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
|
links
To create a second name for a file that already exists, we can
create a link - sometimes called a hard
link - to it using the command ln ('link').
With two arguments, which must be filenames, provided the first
file does exist and the second does not, we can create a link from
the first to the second. For instance, if user sam
whose home directory is /home/ugrad/sam has a file
datafile that I wish to have in my own home directory
under the name samsdata, then the command
ln can be used to create a link between the two:
$ ln /home/ugrad/sam/datafile
samsdata
We say that samsdata has two links, and it
has two names, one samsdata, the other
/home/ugrad/sam/datafile. The file has a single inode,
however. When samsdata is amended, the contents of
/home/ugrad/sam/datafile are changed at exactly the
same time (and vice versa). If we delete
/home/ugrad/sam/datafile, we actually delete that
filename, and the file continues to exist, but with only
one name (samsdata) and one link. The kernel will keep
track of how many names (links) an inode has, and when this drops
to zero the filespace allocated to that inode is released for use
elsewhere. There is one important point to note here - inodes are
unique only within a single filesystem, and therefore you can only
link a file to another file within the same filesystem. We can
check precisely which inodes are allocated to which files by using
option -i ('inode') to ls:
$ ls -i
total 561
241563 myfile 43532 dir1 86475 dir2
567721 prog.c 563341 foo 563341 bar
In this example, files foo and bar have the same
inode, namely 563341, and have therefore been linked.
Note that two linked files do not necessarily have to be in the
same directory.
At this point, it is worth discussing briefly what a directory
actually is. If you type ls while in a particular
directory, any directories contained within it (referred to as
subdirectories) will appear as if they were files.
In a sense, this is correct - every directory can be considered as
a file, each with its own inode. This 'file' contains - in a form
that need not concern us - information as to where the files in
that subdirectory are stored. By typing cd followed by
the name of a directory, the file representing that directory is
examined, and the data in the file that indicates where it is
stored is retrieved and used to work out where the new current
directory is stored.
Worked example
What is the inode of your home directory?
Solution: First of all, type cd to
change to your home directory. If you then type ls -i
the inodes of the files contained in that directory will be given;
the manual page for ls indicates that option
-d ('directory') will list directories like other
files, rather than listing their contents. So you require
$ ls -id
|