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Bash Variables & Parameters

Variables are essential for storing and manipulating data in Bash scripts. Bash provides several types of variables and special parameters that give you access to script arguments and other useful information.

Basic Variables

Variable Assignment


#!/bin/bash

# Assign a value to a variable
name="John Doe"

# Use the variable
echo "Hello, $name"

Rules for Variable Names

  • Can contain letters, numbers, and underscores
  • Must start with a letter or underscore
  • Are case-sensitive
  • By convention, use uppercase for constants and lowercase for variables

Variable Expansion

Basic Expansion


fruit="apple"
echo "I have an $fruit"  # I have an apple

Braces for Clarity


fruit="apple"
echo "I have ${fruit}s"  # I have apples (clearer than $fruits)

Default Values


# Use default if variable is unset or empty
echo "${name:-Guest}"  # Guest if name is unset
echo "${name:=Guest}"  # Guest if name is unset, and assigns it

Special Parameters

Parameter Description
$0 Name of the script
$1, $2, ... $9 Positional parameters (arguments)
$# Number of arguments
$* All arguments as a single string
$@ All arguments as separate strings
$? Exit status of last command
$$ Process ID of the script
$! Process ID of last background command

Command Line Arguments

Accessing Arguments


#!/bin/bash

echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Total arguments: $#"

Shifting Arguments


#!/bin/bash

echo "First argument: $1"
shift  # Shift arguments left
echo "After shift, first argument: $1"

Arrays

Creating Arrays


# Indexed array
fruits=("Apple" "Banana" "Cherry")

# Associative array (Bash 4+)
declare -A user
user=([name]="John" [age]=30)

Accessing Array Elements


# Indexed array
echo ${fruits[0]}  # Apple
echo ${fruits[@]}  # All elements
echo ${#fruits[@]} # Number of elements

# Associative array
echo ${user[name]} # John

Environment Variables

Common Environment Variables


echo "User: $USER"
echo "Home: $HOME"
echo "Path: $PATH"
echo "Current directory: $PWD"
echo "Shell: $SHELL"

Setting Environment Variables


# For current session
export MY_VAR="value"

# For just the script
MY_SCRIPT_VAR="script_value" ./script.sh

Quoting Rules

Different Quote Types


# Single quotes - no expansion
echo '$USER'  # $USER

# Double quotes - expansion happens
echo "$USER"  # john

# No quotes - word splitting
echo hello   world  # hello world

Parameter Expansion Operations

Expression Description
${var:-default} Use default if var is unset or empty
${var:=default} Use default if var is unset or empty, and assign it
${var:?message} Display message and exit if var is unset
${var:+alternate} Use alternate if var is set
${#var} Length of var
${var:offset:length} Substring expansion
${var#pattern} Remove shortest leading pattern
${var##pattern} Remove longest leading pattern
${var%pattern} Remove shortest trailing pattern
${var%%pattern} Remove longest trailing pattern
${var/pattern/replacement} Replace first match
${var//pattern/replacement} Replace all matches

Best Practices

  • Quote variables to prevent word splitting and globbing
  • Use ${var} instead of $var when adjacent text could cause confusion
  • Use readonly for constants
  • Prefer local variables in functions
  • Use descriptive variable names

Next Steps

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