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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 DEBIAN

smrsh

SMRSH(8)						      System Manager's Manual							  SMRSH(8)

NAME

       smrsh - restricted shell for sendmail

SYNOPSIS

       smrsh -c command

DESCRIPTION

       The smrsh program is intended as a replacement for sh for use in the ``prog'' mailer in sendmail(8) configuration files.  It sharply limits
       the commands that can be run using the ``|program'' syntax of sendmail in order to improve the over all security of your system.   Briefly,
       even  if  a  ``bad guy'' can get sendmail to run a program without going through an alias or forward file, smrsh limits the set of programs
       that he or she can execute.

       Briefly, smrsh limits programs to be in a single directory, by default /usr/adm/sm.bin, allowing the system administrator to choose the set
       of  acceptable commands, and to the shell builtin commands ``exec'', ``exit'', and ``echo''.  It also rejects any commands with the charac-
       ters ``', `<', `>', `;', `$', `(', `)', `
' (carriage return), or `
' (newline) on the command line to prevent ``end run''  attacks.	It
       allows ``||'' and ``&&'' to enable commands like: ``"|exec /usr/local/bin/filter || exit 75"''

       Initial	pathnames  on programs are stripped, so forwarding to ``/usr/ucb/vacation'', ``/usr/bin/vacation'', ``/home/server/mydir/bin/vaca-
       tion'', and ``vacation'' all actually forward to ``/usr/adm/sm.bin/vacation''.

       System administrators should be conservative about populating the sm.bin directory.  For example, a reasonable  additions  is  vacation(1),
       and  the  like.	No matter how brow-beaten you may be, never include any shell or shell-like program (such as perl(1)) in the sm.bin direc-
       tory.  Note that this does not restrict the use of shell or perl scripts in the sm.bin directory (using the ``#!'' syntax); it simply  dis-
       allows  execution  of  arbitrary  programs.   Also,  including mail filtering programs such as procmail(1) is a very bad idea.  procmail(1)
       allows users to run arbitrary programs in their procmailrc(5).

COMPILATION

       Compilation should be trivial on most systems.  You may need to use -DSMRSH_PATH="path" to adjust the default search  path  (defaults	to
       ``/bin:/usr/bin:/usr/ucb'') and/or -DSMRSH_CMDDIR="dir" to change the default program directory (defaults to ``/usr/adm/sm.bin'').

FILES

       /usr/adm/sm.bin - default directory for restricted programs on most OSs

       /var/adm/sm.bin - directory for restricted programs on HP UX and Solaris

       /usr/libexec/sm.bin - directory for restricted programs on FreeBSD (>= 3.3) and DragonFly BSD

SEE ALSO

       sendmail(8)

							   $Date: 2004/08/06 03:55:35 $ 						  SMRSH(8)
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