Shell by Example: Environment Variables POSIX 

#!/bin/sh

Environment variables are key-value pairs that configure how programs behave. They’re inherited by child processes.

Access environment variables with $ prefix:

 

echo "Current user: $USER"
echo "Home directory: $HOME"
echo "Current shell: $SHELL"
echo "Current path: $PATH"

Check if a variable is set:

 

if [ -n "$HOME" ]; then
  echo "HOME is set to: $HOME"
fi

if [ -z "$UNDEFINED_VAR" ]; then
  echo "UNDEFINED_VAR is not set"
fi

Set a shell variable (not exported):

 

MY_VAR="hello"
echo "Shell variable: $MY_VAR"

Export makes it available to child processes:

 

export MY_EXPORTED="world"

Check if it’s exported by running a subshell:

 

sh -c 'echo "In child - MY_VAR: ${MY_VAR:-<unset>}"'
sh -c 'echo "In child - MY_EXPORTED: ${MY_EXPORTED:-<unset>}"'

Set and export in one line:

 

export GREETING="Hello, World!"
echo "Greeting: $GREETING"

Common environment variables:

 

echo "Common environment variables:"
echo "  USER=$USER"
echo "  HOME=$HOME"
echo "  PWD=$PWD"
echo "  PATH=$PATH"
echo "  SHELL=$SHELL"
echo "  TERM=$TERM"
echo "  LANG=$LANG"

Modify PATH temporarily:

 

echo "Original PATH has $(echo "$PATH" | tr ':' '\n' | wc -l | tr -d ' ') entries"

PATH="$HOME/bin:$PATH"
echo "After prepending \$HOME/bin: PATH starts with ${PATH%%:*}"

Set variable for a single command:

 

GREETING="Hola" sh -c 'echo "Temporary: $GREETING"'
echo "After command: GREETING=${GREETING:-<unset>}"

Unset a variable:

 

export TEMP_VAR="temporary"
echo "Before unset: $TEMP_VAR"
unset TEMP_VAR
echo "After unset: ${TEMP_VAR:-<unset>}"

Use env to view all environment variables:

 

echo "First 5 environment variables:"
env | head -5

Use env to run command with modified environment:

 

env MYVAR=test sh -c 'echo "MYVAR=$MYVAR"'

Clear environment and run with specific vars:

 

env -i PATH="$PATH" sh -c 'echo "Clean env, PATH set: ${PATH:+yes}"'

Default values for variables:

 

# ${VAR:-default} - use default if unset or empty
echo "DB_HOST: ${DB_HOST:-localhost}"

# ${VAR:=default} - use default and assign if unset
: "${CACHE_DIR:=/tmp/cache}"
echo "CACHE_DIR: $CACHE_DIR"

# ${VAR:?message} - error if unset or empty
# : "${REQUIRED_VAR:?must be set}"  # Would exit if unset

# ${VAR:+value} - use value if var is set
echo "USER is set: ${USER:+yes}"
echo "UNSET is set: ${UNSET_VAR:+yes}"

Export all variables automatically (not recommended):

 

# set -a  # or set -o allexport
# source config.env
# set +a

Check if command exists in PATH:

 

command_exists() {
  command -v "$1" >/dev/null 2>&1
}

if command_exists git; then
  echo "git is installed"
fi

Configuration file pattern:

 

load_config() {
  # Default values
  APP_PORT="${APP_PORT:-3000}"
  APP_HOST="${APP_HOST:-0.0.0.0}"
  APP_DEBUG="${APP_DEBUG:-false}"

  echo "Config:"
  echo "  APP_PORT=$APP_PORT"
  echo "  APP_HOST=$APP_HOST"
  echo "  APP_DEBUG=$APP_DEBUG"
}

echo "Default config:"
load_config

echo "Overridden config:"
APP_PORT=8080 APP_DEBUG=true load_config
Bash

Bash provides additional features

Read-only export:

 
declare -rx CONST_VAR="cannot change"

List all exported variables:

 
export -p

Check if variable is exported:

 
if declare -p VAR 2>/dev/null | grep -q 'declare -x'; then
    echo "VAR is exported"
fi

Security: Don’t put secrets in environment variables that might be logged. Use files with restricted permissions.

 

echo "Environment variable demo complete"

« Getopts | Reading Files »

by Jose Diaz-Gonzalez | inspired by Go by Example | source