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When comparing the symbolic link and the original file, you will notice a clear difference between them (see Listing 4-6). First, the symbolic link and the original file have different inodes: the original file is just a name that is connected directly to the inode, and the symbolic link refers to the name. The latter can be seen from the ls -il (-i displays the inode number) output: after the name of the symbolic link, an arrow is used to indicate what file you are really working on. Also you can see that the size of the symbolic link is significantly different from the size of the real file. The size of the symbolic link is the number of bytes in the name of the file it refers to, because no other information is available in the symbolic link. Also, you can see that the permissions on the symbolic link are completely open. This is because the permissions are not managed here, but on the original file instead. Finally, you can see that the file type of the symbolic link is set to l, which indicates that it is a symbolic link. Listing 4-6. Showing the Differences Between Symbolic and Hard Links root@ubuntu:~# ln -s /etc/hosts symhosts root@ubuntu:~# ln /etc/hosts hardhosts root@ubuntu:~# ls -il /etc/hosts hardhosts symhosts 15024138 -rw-r--r-- 2 root root 253 2007-06-05 00:20 /etc/hosts 15024138 -rw-r--r-- 2 root root 253 2007-06-05 00:20 hardhosts 13500422 lrwxrwxrwx 1 root root 10 2007-07-02 05:45 symhosts -> /etc/hosts You may ask what happens to the symbolic link when the original file is removed. Well, that isn t hard to predict! The symbolic link fails. Ubuntu Server will show this when displaying file properties with the ls command; you ll get a File not found error message when you try to open it.

Caution Many groups are listed in the Main Group drop-down list. Nearly all of these relate to the way the Linux operating system works and can be ignored (you can see the list of groups in Table 21-1). You should never delete any of these groups or add new users to them. This may make the system unstable and/or insecure.

To decide whether someone has entered an Open competition, we need to find just one matching entry. To decide he has not entered an Open competition, we need to check all the Open entries to make sure there is not one for him. In terms of our joined tables in Figure 4-2, finding those people who have entered an Open tournament is a simple relational algebra select operation. Remember that for a select operation we look at each row independently and decide whether it should be included in the returned set of rows. However, to find people who have not entered an Open tournament, we need to investigate every row in the table to ensure that there is not an entry for a particular member. This is a much more complex task than a simple select. We can see from the third row in Figure 4-2 that member 258 has entered a non-Open tournament, but we have to search further down the table to see that he has entered an Open tournament at a different time. In fact, we also need to consider the members who have never entered any tournaments at all. These members IDs will not appear in the Entry table at all, so we also have to investigate another table, the Member table, to find the complete list. The type of query described in the previous paragraph can be expressed in the relational algebra using one of the set operations that we will look at in 7. However, we can also use relational calculus to help us write the SQL statement for this type of query. To do that, we need first to introduce the EXISTS keyword.

Deleting a user is simply a matter of highlighting the username in the list within the main Users and Groups window and clicking the Delete button. Note that you can choose to either delete the user s Home folder or to keep the files. You might want to access the files yourself and make a backup of them before completely removing the Home folder.

Every file on a Linux file system has an inode. As explained earlier, all of a file s administrative data is kept in its inode. Your computer actually works entirely with inodes, and the file names are only a convenience for people who are not too good at remembering numbers. Every name that is connected to an inode can be considered a hard link. So, when you create a hard link for a file, all you really do is add a new name to an inode. To do this, use the ln command without any options; ln /etc/hosts ~/computers will create such a hard link.