getting full path from relative path

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# 1  
Old 03-14-2007
getting full path from relative path

given a relative path, how do i convert it into a full one. i.e. if i am in


and i am given foo/bar then to get a full path i can just concatinate it with pwd, but what if i am given "../mama"

how do i programmatically convert:

/home/polypus and ../mama into /home/mama

in a way that will always work no matter what relative path is passed?

my first thought is to just cd to the directory and set a variable

cd $rev_path
cd $original_dir

it seems there must be a unix command which does this more elegantly?

# 2  
Old 03-15-2007
This is why relative paths have problems sometimes. There isn't a good way to handle absolutely any relative path, because all relative paths have an assumption:

the current working directory is "x".

You are assuming /home/polypus. Unix tools are designed to do one thing very well. It's hard to make a bulletproof tool because of the cwd assumption.

Obviously it may not be true.

So, there is no really elegant way to deal with it. You're onto one way to deal with it.
For programming, you should consider avoiding relative paths unless you want the code to work if and only if it runs from a certain directory.
# 3  
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.
# 4  
Old 03-17-2007
not a 'silver bullet', but.....

thisFILE=$(whence ${0})
echo "here is my absolute pathname: [${thisFILE}]"

# 5  
Old 03-25-2007

thanks guys for explanations and help
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