| (hold) command-s | start up in single user mode |
| df | display disk free space, along with disk names (most useful when you are not in single user mode, so you can see the full drive list) |
| /sbin/fsck -fy | run a filesystem consistency check and interactive repair, assuming yes to all questions (the -f means check a disk even if it is clean) |
| /sbin/fsck -fn | run a filesystem consistency check and interactive repair, assuming no to all questions, and do not open the disk for writing |
| reboot | reboot system from the terminal |
| exit | startup without rebooting |
| diskutil verifyVolume / | Verify the disk structure of a volume. Ownership of the affected disk is required. |
If you are trying to run fsck on a disk other than the main boot disk, you may see this error:
BAD SUPER BLOCK: MAGIC NUMBER WRONG
…so run this instead:
/sbin/fsck_hfs -fy /dev/rdisk0s3
Other Operating System references talk about using newfs -N /dev/diskname to get a list of super-block backups, any of which can be used with the -b flag of fsck to make things work. This is not the case with Mac / HFS, so use fsck_hfs instead.