Interview › Scripting (Bash, Groovy)
What is parameter expansion, and what does ${var:-default} do? [Intermediate]
Answer
Parameter expansion transforms or substitutes variable values. ${var:-default} expands to default if var is unset or empty, without assigning the default to var.
Technical explanation
Use ${var-default} if only unset should use the default and empty string should be respected.
Use ${var:=default} to assign the default when unset or empty.
These expansions make scripts safer under set -u and easier to configure through environment variables.
Hands-on example
log_level=${LOG_LEVEL:-info}
timeout_seconds=${TIMEOUT_SECONDS:-30}
echo "log_level=$log_level timeout=$timeout_seconds"
# With set -u, this avoids unbound variable errors for optional inputs.
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