LPI Linux Certification in a Nutshell (28 page)

-a

Mounts all of the partitions specified in
/etc/fstab
, except those with the
noauto
option.

-h

Displays help on the
mount
command.

-o
mount_options


Specifies mount options on the command line.

-r

Mounts the filesystem as read-only.

-t
fstype


Specifies that the filesystem to be mounted is of type
fstype
. This option is typically
used interactively when no entry for the mount exists in
/etc/fstab
.

-v

Sets verbose mode.

-w

Mounts the filesystem in read/write mode.

async

Establishes asynchronous I/O to the mounted filesystem.
The opposite is
sync
.

auto

Enables a mount specification in
/etc/fstab
to be processed with the
-a
command-line option, as needed at boot
time. The opposite is
noauto
.

defaults

Implies
rw
,
suid
,
dev
,
exec
,
auto
,
nouser
, and
async
.
It is commonly found on
/etc/fstab
entries for
ext2
and
ext3
mount points.

dev

Interprets character or block special devices on the
filesystem.

exec

Enables the execution of programs contained on the
mounted partition. The opposite is
noexec
.

noauto

Prohibits automatic mounting with the
-a
option. This is usually specified for
removable media.

noexec

Prohibits the execution of executable programs, a
potential security measure.

nosuid

Disables the effect of suid or sgid bits on executable
files.

nouser

Forbids nonroot users from mounting and unmounting the
filesystem. See
user
and
users
for the opposite effect.

ro

Equivalent to specifying the command-line option
-r
.

rw

Equivalent to specifying the command-line option
-w
.

suid

Enables the effect of suid and sgid bits on executable
files.

sync

Establishes synchronous I/O to the mounted filesystem.
The opposite is
async
.

user

Allows an ordinary user to mount the filesystem but
prohibits other ordinary users from unmounting it. This is
useful for removable media that an individual requires control
over. See also
users
.

users

Allows any user to mount and unmount the
filesystem.

ext2

The standard Linux filesystem.

ext3

A journaling filesystem that is backward-compatible with
ext2
.

msdos

The MS-DOS FAT filesystem, limited to “8.3” filenames
(eight characters, a dot, and a three-character
extension).

vfat

Virtual FAT, used instead of
msdos
when long filenames must be preserved. For example, you may
wish to have access to Windows partitions on systems
configured to boot both Linux and Windows.

ntfs

The native MS Windows partition since Windows
2000.

iso9660

The CD-ROM format.

nfs

Remote servers.

swap

Swap partitions.

proc

This type represents the
proc
filesystem, which is not really a filesystem at all. The
virtual files found in this virtual filesystem provide a
window into the kernel. It is usually mounted on
/proc
.

-a

Unmounts all of the filesystems described in
/etc/mtab
. This file is maintained by the
mount
and
umount
commands and contains an up-to-date list of mounted
filesystems. This option is typically used at shutdown
time.

-t
fstype


Unmounts only filesystems of type
fstype
.

On the Exam

Be sure that you understand how to use
mount
and mount points and how
/etc/fstab
is used when
mounting files.

Per-user hard limit

The
hard limit
is the maximum amount of
space an individual user can have on the system. Once the user
reaches his quota limit, he won’t be allowed to write files to the
disk.

Per-user soft limit

Each user is free to store data on the filesystem
until reaching her
soft limit
. The soft limit
implements a sort of warning zone, instructing the user to clean
up while still allowing her to work. When the amount of data
exceeds this limit but does not exceed the hard limit, a message
is printed on the user’s terminal, indicating that her quota has
been exceeded; however, the write operation will succeed.

Per-group hard limit

This is the final limit set for a group by the quota
system. Once this limit has been reached, none of the users within
that group will be allowed to write files to the disk—even if the
user’s individual limits are not exceeded.

Per-group soft limit

This limit behaves in the same way as a user’s soft
limit but is enforced based on group ownership instead of
individual ownership.

Grace period

Once a soft limit is reached, the user or group enters the
grace period
. After the grace
period expires, the soft limit becomes a hard limit until enough
files are deleted to eliminate the over-quota situation. The grace
period may be specified for any number of months, weeks, days,
hours, minutes, or seconds. A typical value is seven days.

When a disk write exceeds a hard limit or an expired soft limit,
only part of the write operation will complete, leaving a truncated
and probably useless file. The messages reported to the user when a
quota is exceeded may be lost if the shell he is using is hidden (for
example, if the user is writing to the disk through an SMB share from
a Windows system). This could confuse the user because the error
message generated by the application indicates that the disk is full
or write-protected.

1. Set options in
   /etc/fstab
   . On the line
   containing the
   /home
   filesystem, add the
   usrquota
and
   grpquota
options to the existing
   default
option, like this:
   

   /dev/sda9   /home    ext2    defaults,usrquota,grpquota  1  2
   

   These options tell quota configuration utilities which
   partitions should be managed when the utilities reference
   /etc/fstab
   .
   

2. Create the
   quota.user
   and
   quota.group
   files at the top of the
   /home
   filesystem and set their protection bits
   for root access only:
   

   #
   touch /home/quota.user /home/quota.group
   #
   chmod 600 /home/quota.user /home/quota.group
   

   These two files are the databases for user and group quotas.
   Each filesystem with quotas uses its own quota databases. When
   quotas are enabled, these files will contain binary data (that is,
   they’re not text files). Note that if you want end users to be able
   to examine quotas on groups to which they belong,
   quota.group
   will need a protection mode of 644
   instead of 600.
   

3. Run
   quotacheck
   to initialize the
   databases:
   

   #
   quotacheck -avug
   Scanning /dev/sda9 [/home] done
   Checked 236 directories and 695 files
   Using quotafile /home/quota.user
   Using quotafile /home/quota.group
   

4. Then, verify that your quota database files have been
   initialized by noting that they are no longer of size zero (here
   they are 16,192 bytes each):
   

   #
   ls -al /home/quota.*
   -rw-------  1 root  root 16192 Dec 27 19:53 /home/quota.group
   -rw-------  1 root  root 16192 Dec 27 19:53 /home/quota.user
   

5. Run
   quotaon
   to enable the quota
   system:
   

   #
   quotaon -a
   

6. Verify that your system’s initialization script
   (
   /etc/rc.d/rc.sysinit
   or similar) will turn on
   quotas when your system boots. Something along these lines is
   appropriate, although your system may be very different:
   

   if [ -x /sbin/quotacheck ]; then
   echo "Checking quotas."
   /sbin/quotacheck -avug
   echo " Done."
   fi
   if [ -x /sbin/quotaon ]; then
   echo "Turning on quotas."
   /sbin/quotaon -avug
   fi
   

7. Add a command script to a system
   crontab
   directory (such as the directory
   /etc/crontab.weekly
   ) to execute
   quotacheck
   on a routine basis. An executable
   script file like the following will work:
   

   #!/bin/bash
   exec /sbin/quotacheck -avug
   

   If you prefer, you could instead put
   /sbin/quotacheck
   in
   root
   ’s
   crontab
   file (using the
   crontab
   -e
   command) for weekly execution, like this:
   

   # run quotacheck weekly
   0 3 * * 0   /sbin/quotacheck -avug
   

On the Exam

A general understanding of quotas is necessary for the exam. In
particular, you should know the function of each command. Also
remember that quotas are set on a per-filesystem basis.