Aaron Kulkis wrote:
Bill Anderson wrote:
Bob S wrote:
Hello SuSE users,
While searching for files in /usr/bin I noticed something that seems
odd to me. There is a folder in there named X11. It is a link to the
same folder. If you open it, it shows the exact same thing as
/usr/bin. You can continue opening the X11 directory until you have
a /usr/bin//X11/X11/X11/X11 file open on ad-infitum. They all show
the exact same files and megabytes. What is this? Can somebody
explain the purpose?
Bob S
Not sure of the reason, but it only impacts on the logical path pwd
-L. The physical path (pwd -P) remains as /usr/bin. The impact is to
have all X11 binaries appear in /usr/bin instead of /usr/bin/X11.
Like many symbolic links, it makes the applications happy even when
the underlying structure changes. For those of us who use UNIX, we
are used to /bin being a symbolic link to /usr/bin,
since when? there are several programs which have always
been in /bin because they are needed in runlevel 0 and
run level S, neither of which have /usr mounted, but which
are not administrative only, (and thus do NOT reside in
/sbin or /usr/sbin).
Since the early 1990s, /bin has been a symbolic link to /usr/bin. If you
go to runlevel S, you will find that there are a minimal number of
utilities in /usr/bin. These are hidden if you have a separate mount for
/usr. The intent was for /usr to be a static directory.
examples being: cp, chown, dd, grep, awk, cat, chmod, chgrp,
date, kill, ln, ls, mount, umount, rm, rmdir, sed, stty,
tar, shells (sh/ksh/csh/tcsh/bash), false, true, uname,
ed, ex, vi
I think you need to get on a Unix box, and check out the actual
structure. Linux has never followed this path, and holds to the old Unix
structure. I have been working with Unix since 1978, and have been
through a number of file structure changes.
The point is that symbolic links are used to for backwards compatibility.
As a side light, a number of the utilities that you mention are now
shell built-ins, which take precedence over the equivalent command. For
example, pwd is a built-in that has the -L and -P options for ksh and
bash. The /usr/bin command does not have these options, and exists for
Bourne shell compatibility. You might also note that under Linux it is
/usr/bin/sh, /usr/bin/ksh, and /bin/bash.
Most people are not aware that the default behavior for ksh and bash is
to show the logical path, so that cd .. behaves as expected. It is the
shell that tracks the logical path. The system calls still refer to the
physical path, as does /bin/pwd. For those interested in understanding
this behavior try cd -P /usr/tmp and then do the pwd command. The shell
built-in for cd also support the -P and -L options.
Bill Anderson
WW7BA
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