Parameter_expansion

Chapter 28: Parameter Expansion in Bash

Overview

Parameter expansion is a powerful feature that allows you to manipulate variable values in sophisticated ways without calling external commands. This chapter covers all parameter expansion techniques essential for efficient bash scripting.

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What is Parameter Expansion?

Parameter expansion is the mechanism by which the shell replaces a parameter (variable) with its value. It goes far beyond simple $variable substitution.

┌────────────────────────────────────────────────────────────────┐
│                  Parameter Expansion Types                         │
├────────────────────────────────────────────────────────────────┤
│                                                                │
│   Basic:                                                       │
│   ──────                                                      │
│   $VAR          → value                                        │
│   ${VAR}        → value                                        │
│                                                                │
│   Default Values:                                              │
│   ─────────────                                               │
│   ${VAR:-value}    → value if unset/null                      │
│   ${VAR:=value}    → assign if unset/null                     │
│   ${VAR:+value}    → alt value if set                         │
│   ${VAR:?message}  → error if unset/null                       │
│                                                                │
│   String Operations:                                           │
│   ───────────────                                             │
│   ${#VAR}         → length                                     │
│   ${VAR:offset}   → substring from offset                      │
│   ${VAR:offset:len} → substring                               │
│                                                                │
│   Pattern Removal:                                             │
│   ───────────────                                             │
│   ${VAR#pattern}    → remove shortest prefix                  │
│   ${VAR##pattern}   → remove longest prefix                   │
│   ${VAR%pattern}    → remove shortest suffix                  │
│   ${VAR%%pattern}   → remove longest suffix                   │
│                                                                │
│   Pattern Replacement:                                          │
│   ──────────────────                                          │
│   ${VAR/pattern/string}  → replace first                       │
│   ${VAR//pattern/string} → replace all                        │
│   ${VAR/#pattern/string} → replace prefix                     │
│   ${VAR/%pattern/string} → replace suffix                     │
│                                                                │
└────────────────────────────────────────────────────────────────┘

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Basic Parameter Expansion

Simple Expansion

# Basic variable
name="Alice"
echo $name
echo ${name}

# In strings
echo "Hello, $name"
echo "Hello, ${name}"

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Default Values

Use Default If Unset

# ${parameter:-default}
# Returns default if parameter is unset or null

unset var1
var2=""

echo "${var1:-default}"   # default (var1 is unset)
echo "${var2:-default}"     # default (var2 is null)
echo "${var3:-default}"    # default (var3 never set)

# Common use
echo "${CONFIG_FILE:-/etc/default.conf}"

Assign Default If Unset

# ${parameter:=default}
# Assigns default if parameter is unset or null

unset var1
echo "${var1:=assigned}"   # assigned, var1 now has value

var2=""
echo "${var2:=assigned}"  # assigned, var2 now has value

# Common use
: "${DATABASE_HOST:=localhost}"
# : is a no-op, but expansion happens

Alternate Value If Set

# ${parameter:+alternate}
# Returns alternate if parameter is set and non-null

var1="value"
var2=""

echo "${var1:+alternate}"   # alternate (var1 is set)
echo "${var2:+alternate}"   # empty (var2 is null)
echo "${var3:+alternate}"   # empty (var3 is unset)

# Practical use
DEBUG_MODE=1
echo "${DEBUG_MODE:+--debug-flag}"  # --debug-flag if set

Error If Unset

# ${parameter:?message}
# Errors out if parameter is unset or null

unset important_var

# This will print error and exit
# echo "${important_var:?Error: Variable not set}"

# More specific message
# echo "${important_var:?Please set the important_var variable}"

# Practical use in scripts
: "${DATABASE_URL:?Error: DATABASE_URL must be set}"

---

String Length

Get Variable Length

# ${#parameter}

name="Hello"
echo ${#name}    # 5

path="/home/user/documents"
echo ${#path}    # 22

# Array length (first element)
arr=(one two three)
echo ${#arr}     # 3 (length of first element)

# Array length (all elements)
echo ${#arr[@]}  # 3

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Substring Operations

Extract Substring

# ${parameter:offset}
# ${parameter:offset:length}

text="Hello World"

# From offset (0-based)
echo "${text:6}"      # World (from position 6)

# With length
echo "${text:0:5}"   # Hello (first 5 chars)
echo "${text:6:5}"    # World (5 chars from position 6)

# Negative offset (from end)
echo "${text:-6}"    # Hello World (treats -6 as literal with :-)
echo "${text: -5}"   # World (space before -5 is required)
echo "${text: -5:2}" # Wo

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Pattern Removal

Remove Pattern from Beginning

# ${parameter#pattern} - shortest match
# ${parameter##pattern} - longest match

path="/home/user/documents/file.txt"

# Remove shortest match
echo "${path#*/}"    # home/user/documents/file.txt (until /)

# Remove longest match
echo "${path##*/}"   # file.txt (last /)

# With patterns
filename="/var/log/nginx/access.log"
echo "${filename##*/}"  # access.log

Remove Pattern from End

# ${parameter%pattern} - shortest match
# ${parameter%%pattern} - longest match

path="/home/user/documents/file.txt"

# Remove shortest match
echo "${path%/*}"    # /home/user/documents (after last /)

# Remove longest match
echo "${path%%/*}"   # (first /)

# Extension removal
filename="document.tar.gz"
echo "${filename%.*}"       # document.tar
echo "${filename%%.*}"      # document

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Pattern Replacement

Replace First Match

# ${parameter/pattern/string}

text="hello world hello"

