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2.7.2 Variables

Table of Contents

Understanding Variables in Shell Scripts

Variables in shell scripts let you store and reuse values such as text, numbers, filenames, or command output. This chapter focuses on how variables work in typical POSIX shells, with Bash as the main example.

Creating and Assigning Variables

In shell scripts, variable assignment has a strict syntax:

Examples: 

name=Alice
greeting="Hello, world"
number=42
file_path=/etc/passwd

These assignments:

Quoting in Assignments

Quotes affect how the shell interprets the value:

Examples: 

text=Hello world         # ERROR: word splitting; 'Hello' and 'world' treated separately
text="Hello world"       # Correct: single value with a space
pattern=*.txt            # Will expand when used, not during assignment
literal_pattern='*.txt'  # Exactly the characters *, ., t, x, t
name=Bob
msg='Hi $name'           # Literal: $name
echo "$msg"              # Output: Hi $name
msg="Hi $name"           # Variable expands inside double quotes
echo "$msg"              # Output: Hi Bob

Using Variables

To use the value stored in a variable, prefix its name with $: 

name=Alice
echo $name
echo "User name: $name"

To avoid ambiguity between the variable name and following characters, use braces: 

user=alice
echo "User is $user01"       # Likely empty + '01'
echo "User is ${user}01"     # 'alice01'

Always quote variables containing user input or paths to avoid unwanted word splitting or globbing: 

file="My Document.txt"
rm "$file"           # Safe
rm $file             # Dangerous: treated as 'My' and 'Document.txt'

Variable Naming Conventions

Common conventions:

Examples: 

backup_dir=/backups
LOGFILE=/var/log/my_backup.log

Types of Variables in Shell Scripts

Most shells treat variables as strings by default, but they are used in different ways:

Shell (Local) Variables

These exist only in the current shell session or script. 

name=Alice
echo "$name"          # Works
bash -c 'echo "$name"'  # Empty: not exported

Environment Variables (Exported)

Environment variables are visible to child processes. To turn a shell variable into an environment variable, use export: 

name=Alice
export name
bash -c 'echo "$name"'   # Prints: Alice

You can also assign and export in one step: 

export EDITOR=nano

Special Parameters and Predefined Variables

Shells provide built-in variables and parameters that have special meaning. Examples (just awareness here; most have their own uses explained elsewhere):

Example (positional parameters): 

#!/bin/bash
echo "Script name: $0"
echo "First arg: $1"
echo "Number of args: $#"

Reading Input into Variables

Use read to get input from the user: 

#!/bin/bash
echo -n "Enter your name: "
read name
echo "Hello, $name!"

Common read options:

Example: 

#!/bin/bash
read -p "Username: " user
read -s -p "Password: " pass
echo
echo "You entered user '$user' and a hidden password."

Command Substitution: Assigning Command Output to Variables

You can store the output of a command in a variable using command substitution.

Preferred modern syntax: 

today=$(date)
files=$(ls)

Legacy syntax (avoid in new scripts due to nesting/escaping issues): 

today=`date`

Example: 

#!/bin/bash
current_time=$(date +"%Y-%m-%d %H:%M:%S")
user=$(whoami)
echo "Backup started at $current_time by user $user"

Always quote the result when using it, unless you intentionally want word splitting: 

files=$(ls)
echo "$files"        # Prints all filenames in one block

Arithmetic with Variables

The shell stores variables as strings, but you can perform integer arithmetic using special syntax.

`$(())` Arithmetic Expansion

This evaluates arithmetic expressions: 

a=5
b=3
sum=$((a + b))
prod=$((a * b))
echo "$sum"          # 8
echo "$prod"         # 15

You can also use numbers directly: 

x=$((10 + 2 * 3))    # 16

Variables inside $(( )) don’t need $: 

count=7
echo $((count + 1))  # 8

`let` and `(( ))` for In-Place Arithmetic (Bash)

In Bash, you can do: 

count=0
((count++))          # Increment
((count += 5))       # Add 5
echo "$count"

Or: 

let "count = count + 1"

Prefer $(( )) or (( )) for clarity.

Exporting Variables for Child Processes

To make a variable available to programs that your script runs, use export.

Without export: 

#!/bin/bash
MYVAR=hello
env | grep MYVAR      # Likely empty, not part of environment

With export: 

#!/bin/bash
export MYVAR=hello
env | grep MYVAR      # Shows MYVAR=hello

This is useful for configuring commands that read their settings from environment variables: 

#!/bin/bash
export PAGER=less
man bash              # 'less' is used as pager

Local Variables in Functions (Bash)

In Bash, you can limit a variable to a function scope using local: 

#!/bin/bash
name="Outer"
show_name() {
    local name="Inner"
    echo "Inside function: $name"
}
show_name
echo "Outside function: $name"

Output: 

Inside function: Inner
Outside function: Outer

Without local, the function would overwrite the outer name.

Parameter Expansion Tricks (Defaults and Safety)

Bash and POSIX shells support parameter expansion operators that are especially useful with variables. Common patterns:

Examples: 

echo "User: ${USER:-unknown}"     # If USER not set, prints 'unknown'
name=${NAME:-Guest}               # Uses 'Guest' if NAME not set
logfile=${LOGFILE:=/tmp/app.log}  # Ensures LOGFILE has a default
echo "Logging to $logfile"
: "${HOME:?HOME is not set}"      # Exit if HOME is missing

The last form is often used at the top of scripts to enforce required variables.

Deleting (Unsetting) Variables

To completely remove a variable: 

name=Alice
unset name
echo "$name"          # Empty

You can also unset environment variables: 

export API_KEY=123
unset API_KEY

Practical Tips for Beginners

These basics will be used heavily in conditionals, loops, and functions, which build on the variable concepts introduced here.

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