A system administrator is writing a backup script. The script should use the directory specified in the BACKUP_DIR environment variable. If BACKUP_DIR is not set or is empty, the script should default to using /var/backups for the current operation without permanently changing the BACKUP_DIR variable.
Which of the following commands correctly assigns the appropriate directory to the target_dir variable?
The correct command is target_dir=${BACKUP_DIR:-/var/backups}.
The ${parameter:-word} expansion checks if the variable BACKUP_DIR is unset or null. If it is, the expansion returns the default value /var/backups. Importantly, it does not change the value of BACKUP_DIR itself.
The ${parameter:=word} syntax would also use the default value, but it would also assign /var/backups back to the BACKUP_DIR variable if it was unset or null, which is not desired according to the scenario.
The ${parameter:+word} syntax does the opposite of what is needed; it expands to the word only if the parameter is set and not null.
The ${parameter:?word} syntax would cause the script to exit with an error message if BACKUP_DIR were unset or null, which is intended for error handling, not for providing a default value.
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