# Replace first occurrence
echo "${text/hello/bye}"    # bye world hello

# Case insensitive
echo "${text//[Hh]ello/bye}" # bye world bye

# If string is empty
echo "${text/pattern/}"     # Removes first match
echo "${text//pattern/}"   # Removes all matches

Replace All Matches

# ${parameter//pattern/string}

text="hello world hello"

echo "${text//hello/bye}"  # bye world bye
echo "${text//l/_}"        # he__o wor__d he__o

Replace Prefix

# ${parameter/#pattern/string}

filename="prefix_file.txt"
echo "${filename/#prefix_/}"   # file.txt

url="https://example.com"
echo "${url#https://}"          # example.com

Replace Suffix

# ${parameter/%pattern/string}

filename="document.txt"
echo "${filename%.txt}"        # document

url="example.com/index.html"
echo "${url%.html}"            # example.com/index

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Arrays and Parameter Expansion

Array Length

# Array
arr=(one two three four)

echo ${#arr}         # 3 (length of first element)
echo ${#arr[@]}      # 4 (number of elements)
echo ${#arr[*]}      # 4

Array Slicing

arr=(a b c d e f)

# All elements from index
echo ${arr[@]:2}     # c d e f

# Elements from index with count
echo ${arr[@]:2:2}   # c d

Array Element Removal

arr=(one two three two four)

# Remove first matching element
unset 'arr[1]'       # Remove element at index 1
arr=( "${arr[@]/two}" )  # Remove by value

# Remove all matching
arr=( "${arr[@]//two}" )

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Special Parameters

Positional Parameters

#!/bin/bash
# $0, $1, $2, etc.

echo $0    # Script name
echo $1    # First argument
echo $2    # Second argument
echo $#    # Number of arguments
echo $@    # All arguments
echo $*    # All arguments (single string)

# Shift
shift       # $2 becomes $1
shift 2     # Shift by 2

Special Variables

echo $$      # Current PID
echo $!      # PID of last background job
echo $?      # Exit status of last command
echo $!      # PID of last background process

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Practical DevOps Examples

Path Manipulation

#!/usr/bin/env bash

# Get filename from path
filepath="/var/log/nginx/access.log"
filename="${filepath##*/}"    # access.log

# Get directory from path
dir="${filepath%/*}"          # /var/log/nginx

# Get extension
ext="${filename##*.}"        # log

# Base name without extension
base="${filename%.*}"        # access

Configuration with Defaults

#!/usr/bin/env bash
# Application configuration with sensible defaults

export APP_NAME="${APP_NAME:-myapp}"
export APP_ENV="${APP_ENV:-production}"
export APP_PORT="${APP_PORT:-8080}"
export LOG_LEVEL="${LOG_LEVEL:-info}"
export MAX_CONNECTIONS="${MAX_CONNECTIONS:-100}"
export TIMEOUT_SECONDS="${TIMEOUT_SECONDS:-30}"

echo "Starting $APP_NAME in $APP_ENV mode"
echo "Port: $APP_PORT, Log: $LOG_LEVEL"

Input Validation

#!/usr/bin/env bash

validate_required() {
    local var_name="$1"
    local var_value="${!var_name}"  # Indirect expansion

    if [[ -z "$var_value" ]]; then
        echo "Error: $var_name is required but not set"
        return 1
    fi
}

# Usage
DATABASE_URL="postgres://localhost/db"
validate_required "DATABASE_URL"

String Manipulation

#!/usr/bin/env bash

# Convert to lowercase
text="HELLO WORLD"
echo "${text,,}"   # hello world

# Convert to uppercase
text="hello world"
echo "${text^^}"   # HELLO WORLD

# Replace patterns
config="DEBUG=true"
echo "${config,,}"  # debug=true

# Remove quotes
value='"quoted string"'
echo "${value//\"}"  # quoted string

Username from Email

#!/usr/bin/env bash

email="john.doe@example.com"

# Get username (before @)
username="${email%%@*}"     # john.doe

# Get domain (after @)
domain="${email#*@}"       # example.com

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Advanced Techniques

Indirect Expansion

var1="value1"
var_name="var1"

# Get value by name
echo "${!var_name}"         # value1

# For arrays
arr=(one two three)
i=0
echo "${!arr[$i]}"          # one (element at index 0)

Case Modification

# Bash 4+

text="Hello World"

# Lowercase
echo "${text,,}"            # hello world

# Uppercase
echo "${text^^}"            # HELLO WORLD

# Lowercase first char
echo "${text,}"             # hello World

# Uppercase first char
echo "${text^}"             # Hello World

DevOps-Specific Examples

#!/usr/bin/env bash

# Parse Kubernetes resource name
resource="deployment/myapp-5d9f7b8-x"
name="${resource##*/}"       # myapp-5d9f7b8-x
deployment="${resource%%/*}" # deployment

# Docker image tag
image="registry.example.com/app:v1.2.3"
registry="${image%%/*}"     # registry.example.com/app
tag="${image##*:}"          # v1.2.3
name="${image%:*}"          # registry.example.com/app

# Semver comparison
version="v1.2.3"
major="${version%%.*}"      # v1
minor="${version#*.}"       # 2.3
minor="${minor%%.*}"        # 2
patch="${version##*.}"      # 3

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Summary

In this chapter, you learned:

• ✅ Basic parameter expansion
• ✅ Default value operators (:-), (:=), (:+), (:?)
• ✅ String length with # operator
• ✅ Substring extraction
• ✅ Pattern removal (#, ##, %, %%)
• ✅ Pattern replacement (/, //, #/, %/)
• ✅ Arrays and parameter expansion
• ✅ Special parameters (positional, $, ?, !)
• ✅ Practical DevOps examples
• ✅ Advanced techniques (indirect expansion, case modification)

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Next Steps

Continue to the next chapter to learn about Command Substitution.

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