Setting Environment Variables In Bash

Setting environment variables in Bash is one of the most fundamental skills for working with Linux command lines and shell scripts. Bash provides several ways to define and manage variables.

Setting a Variable for the Current Shell

MY_VAR="hello world"

Important: No spaces around the = sign. MY_VAR = "value" will produce an error because Bash interprets it as a command MY_VAR with arguments.

Access the variable with $:

echo $MY_VAR
# Output: hello world

Exporting to Child Processes

Variables set without export are only visible to the current shell. To make them available to child processes (scripts, applications launched from this shell):

export MY_VAR="hello world"
# Or export an existing variable
MY_VAR="hello world"
export MY_VAR

Without export, a script called from this shell won’t see the variable:

TEST="only here"
bash -c 'echo $TEST'    # Empty - child process doesn't see it

export TEST="everywhere"
bash -c 'echo $TEST'    # "everywhere" - child sees it

Common Patterns

Add to PATH:

export PATH="/usr/local/bin:$PATH"
export PATH="$PATH:$HOME/.local/bin"  # Append instead of prepend

Set a development variable:

export JAVA_HOME=/usr/lib/jvm/java-17-openjdk
export GOPATH=$HOME/go
export EDITOR=vim

Making Variables Persistent

Variables set in a terminal only last for that session. To persist them:

Per-user (most common): Add to ~/.bashrc:

echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.bashrc
source ~/.bashrc

Login shells only: Use ~/.bash_profile or ~/.profile instead. These run once at login, while ~/.bashrc runs for each new interactive shell.

System-wide: Add to /etc/profile.d/:

echo 'export JAVA_HOME=/usr/lib/jvm/java-17' | sudo tee /etc/profile.d/java.sh
source /etc/profile.d/java.sh

Setting Variables for a Single Command

# Variable only for this one command
DEBUG=1 ./my-script.sh

# Multiple variables
NODE_ENV=production PORT=3000 node server.js

This is the preferred pattern for passing configuration to scripts and daemons — the variable doesn’t pollute your shell environment.

Special Variable Syntax

Bash provides several variable expansion features:

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

# Set default and assign
echo ${MY_VAR:="default"}

# Error if unset
echo ${MY_VAR:?"MY_VAR is not set"}

# Alternative value if SET
echo ${MY_VAR:+"was set to $MY_VAR"}

# String length
echo ${#MY_VAR}

# Substring
echo ${MY_VAR:0:5}      # First 5 characters
echo ${MY_VAR: -3}      # Last 3 characters

# Replace
echo ${MY_VAR/hello/goodbye}   # Replace first match
echo ${MY_VAR//o/0}            # Replace all matches

Unsetting Variables

unset MY_VAR

This removes the variable entirely from the current shell and its export list.

Listing Environment Variables

# All exported variables
env
printenv

# All variables (including non-exported)
set

# Search for a specific variable
printenv PATH
echo $PATH

Variable Scope and Subshells

Child processes inherit exported variables, but changes in a child don’t affect the parent:

export MY_VAR="parent"
bash -c 'MY_VAR="child"; echo "Child: $MY_VAR"'
echo "Parent: $MY_VAR"
# Output:
# Child: child
# Parent: parent

This is a common gotcha when trying to set variables from inside a pipeline or subshell — the parent shell won’t see the change.

Quick Reference

VAR=value — Set for current shell
export VAR=value — Set and export to children
VAR=value command — Set for single command only
~/.bashrc — Persist per-user
/etc/profile.d/ — Persist system-wide
unset VAR — Remove variable

Integer and Array Variables

Bash supports typed variables:

# Integer variable (faster arithmetic)
declare -i COUNT=0
((COUNT++))
echo $COUNT  # 1

# Read-only variable
declare -r CONFIG_PATH="/etc/myapp"
CONFIG_PATH="/new/path"  # Error: readonly variable

# Array
FILES=("file1.txt" "file2.txt" "file3.txt")
echo ${FILES[0]}     # file1.txt
echo ${FILES[@]}     # All elements
echo ${#FILES[@]}    # Array length

# Associative array (Bash 4+)
declare -A CONFIG
CONFIG[host]="example.com"
CONFIG[port]="443"
echo ${CONFIG[host]}  # example.com

Environment Variables in Scripts

When writing shell scripts, follow these best practices:

#!/bin/bash
# Check if a variable is set
if [ -z "$JAVA_HOME" ]; then
    echo "Error: JAVA_HOME is not set" >&2
    exit 1
fi

# Use with default
DB_HOST=${DB_HOST:-"localhost"}
DB_PORT=${DB_PORT:-"5432"}

# Quote variables to handle spaces and empty values
echo "Connecting to $DB_HOST:$DB_PORT"

Always quote variable expansions ("$VAR" not $VAR) to prevent word splitting and glob expansion issues with values containing spaces or special characters.

Setting Environment Variables in Bash