Files
Overview
The UNIX directory
hierarchy
Filesystems
Manipulating files
Creating
directories
Creating files
links
'Dot' files
Protecting files
Groups
File access
control
Changing
privileges
File
contents
Text files
Comparing
files
Filtering
files
Non-text files
Printing
files
File archives and file
compression
Other relevant commands
Summary
Exercises
|
Filesystems
UNIX does not have the notion of 'a disk' that the programmer is
allowed to work with. Not only is this concept somewhat vague, but
developments in hardware and in storage devices may well mean that
thinking in terms of disks might be inappropriate in the future.
Instead, it uses the concept of a filesystem,
within which inodes are unique, and which is named and known by the
machine, and associated with a specific directory in the file
hierarchy. Each filesystem is set up with an allocated amount of
storage space which the user cannot change.
When manipulating files, you will occasionally get error
messages telling you that a filesystem is 'full'. In order to
discover the amount of free disk space we can use the command
df ('disk free'), which will also give information on
which filesystems are set up for the system:
$ df
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/id000a 10637 10103 -529 106% /
/dev/id001b 186723 155666 12385 93% /usr
/dev/id000f 93033 42924 40806 51% /export
/dev/id000d 46508 15384 26474 37% /var
/dev/id001a 373463 266931 69186 79% /usr
/dev/id001h 124263 93306 18531 83% /usr/local
/dev/id001e 57802 39182 12840 75% /var/tmp
/dev/id000g 747582 600260 72564 89% /home/staff
/dev/id001f 61803 50714 4909 91% /home/ugrad
/dev/sd8e 863422 700676 76404 90% /home/seng
/dev/id002h 560203 456846 47337 91% /home/res
/dev/sd8d 878162 562567 227779 71% /home/acad
/dev/id001d 186723 184473 2250 99% /ex/swap2
/dev/id002f 524542 393609 130933 75% /ex/swap
/dev/id000e 93033 80079 3651 96% /ex/root
We see the storage for the machine divided into the filesystems,
and their sizes are listed together with how much of each is in
use, and where within the directory hierarchy it is placed. Notice
that, in this example, /dev/id000a is more than 100%
full - this is not a printer's error! For each filesystem, normally
only 90% of the physical space on that device is available. This
gives UNIX leeway to warn a user if they attempt to use more of a
filesystem than actually exists. Without this extra space, the
kernel might find itself without enough workspace to continue, and
the system might crash. However, the super-user is allowed to use
the final 10% of a filesystem, and the capacity is measured
relative to the normally allowed 90% of the filesystem. Although
most UNIX systems are set up in this way, the 90% is not cast in
stone, and the super-user may change it as local circumstances
dictate.
|