?error}     # Print error if unset

# Remove pattern
${VAR#pattern}        # Remove shortest match from start
${VAR##pattern}       # Remove longest match from start
${VAR%pattern}         # Remove shortest match from end
${VAR%%pattern}       # Remove longest match from end

Array from Environment

# Export array-like variable
export MY_LIST="one:two:three"

# Read as array
IFS=':' read -ra ARR <<< "$MY_LIST"
echo "${ARR[0]}"  # one

# Alternative: multiple exports
export MY_VAR_1="value1"
export MY_VAR_2="value2"

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Real-World DevOps Examples

Application Configuration

#!/usr/bin/env bash
# Application startup with environment

# Configuration
export APP_NAME="myapp"
export APP_ENV="${APP_ENV:-production}"
export APP_PORT="${APP_PORT:-8080}"
export APP_HOST="${APP_HOST:-0.0.0.0}"
export LOG_LEVEL="${LOG_LEVEL:-info}"

# Database
export DB_HOST="${DB_HOST:-localhost}"
export DB_PORT="${DB_PORT:-5432}"
export DB_NAME="${DB_NAME:-appdb}"
export DB_USER="${DB_USER:-appuser}"

# Start application
echo "Starting $APP_NAME in $APP_ENV mode"
./bin/$APP_NAME

Secrets Management

#!/usr/bin/env bash
# Load secrets from file

SECRETS_FILE="/etc/myapp/secrets"

if [[ -f "$SECRETS_FILE" ]]; then
    set -a  # Auto-export
    source "$SECRETS_FILE"
    set +a
fi

# Now use variables
echo "Database: $DB_PASSWORD" | head -c 5
# ****

Docker Environment

#!/usr/bin/env bash
# Docker environment variables

# Docker sets these automatically
echo "Image: $IMAGE_NAME"
echo "Container: $HOSTNAME"

# User-defined
echo "Config: $MY_CONFIG"

# All env in container
env | sort

Kubernetes Environment

#!/usr/bin/env bash
# Kubernetes environment variables

# Kubernetes injects:
echo "Pod: $POD_NAME"
echo "Namespace: $NAMESPACE"
echo "Node: $NODE_NAME"

# Container resources
echo "CPU limit: $CPU_LIMIT"
echo "Memory limit: $MEMORY_LIMIT"

# ConfigMap values
echo "Config: $CONFIG_KEY"

# Secret values
echo "Secret: $SECRET_KEY"

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PATH Management

Adding to PATH

# Add to beginning (takes precedence)
export PATH="/usr/local/bin:$PATH"

# Add to end
export PATH="$PATH:/usr/local/bin"

# Add multiple
export PATH="/opt/bin:/usr/local/bin:$PATH"

# Conditional addition
if [[ ":$PATH:" != *":/custom/bin:"* ]]; then
    export PATH="/custom/bin:$PATH"
fi

Finding Commands

# Find command location
which python
which -a python     # All occurrences
whereis python     # Binary, source, man pages

# Check if in PATH
command -v python  # Returns path or empty
type python        # Shows type

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Environment Files

/etc/environment

# Simple format (no export)
VAR1=value1
VAR2=value2

/etc/profile.d/

# Create custom.sh
sudo tee /etc/profile.d/custom.sh <<'EOF'
export CUSTOM_VAR="value"
export PATH="/opt/custom/bin:$PATH"
EOF

# Makes available to all users on login

systemd Service Environment

# In service file
[Service]
Environment="VAR1=value1"
EnvironmentFile=/etc/myapp/env
ExecStart=/usr/bin/myapp

# Environment file format
# VAR=value
# Lines starting with # are comments

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Security Considerations

Sensitive Data

# Never store secrets in environment variables that persist
# - .bashrc
# - .bash_history
# - Log files

# Use secret management tools instead:
# - HashiCorp Vault
# - AWS Secrets Manager
# - Kubernetes Secrets

# Example: Kubernetes Secret
# kubectl create secret generic db-creds \
#   --from-literal=username=admin \
#   --from-literal=password=secret

Best Practices

.env

# Don't put secrets in version control
# .gitignore:
# Use .env files with loading mechanism
if [[ -f .env ]]; then
    set -a
    source .env
    set +a
fi

# .env should contain:
# APP_KEY=xxxxx
# DATABASE_PASSWORD=xxxxx
# API_TOKEN=xxxxx

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Debugging Environment

Show Environment Changes

#!/usr/bin/env bash
# Debug environment

echo "=== Original Environment ==="
env | sort > /tmp/env_before

# Run command
"$@"

echo "=== New Environment ==="
env | sort > /tmp/env_after

# Compare
diff /tmp/env_before /tmp/env_after

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Summary

In this chapter, you learned:

• ✅ What environment variables are
• ✅ Viewing variables (printenv, env)
• ✅ Common system variables
• ✅ Setting variables (temporary and persistent)
• ✅ Variable scope and inheritance
• ✅ Special operations (unset, readonly, declare)
• ✅ Parameter expansion
• ✅ Real-world DevOps examples
• ✅ PATH management
• ✅ Environment files (/etc/environment, profile.d/)
• ✅ Security considerations

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Next Steps

Continue to the next chapter to learn about Shell Expansion.

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