🔍 Advanced Expansions

Shell expansions transform text and compute values inline — mastering them enables concise, powerful scripts.

🧭 Expansion Types Overview

Shells perform several types of expansions in order:

1. Brace Expansion — {a,b}{1,2}
2. Tilde Expansion — ~, ~/Documents
3. Parameter Expansion — $var, ${var}, ${var:-default}
4. Command Substitution — $(command), `command`
5. Arithmetic Expansion — $((expression))
6. Word Splitting — Splitting on $IFS
7. Filename Expansion — Globbing *.txt

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🧪 Parameter Expansion Deep Dive

Default Value Substitution

# If var is unset or empty, use default
echo "${var:-default}"        # Use default
: "${EDITOR:=nano}"           # Set if unset
echo "${var:?Error message}"  # Exit if unset
echo "${var:+alternative}"    # Use alternative if set

String Manipulation

filename="/home/user/document.pdf"

# Remove shortest match from beginning
echo "${filename#/*/}"        # user/document.pdf

# Remove longest match from beginning
echo "${filename##/*/}"       # document.pdf

# Remove shortest match from end
echo "${filename%.*}"         # /home/user/document

# Remove longest match from end
echo "${filename%%.*}"        # /home/user/document

# Replace first occurrence
echo "${filename/\/home/\/opt}"  # /opt/user/document.pdf

# Replace all occurrences
echo "${filename//\//_}"      # _home_user_document.pdf

Substring Extraction

string="Hello World"

# Extract substring
echo "${string:0:5}"          # Hello
echo "${string:6}"            # World
echo "${string: -5}"          # World (note space before -)

# Length
echo "${#string}"             # 11

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🧠 Pattern-Based Expansions

Case Conversion (Bash 4.0+)

str="Hello World"

echo "${str^^}"               # HELLO WORLD
echo "${str,,}"               # hello world
echo "${str^}"                # Hello World (first char)
echo "${str,}"                # hello World (first char)

Indirect Variable References

varname="USER"
value="${!varname}"           # Equivalent to $USER

# Dynamic configuration access
DB_HOST="localhost"
DB_PORT="5432"

for var in DB_HOST DB_PORT; do
    echo "${var}: ${!var}"
done

⚠️ Not POSIX — avoid in portable scripts.

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🧪 Command Substitution

Modern Syntax

# Preferred method
today=$(date +%Y-%m-%d)
files=$(find . -name "*.log")

# Nested substitution
result=$(grep -l "$(hostname)" configs/*.conf)

Legacy Syntax (Avoid)

# Old style - harder to nest
today=`date +%Y-%m-%d`

Handling Special Characters

# Command substitution removes trailing newlines
output=$(printf "line1\nline2\n")
echo "[$output]"  # [line1
                  # line2]

# Preserve all output
output=$(command; echo x)
output=${output%x}

---

🧠 Arithmetic Expansion

Basic Arithmetic

# Simple calculations
sum=$((2 + 3 * 4))            # 14
index=$((index + 1))

# Bit operations
mask=$((0xFF & 0x0F))         # 15
shifted=$((8 << 2))           # 32

Conditional Expressions

# Ternary-like operator
result=$(( a > b ? a : b ))

# Boolean logic
enabled=$(( flag == 1 ? 1 : 0 ))

Array Indexing (Bash)

arr=(apple banana cherry)
index=1
echo "${arr[$index]}"         # banana
echo "${arr[$((index + 1))]}" # cherry

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🧪 Brace Expansion

Sequence Generation

# Numeric ranges
echo file{1..5}.txt           # file1.txt file2.txt ... file5.txt

# Zero-padded sequences
echo file{01..05}.txt         # file01.txt file02.txt ... file05.txt

# Reverse sequences
echo {5..1}                   # 5 4 3 2 1

# Character ranges
echo {a..e}                   # a b c d e

Cartesian Products

# Generate combinations
echo {a,b}{1,2}               # a1 a2 b1 b2

# Complex structures
mkdir -p project/{src,tests,docs}/{main,util,shared}

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🧠 Word Splitting and IFS

Default Behavior

# Default IFS: space, tab, newline
words="one two three"
for word in $words; do
    echo "Word: [$word]"
done

Custom IFS

# Split on comma
data="apple,banana,cherry"
IFS=',' read -ra fruits <<< "$data"
for fruit in "${fruits[@]}"; do
    echo "Fruit: [$fruit]"
done

# Temporary IFS change
old_ifs="$IFS"
IFS=':'
read user pass uid gid rest < /etc/passwd
IFS="$old_ifs"

Preventing Splitting

# Quote to prevent splitting
for item in "$words"; do
    echo "Item: [$item]"      # [one two three]
done

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🧪 Filename Expansion (Globbing)

Basic Patterns

# Wildcards
ls *.txt                      # All .txt files
ls file?.log                  # file1.log, fileA.log
ls [a-c]*.sh                  # Files starting with a,b,c

Extended Globs (Bash)

shopt -s extglob

# Match anything except pattern
ls !(temp).txt                # All .txt except temp.txt

# Match one of patterns
ls @(file1|file2).txt         # file1.txt or file2.txt

# Match zero or one occurrence
ls file?(.bak).txt            # file.txt or file.bak.txt

Recursive Globbing (Bash 4.3+)

shopt -s globstar
ls **/*.txt                   # All .txt files recursively

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🧠 Expansion Order Examples

Complex Nesting

# Expansion order demonstration
files=(*.txt)
echo "First file: ${files[0]##*/}"  # Basename of first .txt file

# Nested parameter expansion
config="${CONFIG_FILE:-${HOME}/.config/app.conf}"

Timing Considerations

# Expensive expansion - evaluated every time
for i in {1..1000}; do
    echo "Date: $(date)"        # Slow - 1000 date calls
done

# Better approach - cache result
current_date=$(date)
for i in {1..1000}; do
    echo "Date: $current_date"  # Fast - 1 call
done

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🧾 Portability Notes

Feature	POSIX	Bash	Zsh
${var:-default}	✅	✅	✅
${var:=default}	✅	✅	✅
${var:?error}	✅	✅	✅
${var:+alt}	✅	✅	✅
${var^^} (case)	❌	✅	✅
${!var} (indirect)	❌	✅	✅
$((arithmetic))	✅	✅	✅
{1..5} (brace)	❌	✅	✅
Extended globs	❌	✅	✅

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🧾 Summary

• Master parameter expansions for robust default handling
• Use brace expansion for generating sequences and structures
• Prefer $() over backticks for command substitution
• Leverage arithmetic expansion for calculations
• Control word splitting with careful quoting and IFS
• Understand expansion order to predict behavior
• Test portability with multiple shell implementations

👉 Continue to: Error Semantics