Question

A backup script must use the value of the environment variable BACKUP_DIR if it is set, but fall back to /var/backups if the variable is unset or empty, without permanently assigning the variable. Which line accomplishes this correctly?

Accepted Answer

TARGET="${BACKUP_DIR:-/var/backups}"

Answer

TARGET="${BACKUP_DIR:=/var/backups}"

Answer

TARGET="${BACKUP_DIR:?/var/backups}"

Answer

TARGET="${BACKUP_DIR:+/var/backups}"