FSTAB(5) File Formats Manual FSTAB(5)

fstabstatic information about the filesystems

#include <fstab.h>

The file fstab contains descriptive information about the various file systems. fstab is only read by programs, and not written; it is the duty of the system administrator to properly create and maintain this file, using the vifs(8) command. Each filesystem is described on a separate line; fields on each line are separated by tabs or spaces. The order of records in fstab is important because diskarbitrationd(8), fsck(8), mount(8), and umount(8) sequentially iterate through fstab doing their thing.

The first field, (fs_spec), describes the block special device, the local filesystem, or the remote filesystem to be mounted. The diskarbitrationd program supports the identification of a local filesystem uniquely by its UUID or by its volume name, irrespective of hardware configuration and of hardware parallelism, using the constructs ``UUID'' and ``LABEL''. For APFS volumes, this field should never be the block special device as it is not constant. Only the constructs ``UUID'' and ``LABEL'' should be used.

The second field, (fs_file), describes the mount point for the filesystem. For swap partitions, this field should be specified as ``none''.

The third field, (fs_vfstype), describes the type of the filesystem. The system currently supports different filesystem types, including the following:

APFS is the Mac OS X default filesystem since version 10.13 (High Sierra).
HFS+ is the previous Mac OS X default filesystem.
a Sun Microsystems compatible ``Network File System''
a DOS compatible filesystem
a CD-ROM filesystem (as per ISO 9660)
an implementation of /dev/fd
a translucent filesystem

The fourth field, (fs_mntops), describes the mount options associated with the filesystem. It is formatted as a comma separated list of options. It contains at least the type of mount (see fs_type below) plus any additional options appropriate to the filesystem type.

The option ``auto'' can be used in the ``noauto'' form to cause a file system not to be mounted automatically (with ``mount -a'', or system boot time).

The type of the mount is extracted from the fs_mntops field and stored separately in the fs_type field (it is not deleted from the fs_mntops field). If fs_type is ``rw'' or ``ro'' then the filesystem whose name is given in the fs_file field is normally mounted read-write or read-only on the specified special file. If fs_type is ``sw'' then the special file is made available as a piece of swap space by the swapon(8) command at the end of the system reboot procedure. The fields other than fs_spec and fs_type are unused. If fs_type is specified as ``xx'' the entry is ignored. This is useful to show disk partitions which are currently unused.

The fifth field, (fs_freq), is used for these filesystems by the dump(8) command to determine which filesystems need to be dumped. If the fifth field is not present, a value of zero is returned and dump will assume that the filesystem does not need to be dumped.

The sixth field, (fs_passno), is used by the fsck(8) program to determine the order in which filesystem checks are done at reboot time. The root filesystem should be specified with a fs_passno of 1, and other filesystems should have a fs_passno of 2. Filesystems within a drive will be checked sequentially, but filesystems on different drives will be checked at the same time to utilize parallelism available in the hardware. If the sixth field is not present or zero, a value of zero is returned and fsck will assume that the filesystem does not need to be checked.

#define	FSTAB_RW	"rw"	/* read-write device */
#define	FSTAB_RO	"ro"	/* read-only device */
#define	FSTAB_SW	"sw"	/* swap device */
#define	FSTAB_XX	"xx"	/* ignore totally */

struct fstab {
	char	*fs_spec;	/* block special device name */
	char	*fs_file;	/* filesystem path prefix */
	char	*fs_vfstype;	/* type of filesystem */
	char	*fs_mntops;	/* comma separated mount options */
	char	*fs_type;	/* rw, ro, sw, or xx */
	int	fs_freq;	/* dump frequency, in days */
	int	fs_passno;	/* pass number on parallel fsck */

The proper way to read records from fstab is to use the routines getfsent(3), getfsspec(3), getfstype(3), and getfsfile(3).

UUID=2A1B02AD-467D-403A-8CCD-B87E50AD3DA2 none    apfs  rw
UUID=DF000C7E-AE0C-3B15-B730-DFD2EF15CB91 /export apfs  ro
UUID=FAB060E9-79F7-33FF-BE85-E1D3ABD3EDEA none    hfs   rw,noauto
LABEL=The\040Volume\040Name\040Is\040This none    msdos ro

The file fstab resides in /etc.

getfsent(3), diskarbitrationd(8)

The fstab file format appeared in 4.0BSD.

March 28, 2002 Darwin