Page 238 - DCAP103_Principle of operating system

Unit 6: File Management

the command cp mailbox mailbox.bak do exactly the same thing if the working directory is /usr/ Notes
ast/. The relative form is often more convenient, but it does the same thing as the absolute form.
Some programs need to access a specific file without regard to what the working directory
is. In that case, they should always use absolute path names. For example, a spelling checker
might need to read /user/lib/dictionary to do its work. It should use the full, absolute path
name in this case because it does not know what the working directory will be when it is
called. The absolute path name will always work, no matter what the working directory is of
course, if the spelling checker needs a large number of files from /user/lib/, an alternative
approach is for it to issue a system call to change its working directory to /user/lib/, and
then use just dictionary as the first parameter to open. By explicitly changing the working
directory, it knows for sure where it is in the directory tree, so it can then use relative paths.
Each process has its own working directory, so when a process changes its working directory
and later exits, no other processes are affected and no traces of the change are left behind in the
file system. In this way it is always perfectly safe for a process to change its working directory
whenever that is convenient on the other hand, if a library procedure changes the working
directory and does not change back to where it was when it is finished, the rest of the program
may not work since its assumption about where it is may now suddenly be invalid. For this
reason, library procedures rarely change the working directory, and when they must, they
always change it back again before returning.
Most operating systems that support a hierarchical directory system have two special entries in
every directory, “.” and “..”, generally pronounced “dot” and “dotdot.” Dot refers to the current
directory; dotdot refers to its parent. To see how these are used, consider the UNIX file tree of
Fig. 6.15 A certain process has /usr/ast/ as its working directory. It can use .. to go up the tree.
For example, it can copy the file /usr/lib/dictionary to its own directory using the command:
cp ../lib/dictionary.
Figure 6.15: A UNIX Directory Tree

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