Variables in Linux Shell Scripting - System, User & Environment Variables

Variables are an essential building block of every programming and scripting language. In Linux, a variable is a named storage pointer referencing a block of memory whose stored content can be read or modified during program execution. Unlike statically typed languages like C, Java, or C++, Linux shell scripting is dynamically and untyped: variables hold ASCII text strings by default, which can be interpreted as integers depending on the arithmetic context. Linux distinguishes between two broad categories of variables: System (Environment) Variables and User-Defined Variables.

1. System (Environment) Variables

An environment variable is a global setting managed by the Linux operating system kernel and shell session. They are available to any child process or executable program invoked from the shell. By convention, system environment variables are named in UPPERCASE letters. The table below lists the most important system variables:

2. User-Defined Variables

Shells allow programmers to create customized local variables for storing numbers, strings, and command outputs. By convention, user-defined variables are named in lowercase or camelCase to avoid accidental collisions with system environment variables.

Naming Rules & Assignment Syntax

• Variable names can contain letters (a-z, A-Z), numbers (0-9), and underscores (_).
• Names cannot start with a number (e.g., 1var is invalid).
• Variable names are case-sensitive: $Count, $COUNT, and $count are three distinct variables.
• CRITICAL RULE: There must be NO SPACES around the equal sign (=)!

# CORRECT: No spaces around '='
count=10
company='Veewom Technology'
pi=3.14159

# INCORRECT (Will trigger "command not found" errors!):
count = 10        # Error!
company = 'Veewom' # Error!

Referencing Variable Values

To extract or print the value of a variable, prefix its name with the dollar symbol ($) or wrap it in curly braces (${}):

#!/bin/bash
name="Linus Torvalds"
project="Linux Kernel"

echo "Creator: $name"
echo "Project: ${project}_v6.x"  # Braces protect the variable name from following text

3. Read-Only Variables & Unsetting Variables

You can protect sensitive variables from accidental overwrite or remove variables from memory when no longer needed:

#!/bin/bash
# 1. Read-Only Variable (Constant)
readonly API_PORT=8080
echo "Port: $API_PORT"

# Trying to reassign a readonly variable causes a bash error:
# API_PORT=9000   # bash: API_PORT: readonly variable

# 2. Unsetting (Deleting) a variable
temp_data="Temporary buffer"
echo "Before unset: $temp_data"

unset temp_data   # Removes temp_data from memory
echo "After unset: $temp_data" # Outputs an empty blank line

4. Exporting Variables to Child Processes

By default, user-defined variables are local to the current script or shell process. If your script launches a child script or subshell, that child cannot see your variables unless you export them:

#!/bin/bash
# Local variable: Only visible inside this script
LOCAL_VAR="Visible only here"

# Exported environment variable: Inherited by child processes and scripts
export APP_ENV="production"
export DATABASE_URL="mysql://localhost:3306/veewom_db"

# Now any child script executed here can read $APP_ENV
./run_service.sh

5. Quoting Rules in Shell Scripting

6. Parameter Substitution Default Values

Bash provides convenient shorthand to assign fallbacks when variables might be unset:

• ${var:-fallback} : If var is unset or null, return fallback (does not assign).
• ${var:=fallback} : If var is unset, return fallback AND assign it to var.
• ${var:?error_msg} : If var is unset, print error_msg and abort execution.

#!/bin/bash
# sys_profiler.sh: System audit script demonstrating variables

# Read system variables
cur_user=${USER:-"Unknown User"}
cur_shell=${SHELL:-"/bin/sh"}
cur_home=${HOME:-"/root"}
cur_host=${HOSTNAME:-"localhost"}
cur_path=${PATH}

echo "============================================="
echo "       LINUX SYSTEM PROFILE REPORT"
echo "============================================="
echo "Date & Time    : $(date)"
echo "Current User   : $cur_user"
echo "Host Machine   : $cur_host"
echo "Home Directory : $cur_home"
echo "Active Shell   : $cur_shell"
echo "Working Dir    : $PWD"
echo "---------------------------------------------"

# Prompt user with parameter substitution default
read -p "Enter backup directory [Default: /tmp/backup]: " backup_dir
target_dir=${backup_dir:-"/tmp/backup"}

echo "Configured Backup Target: $target_dir"
echo "============================================="

Frequently Asked Questions (FAQs)