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Full Discussion: getting full path from relative path
Top Forums Shell Programming and Scripting getting full path from relative path Post 302110800 by Perderabo on Thursday 15th of March 2007 11:29:37 AM
Old 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 NETBSD

pwd

PWD(1)							    BSD General Commands Manual 						    PWD(1)

NAME

     pwd -- return working directory name

SYNOPSIS

     pwd [-LP]

DESCRIPTION

     pwd writes the absolute pathname of the current working directory to the standard output.

     The following options are available:

     -L      If the PWD environment variable is an absolute pathname that contains neither "/./" nor "/../" and references the current directory,
	     then PWD is assumed to be the name of the current directory.

     -P      Print the physical path to the current working directory, with symbolic links in the path resolved.

     The default for the pwd command is -P.

     pwd is usually provided as a shell builtin (which may have a different default).

EXIT STATUS

     The pwd utility exits 0 on success, and >0 if an error occurs.

SEE ALSO

     cd(1), csh(1), ksh(1), sh(1), getcwd(3)

STANDARDS

     The pwd utility is expected to be conforming to IEEE Std 1003.1 (``POSIX.1''), except that the default is -P not -L.

BUGS

     In csh(1) the command dirs is always faster (although it can give a different answer in the rare case that the current directory or a con-
     taining directory was moved after the shell descended into it).

     pwd -L relies on the file system having unique inode numbers.  If this is not true (e.g., on FAT file systems) then pwd -L may fail to detect
     that PWD is incorrect.

BSD
								 October 30, 2003							       BSD
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