CODEWITHSUNDEEP
unixpwd
hits_lucky
hits_lucky

Reputation: 337

shell built in pwd versus /bin/pwd

I would like to know the code implementation of built-in pwd and /bin/pwd especially when the directory path is a symbolic link.

Example:

hita@hita-laptop:/home$ ls -l
lrwxrwxrwx  1 root root   31 2010-06-13 15:35 my_shell -> /home/hita/shell_new/hita_shell

hita@hita-laptop:/home$ cd my_shell

hita@hita-laptop:/home/my_shell$ pwd <SHELL BUILT-IN PWD>  
/home/my_shell

hita@hita-laptop:/home/my_shell$ /bin/pwd  
/home/hita/shell_new/hita_shell

The output is different in both the cases. Any clue?

thanks

Upvotes: 2

Views: 3655

Answers (4)

Jonathan
Jonathan

Reputation: 41

The kernel maintains a current directory (by inode) and when you need the current directory, it determines its name by walking up the directory tree (using ..) to find the names of all the path components. This is the 'real' or sometimes called 'physical' working directory. There is a library function getcwd(3) which does this for you; on more-recent Linux systems this is actually a system call, which helps with getting a consistent view should the parent directories be in the process of being renamed.

Some shells, notably bash, maintain a environment variable PWD to keep track of where you are, and if you changed directory through a symbolic link, this environment variable will show that. They call this the 'logical' path.

/bin/pwd shows the result of getcwd(3), ie the real path; if you give it -L it will tell you the value of PWD (unless it's rubbish, then you get the real path). (Gnu's version of /bin/pwd does more work than this to deal with complexities of parent directories without read permission and very long path names.)

Bash's built-in pwd shows you the 'logical' path with whatever symlinks you used to get there; even if it's now rubbish (ie deleted or renamed since you used it). The default of the built-in pwd can be changed with set -o physical (on) or set +o physical (off is plus!) The default prompt (containing the current directory) follows the option too.

# make a directory with a symlink alias
cd /tmp
mkdir real
ln -s real sym
cd sym

pwd  # will say sym
pwd -L # will say sym
pwd -P # will say real
/bin/pwd  # will say real
/bin/pwd -L # will say sym
/bin/pwd -P # will say real

rm /tmp/sym
pwd  # says sym, though link no longer exists
/bin/pwd -L # will say real!

rmdir /tmp/real
pwd  # says sym, though no directory exists
/bin/pwd # says error, as there isn't one

For what it's worth, my opinion is that all the 'logical' business is just adding to the confusion; the old way was the better way. It's true that symbolic links can be confusing, but this makes it more confusing, because any file operations which open .. don't do the same thing as any directory changes which use .. for example in this rather nasty example:

mkdir -p /tmp/dir/subdir
ln -s /tmp/dir/subdir /tmp/a
cd /tmp/a
ls .. # shows contents of /tmp/dir
(cd .. ; ls) # shows contents of /tmp

To avoid all this, you can put the following in your ~/.bashrc set -o physical

Hope that helps!

Kind regards, J.

PS The above is pretty specific to Linux and Gnu bash; other shells and systems are similar but different.

Upvotes: 4

jamessan
jamessan

Reputation: 42747

The shell's builtin pwd has the advantage of being able to remember how you accessed the symlinked directory, so it shows you that information. The standalone utility just knows what your actual working directory is, not how you changed to that directory, so it reports the real path.

Personally, I dislike shells that do what you're describing because it shows a reality different than that which standalone tools will see. For example, how a builtin tool parses a relative path will differ from how a standalone tool parses a relative path.

Upvotes: 2

Richard Fearn
Richard Fearn

Reputation: 25501

The built-in pwd shows symbolic links by default, but won't do if you give it the -P option.

In contrast, the pwd command doesn't show symbolic links by default, but will do if given the -L option.

Upvotes: 1

Gabe
Gabe

Reputation: 86768

The shell keeps track in its own memory what your currenct directory is by concatenating it with whatever you cd to (and eliminating . and .. entries). It does this so that symbolic links don't mess up cd ... The /bin/pwd implementation walks the directory tree upwards trying to find inodes with the right names.

Upvotes: 1

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