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$ ./simc(output here)

One more example—a typical directory:

drwxr-xr-x 2 mdw lib 512 Jul 17 18:23 perl

The leftmost bit is now a d to show that this is a directory. The executable bits are back because you want people to see the contents of the directory.

Files can be in some obscure states that aren't covered here; see the ls manual page for gory details. But now it's time to see how you can change ownership and permissions.

Changing the Owner, Group, and Permissions

As we said, most of the time you can get by with the default security the system gives you. But there are always exceptions, particularly for system administrators. To take a simple example, suppose you are creating a directory under /home for a new user. You have to create everything as root, but when you're done you have to change the ownership to the user; otherwise, that user won't be able to use the files! (Fortunately, if you use the adduser command discussed earlier in "Creating Accounts," it takes care of ownership for you.)

Similarly, certain utilities and programs such as the MySQL database and News have their own users. No one ever logs in as mysql or News, but those users and groups must exist so that the utilities can do their job in a secure manner. In general, the last step when installing software is usually to change the owner, group, and permissions as the documentation tells you to do.

The chown command changes the owner of a file, and the chgrp command changes the group. On Linux, only root can use chown for changing ownership of a file, but any user can change the group to another group to which he belongs.

So after installing some software named sampsoft, you might change both the owner and the group to bin by executing:

# chown bin sampsoft

#chgrp bin sampsoft

You could also do this in one step by using the dot notation:

# chown bin.bin sampsoft

The syntax for changing permissions is more complicated. The permissions can also be called the file's "mode," and the command that changes permissions is chmod. Let's start our exploration of this command through a simple example. Say you've written a neat program in Perl or Tcl named header, and you want to be able to execute it. You would type the following command:

$ chmod +x header

The plus sign means "add a permission," and the x indicates which permission to add.

If you want to remove execute permission, use a minus sign in place of a plus:

$ chmod -x header

This command assigns permissions to all levels: user, group, and other. Let's say that you are secretly into software hoarding and don't want anybody to use the command but yourself. No, that's too cruel—let's say instead that you think the script is buggy and want to protect other people from hurting themselves until you've exercised it. You can assign execute permission just to yourself through the command:

$ chmod u+x header

Whatever goes before the plus sign is the level of permission, and whatever goes after is the type of permission. User permission (for yourself) is u, group permission is g, and other is o. So to assign permission to both yourself and the file's group, enter:

$ chmod ug+x header

You can also assign multiple types of permissions:

$ chmod ug+rwx header

You can learn a few more shortcuts from the chmod manual page in order to impress someone looking over your shoulder, but they don't offer any functionality besides what we've shown you.

As arcane as the syntax of the mode argument may seem, there's another syntax that is even more complicated. We have to describe it, though, for several reasons. First of all, there are several situations that cannot be covered by the syntax, called symbolic mode , that we've just shown. Second, people often use the other syntax, called absolute mode , in their documentation. Third, there are times you may actually find the absolute mode more convenient.

To understand absolute mode, you have to think in terms of bits and octal notation. Don't worry, it's not too hard. A