Variables

Variable expansion allows you to retrieve and manipulate the value stored in a Bash variable using the $ syntax. This powerful feature is essential for automation, text processing, and script flexibility.

Introduction

In shell scripting, variables can store filenames, paths, numbers, or strings. By applying parameter expansion, you can:

• Access variable contents
• Perform string operations
• Provide fallback values
• Calculate lengths and more

filename="file.txt"
number=100
string="Hello World"
command_output=$(ls)

dir_path="/path/to/directory"
greeting="Hello"
name="User"
echo "$greeting, $name! Welcome to $dir_path"

Basic Variable Expansion

Whenever the shell sees $variable_name, it replaces it with the variable’s value:

name="John Doe"
echo $name
# Output:
# John Doe

Using braces helps delimit variable names clearly:

name="John Doe"
echo "Hello, ${name}"
# Output:
# Hello, John Doe

Variable names are case-sensitive and may contain letters, digits, and underscores:

# Invalid:
# Valid:
name_user="John Doe"

Using Curly Braces

Curly braces {} not only disambiguate names but also unlock advanced parameter expansion features.

Avoiding Ambiguity

Without braces, attached text can be misinterpreted:

#!/usr/bin/env bash
height=170

# Wrong: shell looks for 'heightcm'
echo "Your height is: $heightcm"

# Correct: braces isolate 'height'
echo "Your height is: ${height}cm"
# Output:
# Your height is: 170cm

Default Values

Provide a fallback when a variable is unset or empty:

#!/usr/bin/env bash
echo "Hello, ${name:-Unknown}"
# If 'name' is unset:
# Output:
# Hello, Unknown

Parameter Expansion Operators

String Manipulation

Substring Extraction

Pull out part of a string using an offset and length:

#!/usr/bin/env bash
name="John Doe"
echo "Hello, ${name:0:4}"
# Output:
# Hello, John

Substring Replacement

Replace the first occurrence of a pattern:

#!/usr/bin/env bash
path="/home/user/file.txt"
echo "${path/file/data}"
# Output:
# /home/user/data.txt

Length of a Variable

Determine the number of characters in a variable’s value:

#!/usr/bin/env bash
name="John Doe"
echo "${#name}"
# Output:
# 8

Quoting and Word Splitting

By default, unquoted expansions undergo word splitting. To preserve spaces, quote your variables. If you want to iterate over words, leave them unquoted:

#!/usr/bin/env bash
readonly SERVERS="server1 server2 server3"

# Iterate over each hostname:
for server in $SERVERS; do
  echo "${server}.kodekloud.com"
done
# Output:
# server1.kodekloud.com
# server2.kodekloud.com
# As a single string when quoted:
for server in "$SERVERS"; do
  echo "${server}.kodekloud.com"
done
# Output:
# server1 server2 server3.kodekloud.com

Additional Resources

• GNU Bash Manual – Shell Parameter Expansion
• Bash Reference Manual
• Understanding Quoting in Bash