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
pwd
pwd(1) User Commands pwd(1)
NAME
pwd - return working directory name
SYNOPSIS
/usr/bin/pwd
DESCRIPTION
The pwd utility writes an absolute path name of the current working directory to standard output.
Both the Bourne shell, sh(1), and the Korn shells, ksh(1) and ksh93(1), also have a built-in pwd command.
ENVIRONMENT VARIABLES
See environ(5) for descriptions of the following environment variables that affect the execution of pwd: LANG, LC_ALL, LC_MESSAGES, and
NLSPATH.
EXIT STATUS
The following exit values are returned:
0 Successful completion.
>0 An error occurred.
If an error is detected, output will not be written to standard output, a diagnostic message will be written to standard error, and the
exit status will not be 0.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Availability |SUNWcsu |
+-----------------------------+-----------------------------+
|CSI |Enabled |
+-----------------------------+-----------------------------+
|Interface Stability |Committed |
+-----------------------------+-----------------------------+
|Standard |See standards(5). |
+-----------------------------+-----------------------------+
SEE ALSO
cd(1), ksh(1), ksh93(1), sh(1), shell_builtins(1), attributes(5), environ(5), standards(5)
DIAGNOSTICS
``Cannot open ..'' and ``Read error in ..'' indicate possible file system trouble and should be referred to a UNIX system administrator.
NOTES
If you move the current directory or one above it, pwd may not give the correct response. Use the cd(1) command with a full path name to
correct this situation.
SunOS 5.11 2 Nov 2007 pwd(1)