Environment Variables Management

View all environment variables

env
printenv
export -p  # With 'declare -x' format

View specific variable

echo $VARIABLE
printenv VARIABLE
env | grep VARIABLE

Set temporary variable (current shell)

VARIABLE=value

# Use in command
VARIABLE=value command

# Example
LANG=C ls /nonexistent

Export variable (child processes)

export VARIABLE=value
export PATH=/new/path:$PATH

# Or two-step
VARIABLE=value
export VARIABLE

Common environment variables

PATH          # Executable search path
HOME          # User home directory
USER          # Current username
SHELL         # Current shell
PWD           # Present working directory
OLDPWD        # Previous directory
LANG          # Language/locale
TERM          # Terminal type
EDITOR        # Default text editor
PAGER         # Default pager
PS1           # Primary shell prompt

View values

echo $PATH
echo $HOME
echo $USER
printenv PATH HOME USER

Modify PATH

# Append to PATH
export PATH=$PATH:/new/directory

# Prepend to PATH
export PATH=/new/directory:$PATH

# Replace PATH
export PATH=/usr/local/bin:/usr/bin:/bin

Unset variable

unset VARIABLE
unset PATH  # Careful!

Permanent environment variables

User-specific (~/.bashrc or ~/.profile)

# Add to ~/.bashrc
export JAVA_HOME=/usr/lib/jvm/java-11
export PATH=$PATH:$JAVA_HOME/bin

# Apply changes
source ~/.bashrc

System-wide (/etc/environment)

# Edit /etc/environment
sudo vim /etc/environment

# Add lines like:
JAVA_HOME="/usr/lib/jvm/java-11"
CUSTOM_VAR="value"

# Relogin or reboot

Profile scripts

# /etc/profile.d/custom.sh
export CUSTOM_VAR=value

# Make executable
sudo chmod +x /etc/profile.d/custom.sh

ENV vs PRINTENV

# printenv: Shows environment variables
printenv

# printenv with argument
printenv PATH

# env: Can run commands with modified environment
env VARIABLE=value command

# env -i: Clear environment
env -i command

Run with clean environment

# Remove all env vars
env -i bash

# Keep specific vars
env -i HOME=$HOME USER=$USER bash

# Run command
env -i /path/to/command

Set multiple variables

# For command
VAR1=value1 VAR2=value2 command

# Export multiple
export VAR1=value1 VAR2=value2

# Using env
env VAR1=value1 VAR2=value2 command

Variable expansion

# Simple
echo $VARIABLE

# Braces (recommended)  
echo ${VARIABLE}

# Default value if unset
echo ${VARIABLE:-default}

# Set if unset
echo ${VARIABLE:=default}

# Error if unset
echo ${VARIABLE:?error message}

# Use alternative if set
echo ${VARIABLE:+alternative}

Check if variable is set

# In script
if [ -z "$VARIABLE" ]; then
    echo "VARIABLE is not set"
fi

# Or
if [ -n "$VARIABLE" ]; then
    echo "VARIABLE is set"
fi

# Using printenv
if printenv VARIABLE > /dev/null 2>&1; then
    echo "VARIABLE exists"
fi

Process-specific variables

# Show environment of running process
cat /proc/PID/environ | tr '\0' '\n'

# Using ps
ps auxe | grep process-name

Scripting with environment

#!/bin/bash

# Check required variables
: ${DATABASE_URL:?DATABASE_URL must be set}
: ${API_KEY:?API_KEY must be set}

# Set defaults
: ${LOG_LEVEL:=INFO}
: ${PORT:=3000}

echo "Starting application on port $PORT"
echo "Log level: $LOG_LEVEL"

Load from .env file

#!/bin/bash

# Load .env file
if [ -f .env ]; then
    export $(cat .env | grep -v '^#' | xargs)
fi

# Or
set -a
source .env
set +a

.env file format

# .env
DATABASE_URL=postgresql://localhost/mydb
API_KEY=secret123
DEBUG=true
PORT=3000

Common patterns

Java

export JAVA_HOME=/usr/lib/jvm/java-11
export PATH=$PATH:$JAVA_HOME/bin
export CLASSPATH=.:$JAVA_HOME/lib

Node.js

export NODE_ENV=production
export NODE_OPTIONS="--max-old-space-size=4096"

Python

export PYTHONPATH=/path/to/modules:$PYTHONPATH
export VIRTUAL_ENV=/path/to/venv

Go

export GOPATH=$HOME/go
export GOROOT=/usr/local/go
export PATH=$PATH:$GOROOT/bin:$GOPATH/bin

Security considerations

# Never store secrets in environment (visible to all processes)
# Better: Use secret management tools

# Avoid in scripts
export PASSWORD=secret123  # BAD

# Instead, read from secure source
read -sp "Enter password: " PASSWORD
export PASSWORD

# Or use credential files
export DB_PASSWORD=$(cat /etc/secrets/db_password)

Debugging

# Show all exports in current shell
export -p

# Show difference between shells
set | grep -v '^_'

# Trace variable changes
set -x
export VARIABLE=value
set +x

Cleanup script

#!/bin/bash

# Save current state
env > /tmp/env_before.txt

# Do work...

# Compare
env > /tmp/env_after.txt
diff /tmp/env_before.txt /tmp/env_after.txt

Tips & best practices

# 1. Always quote variables
echo "$VARIABLE"  # Good
echo $VARIABLE    # Can break with spaces

# 2. Use meaningful names
export DATABASE_URL  # Good
export DB           # Less clear

# 3. Document in scripts
# Required environment variables:
# - DATABASE_URL: PostgreSQL connection string
# - API_KEY: External API key

# 4. Provide defaults
: ${PORT:=3000}

# 5. Validate
if [ -z "$REQUIRED_VAR" ]; then
    echo "Error: REQUIRED_VAR not set"
    exit 1
fi

Quick reference

# View
env                    # All variables
printenv VARIABLE      # Specific variable
echo $VARIABLE         # Value

# Set
export VAR=value       # This shell + children
VAR=value              # This shell only
VAR=value command      # Command only

# Unset
unset VAR

# Modify PATH
export PATH=$PATH:/new/path

# Permanent (user)
# Add to ~/.bashrc:
export VARIABLE=value

# Permanent (system)
# Add to /etc/environment:
VARIABLE=value