If a directory name is not specified, the user's home directory is used.
If there is not a readable file named ".backup.od" in the specified directory when backup.od runs, all files are copied. If the file exists, only those files that have been modified after the timestamp of ".backup.od" are copied. backup.od creates a file in the specified directory named ".backup.od" each time it runs. So, if an automatic backup is started, it will make a full backup the first time, and incremental backups thereafter.
Full backups are written to files named "fl.month.day.hour.tar", with the list file being written to "fl.month.day.hour.lst". If a full backup exists for the same month, day, and hour, it is overwritten. A message to that effect is written to stdout.
Incremental backups are written to files named "sv.month.day.hour.tar", with the list file being written to "sv.month.day.hour.lst". If an incremental backup exists for the same month, day, and hour, backup.od fails. A message saying that the file exists is written to stdout.
Using the -a time flag, the backup may be started at time rather than at the current time. If the -a flag is specified, backup.od will reschedule itself for time when it completes the backup. Doing this, daily unattended backups may be specified. When the -a flag is used, stdout and stderr are mailed to $USER.
If the -m flag is used, backup.od requests that an optical disk be mounted, and waits until one is inserted before making the tar and list files. The disk is ejected when backup.od is finished.
If the -m flag is not used, the optical disk must be mounted on /tmp/backup, and must have a directory named /tmp/backup/$USER. $USER must have write permission to this directory. The tar files and list files are written to that directory. This allows multiple users to make backups at the same time.
If an uninitialized optical disk is inserted, this command will fail with the message "mount: /dev/od0a on directory: No such device or address.