Parameter Expansion and Default Values

1. Parameter expansion happens before a command runs

When Bash sees a word such as "${name:-guest}", it expands the parameter before the target command receives its arguments. No external program is required for the basic operation.

flowchart TD
  S["Source word: ${NAME:-guest}"] --> E["Bash parameter expansion"]
  E --> Q["Quoting and remaining expansions"]
  Q --> A["Final command argument"]

The braces make the variable boundary explicit and unlock operators that cannot be expressed with simple $name.

2. Unset and null are separate states

Suppose name is unset in one case and assigned an empty string in another. Operators containing a colon, such as :-, usually treat both unset and null as missing. Related operators without the colon, such as -, test only whether the parameter is unset.

unset a
b=""

printf 'a with :- => <%s>\n' "${a:-default}"
printf 'b with :- => <%s>\n' "${b:-default}"

printf 'a with -  => <%s>\n' "${a-default}"
printf 'b with -  => <%s>\n' "${b-default}" 

3. Default values do not necessarily modify the variable

${name:-value} substitutes a fallback for that expansion. It does not assign the fallback to name.

unset REGION

printf 'effective=%s\n' "${REGION:-us-east}"
printf 'stored=%s\n' "${REGION-unset}" 

This is ideal when a default is needed for one expression but you do not want to mutate shell state.

4. := assigns a default when the value is missing

The := form both computes a fallback and stores it in the parameter.

unset CACHE_DIR

printf 'first=%s\n' "${CACHE_DIR:="$HOME/.cache/mytool"}"
printf 'stored=%s\n' "$CACHE_DIR" 

Assignment expansion can be concise during configuration initialization. Do not bury complex side effects inside deeply nested expansions; readability matters more than saving a line.

5. :? turns missing configuration into an immediate failure

The required-value form is useful for deployment scripts that cannot operate safely without a specific input.

# Run this in a disposable shell so the example can exit safely.
bash -c '
  : "${DEPLOY_ENV:?DEPLOY_ENV must be set and non-empty}"
  printf "deploying to %s\n" "$DEPLOY_ENV"
' || printf 'validation failed as expected\n'

DEPLOY_ENV=staging bash -c '
  : "${DEPLOY_ENV:?DEPLOY_ENV must be set and non-empty}"
  printf "deploying to %s\n" "$DEPLOY_ENV"
'

6. :+ substitutes a value only when a parameter is present

The alternate-value form can conditionally add arguments or labels.

DEBUG=true
printf 'debug flag: %s\n' "${DEBUG:+enabled}"

unset DEBUG
printf 'debug flag: %s\n' "${DEBUG:+enabled}" 

For complex conditionals, ordinary if statements are often clearer. Parameter expansion is strongest when the transformation remains obvious.

7. Length and substring operations stay inside Bash

Bash can compute string length and extract substrings directly.

commit="a1b2c3d4e5f6"

printf 'length=%s\n' "${#commit}"
printf 'short=%s\n' "${commit:0:7}"
printf 'tail=%s\n' "${commit: -4}" 

The space in ${commit: -4} prevents :- from being interpreted as the default-value operator.

8. Remove prefixes and suffixes with shell patterns

The # and % forms remove matching prefixes and suffixes. Single symbols remove the shortest match; doubled symbols remove the longest.

path="/srv/releases/app.tar.gz"

printf 'short prefix: %s\n' "${path#*/}"
printf 'long prefix:  %s\n' "${path##*/}"

file="${path##*/}"
printf 'short suffix: %s\n' "${file%.*}"
printf 'long suffix:  %s\n' "${file%%.*}" 

Patterns use shell glob syntax, not regular expressions. That distinction is important when porting logic from grep, sed, or programming languages.

9. Replace text using parameter expansion

Bash supports pattern replacement without launching sed for simple cases.

image="registry.example.com/team/api:latest"

printf '%s\n' "${image/latest/2026.08.09}"

text="one-two-two"
printf '%s\n' "${text//two/2}" 

The single slash replaces the first match; the double slash replaces all matches. Again, the match expression uses shell pattern rules.

10. Hands-on lab: validate and normalize deployment configuration

Create a script that accepts environment variables, applies defaults, requires one critical value, and derives a normalized label.

mkdir -p "$HOME/devops-academy/bash/chapter02/lesson04"
cd "$HOME/devops-academy/bash/chapter02/lesson04"

cat > config.sh <<'EOF'
#!/usr/bin/env bash

: "${APP_NAME:?APP_NAME is required}"

DEPLOY_ENV=${DEPLOY_ENV:-development}
REGION=${REGION:-eu-central-1}
IMAGE_TAG=${IMAGE_TAG:-latest}

safe_name=${APP_NAME// /-}
safe_name=${safe_name,,}

printf 'app=%s\n' "$safe_name"
printf 'environment=%s\n' "$DEPLOY_ENV"
printf 'region=%s\n' "$REGION"
printf 'image_tag=%s\n' "$IMAGE_TAG"
EOF

APP_NAME="Example API" bash config.sh
APP_NAME="Example API" DEPLOY_ENV=staging IMAGE_TAG=abc1234 bash config.sh

11. Knowledge check

12. Summary

Parameter expansion is Bash's built-in toolkit for defaults, validation, string length, extraction, prefix/suffix removal, and lightweight replacement. The most important production distinction is between unset and empty values. Use concise expansions when they make intent clearer, and switch to explicit control flow when compact syntax begins hiding behavior.

13. Further reading

• GNU Bash Reference Manual — Shell Parameter Expansion.
• POSIX Shell Command Language — parameter expansion operators.
• ShellCheck wiki — quoting, undefined variables, and expansion pitfalls.