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Parameter Expansion: Advanced Variable Tricks

Parameter expansion manipulates variables. Defaults. Substrings. Patterns.

Here's the thing: Parameter expansion is powerful. But you don't need it to start. Learn basics first. Then come back here.

Default Values​

${var:-default}                  # Use default if var is empty
${var:=default}                  # Set to default if empty
${var:+alternate}                # Use alternate if var is set
${var:?error}                    # Error if var is empty

Real example:

name=""
echo "${name:-Guest}"            # Output: Guest

port="${PORT:-8080}"            # Use PORT or default to 8080

My take: Default values are useful. Use them for configuration.

String Length​

name="John"
echo "${#name}"                  # Length: 4

My take: String length is useful. Count characters.

Substring​

text="Hello World"
echo "${text:0:5}"               # Hello (start at 0, length 5)
echo "${text:6}"                  # World (from position 6)

My take: Substrings extract parts of strings. Useful for parsing.

Pattern Matching​

file="backup.tar.gz"
echo "${file%.*}"                # backup.tar (remove shortest .* from end)
echo "${file%%.*}"               # backup (remove longest .* from end)
echo "${file#*.}"               # tar.gz (remove shortest * from start)
echo "${file##*.}"               # gz (remove longest * from start)

My take: Pattern matching manipulates strings. Useful for filenames.

Common Uses​

Default Values​

BACKUP_DIR="${BACKUP_DIR:-/backup}"
PORT="${PORT:-8080}"

Extract Filename​

file="/path/to/file.txt"
filename="${file##*/}"           # file.txt
extension="${file##*.}"          # txt

Remove Extension​

file="backup.tar.gz"
name="${file%.*}"                # backup.tar

What You'll Actually Use​

Daily use:

  • Default values (${var:-default})
  • String length (${#var})
  • Pattern matching (for filenames)

The rest? Learn when you need them.

Common Mistakes (I've Made These)​

  1. Wrong syntax: ${var-default} doesn't work. Use ${var:-default} (colon required).

  2. Not quoting: Quote expansions. Spaces break things.

  3. Too complex: Keep it simple. Complex expansions are hard to read.

  4. Not understanding patterns: Pattern matching is tricky. Learn it slowly.

  5. Overusing: Don't use advanced features when simple works.

Real-World Examples​

Configuration with Defaults​

SERVER="${SERVER:-localhost}"
PORT="${PORT:-8080}"
DEBUG="${DEBUG:-false}"

Process Filenames​

file="/var/log/app.log"
filename="${file##*/}"           # app.log
name="${filename%.*}"             # app

What's Next?​

Now that you know parameter expansion, you can manipulate variables powerfully. Or stick with basics. They're enough for most things.

Personal note: I learned parameter expansion slowly. As I needed it. Don't try to learn everything at once. Learn defaults. Use them. Then learn more.