🔧 Extended Parameters and Expansions

Beyond basic variable assignment (var=value), shells offer powerful mechanisms for manipulating parameters, performing substitutions, and handling missing values gracefully.

🧭 Why Extended Expansions Matter

In production scripts, you'll frequently encounter: - Variables that might be unset - Strings that need transformation - Default values that should apply conditionally - Indirect references to variable names

Extended parameter expansions handle all these cases without external tools, making scripts faster and more portable.

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🧮 Basic Parameter Expansion Recap

Before diving into advanced features, let's review the fundamentals:

name="John Doe"

# Length
echo ${#name}           # 8

# Substring extraction
echo ${name:0:4}        # John (from position 0, 4 chars)
echo ${name:5}          # Doe (from position 5 to end)

# Replace first occurrence
echo ${name/John/Jane}  # Jane Doe

# Replace all occurrences
echo ${name//o/0}       # Jane D0e

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🧪 Default Values

Unset or Empty Variables

Provide fallback when variable doesn't exist or is empty:

# If $EDITOR is unset or empty, use "nano"
editor="${EDITOR:-nano}"
echo "Using editor: $editor"

This is extremely useful for configuration:

#!/bin/sh
: "${LOG_LEVEL:=INFO}"
: "${MAX_RETRIES:=3}"
: "${API_URL:=https://api.example.com}"

echo "Log level: $LOG_LEVEL"
echo "Max retries: $MAX_RETRIES"
echo "API URL: $API_URL"

The : builtin is a no-op — it's used here purely for the side effect of parameter expansion.

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Assign Default Permanently

Use := to set the variable if it's unset:

: "${CACHE_DIR:=/tmp/cache}"
mkdir -p "$CACHE_DIR"

Subsequent uses will see the assigned value:

echo "$CACHE_DIR"  # /tmp/cache (not empty anymore)

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Require Variable to Be Set

Fail fast if required variable is missing:

: "${DATABASE_URL:?Error: DATABASE_URL is required}"

If DATABASE_URL is unset or empty, the script exits with:

script.sh: line 5: DATABASE_URL: Error: DATABASE_URL is required

This is perfect for mandatory configuration.

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🧠 Pattern-Based Substitutions

Remove Prefix

Strip shortest matching prefix:

path="/usr/local/bin/script.sh"

# Remove everything up to last slash
basename="${path##*/}"     # script.sh

# Remove everything up to first slash
dirname="${path#/*}"       # usr/local/bin/script.sh

Remove Suffix

Strip shortest matching suffix:

filename="document.tar.gz"

# Remove extension
name="${filename%.*}"      # document.tar

# Remove specific suffix
name="${filename%.gz}"     # document.tar

# Remove longest match (all extensions)
name="${filename%%.*}"     # document

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🧪 Advanced Replacement Patterns

Case Modification

Convert case without external tools:

str="Hello World"

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

⚠️ Bash 4.0+ / Zsh only — not POSIX.

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Search and Replace

Replace patterns within strings:

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

# Replace first occurrence
new="${path/\/home\/user/\/opt/shared}"
# Result: /opt/shared/documents/file.txt

# Replace all occurrences
new="${path//\//_}"
# Result: _home_user_documents_file.txt

Useful for sanitizing paths or generating identifiers.

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🧠 Indirect Variable References

Access a variable whose name is stored in another variable:

varname="USER"
value="${!varname}"   # Equivalent to $USER
echo "$value"         # root (or current user)

Practical example — configuration map:

DB_HOST="db.example.com"
DB_PORT="5432"
DB_NAME="myapp"

for var in DB_HOST DB_PORT DB_NAME; do
    key="${var#DB_}"           # HOST, PORT, NAME
    value="${!var}"
    echo "$key=$value"
done

Output:

HOST=db.example.com
PORT=5432
NAME=myapp

⚠️ Not POSIX — avoid in portable scripts.

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🧪 Array-Like Behavior

Bash supports arrays, but POSIX shells don't. Simulate with positional parameters:

set -- apple banana cherry

echo "First: $1"           # apple
echo "Count: $#"           # 3

# Iterate
for item; do
    echo "Item: $item"
done

Or use delimited strings:

items="apple,banana,cherry"
IFS=',' read -ra arr <<< "$items"
echo "${arr[0]}"           # apple
echo "${arr[@]}"           # apple banana cherry

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🧾 Portability Notes

Feature	POSIX	Bash	Zsh
${var:-default}	✅	✅	✅
${var:=default}	✅	✅	✅
${var:?error}	✅	✅	✅
${var^^} (uppercase)	❌	✅	✅
${var,,} (lowercase)	❌	✅	✅
${!var} (indirect)	❌	✅	✅
Arrays (${arr[@]})	❌	✅	✅

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🧾 Summary

• Use ${var:-default} for optional configuration.
• Use ${var:?error} for mandatory values.
• Pattern substitutions (##, %%, //) are powerful and portable.
• Indirect references and case conversion are convenient but not portable.
• Simulate arrays in POSIX shells with $@ or delimited strings.

👉 Continue to: Conditionals and Tests