03-15-2007
Suppose that I don't have room in /usr for /usr/local but I do have some room in /export. So I make a directory called /export/usr.local and it has a subdirectory called bin. And I do a "ln -s /export/usr.local /usr/local". Now I do a "cd /usr/local" then I do a "cd bin". At this point my view is that I am in /usr/local/bin and a shell with a built-in pwd command may return that path. But /usr/bin/pwd will return /export/usr.local/bin. With more symbolic links in a path, there can be more aliases like this. The value returned by /usr/bin/pwd is the physical path. With good permissions on each directory leading to the current directory, /usr/bin/pwd can return the physical path. A built-in pwd in the same shell that navigated to the current directory may be able to return the particular logical path used to arrive at the current directory. Finding all logical paths to a particular directory would be rather daunting.
Writing a program like /usr/bin/pwd is not very easy in unix. You can stat the . directory to get the inode of the current directory. Then you can open .. and stat each file in the parent directory until you find the same inode. You walk up the chain one .. at a time repeating this process until . and .. are the same inode which means you have reached /. (Actually it is harder than that because you might traverse a mount point.) This is why putting an NFS mounted filesystem in / is very unwise... every /usr/bin/pwd must contact the NFS server to perform the stat.
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LEARN ABOUT OPENSOLARIS
ln
ln(1B) SunOS/BSD Compatibility Package Commands ln(1B)
NAME
ln - make hard or symbolic links to files
SYNOPSIS
/usr/ucb/ln [-fs] filename [linkname]
/usr/ucb/ln [-fs] pathname... directory
DESCRIPTION
The /usr/ucb/ln utility creates an additional directory entry, called a link, to a file or directory. Any number of links can be assigned
to a file. The number of links does not affect other file attributes such as size, protections, data, etc.
filename is the name of the original file or directory. linkname is the new name to associate with the file or filename. If linkname is
omitted, the last component of filename is used as the name of the link.
If the last argument is the name of a directory, symbolic links are made in that directory for each pathname argument; /usr/ucb/ln uses the
last component of each pathname as the name of each link in the named directory.
A hard link (the default) is a standard directory entry just like the one made when the file was created. Hard links can only be made to
existing files. Hard links cannot be made across file systems (disk partitions, mounted file systems). To remove a file, all hard links
to it must be removed, including the name by which it was first created; removing the last hard link releases the inode associated with the
file.
A symbolic link, made with the -s option, is a special directory entry that points to another named file. Symbolic links can span file sys-
tems and point to directories. In fact, you can create a symbolic link that points to a file that is currently absent from the file sys-
tem; removing the file that it points to does not affect or alter the symbolic link itself.
A symbolic link to a directory behaves differently than you might expect in certain cases. While an ls(1) on such a link displays the
files in the pointed-to directory, an `ls -l' displays information about the link itself:
example% /usr/ucb/ln -s dir link
example% ls link
file1 file2 file3 file4
example% ls -l link
lrwxrwxrwx 1 user 7 Jan 11 23:27 link -> dir
When you use cd(1) to change to a directory through a symbolic link, you wind up in the pointed-to location within the file system. This
means that the parent of the new working directory is not the parent of the symbolic link, but rather, the parent of the pointed-to direc-
tory. For instance, in the following case the final working directory is /usr and not /home/user/linktest.
example% pwd
/home/user/linktest
example% /usr/ucb/ln -s /var/tmp symlink
example% cd symlink
example% cd ..
example% pwd
/usr
C shell user's can avoid any resulting navigation problems by using the pushd and popd built-in commands instead of cd.
OPTIONS
-f Force a hard link to a directory. This option is only available to the super-user, and should be used with extreme caution.
-s Create a symbolic link or links.
USAGE
See largefile(5) for the description of the behavior of ln when encountering files greater than or equal to 2 Gbyte ( 2^31 bytes).
EXAMPLES
Example 1 The /usr/ucb/ln command
The commands below illustrate the effects of the different forms of the /usr/ucb/ln command:
example% /usr/ucb/ln file link
example% ls -F file link
file link
example% /usr/ucb/ln -s file symlink
example% ls -F file symlink
file symlink@
example% ls -li file link symlink
10606 -rw-r--r-- 2 user 0 Jan 12 00:06 file
10606 -rw-r--r-- 2 user 0 Jan 12 00:06 link
10607 lrwxrwxrwx 1 user 4 Jan 12 00:06 symlink -> file
example% /usr/ucb/ln -s nonesuch devoid
example% ls -F devoid
devoid@
example% cat devoid
devoid: No such file or directory
example% /usr/ucb/ln -s /proto/bin/* /tmp/bin
example% ls -F /proto/bin /tmp/bin
/proto/bin:
x* y* z*
/tmp/bin:
x@ y@ z@
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Availability |SUNWscpu |
+-----------------------------+-----------------------------+
SEE ALSO
cp(1), ls(1), mv(1), rm(1), link(2), readlink(2), stat(2), symlink(2), attributes(5), largefile(5)
NOTES
When the last argument is a directory, simple basenames should not be used for pathname arguments. If a basename is used, the resulting
symbolic link points to itself:
example% /usr/ucb/ln -s file /tmp
example% ls -l /tmp/file
lrwxrwxrwx 1 user 4 Jan 12 00:16 /tmp/file -> file
example% cat /tmp/file
/tmp/file: Too many levels of symbolic links
To avoid this problem, use full pathnames, or prepend a reference to the PWD variable to files in the working directory:
example% rm /tmp/file
example% /usr/ucb/ln -s $PWD/file /tmp
lrwxrwxrwx 1 user 4 Jan 12 00:16 /tmp/file ->
/home/user/subdir/file
SunOS 5.11 11 Mar 1994 ln(1B)