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Each file and directory in Linux contains a set of permissions that determine who can access them and how You set these permissions to limit access in one of three ways: You can restrict access to yourself alone, you can allow users in a predesignated group to have access, or you can permit anyone on your system to have access; and, you can control how a given file or directory is accessed A file and directory may have read, write, and execute permissions When a file is created, it is automatically given read and write permissions for the owner, enabling you to display and modify the file You may change these permissions to any combination you want A file could have read-only permission, preventing any modifications A file could also have execute permission, allowing it to be executed as a program.

Three different categories of users can have access to a file or directory: the owner, the group, or others The owner is the user who created the file Any file you create, you own You can also permit your group to have access to a file Often, users are collected into groups For example, all the users for a given class or project could be formed into a group by the system administrator A user can give access to a file to other members of the group Finally, you can also open up access to a file to all other users on the system In this case, every user on your system could have access to one of your files or directories In this sense, every other user on the system makes up the "others" category Each category has its own set of read, write, and execute permissions.

The first set controls the user's own access to his or her files-the owner access The second set controls the access of the group to a user's files The third set controls the access of all other users to the user's files The three sets of read, write, and execute permissions for the three categories-owner, group, and other-make a total of nine types of permissions As you saw in the previous section, the ls command with the -l option displays detailed information about the file, including the permissions In the next example, the first set of characters on the left is a list of the permissions set for the mydata file:.

Notice the use of the line S = rSense(). This saves the sensors condition in the variable S. We then use this variable to test for various situations. We could have written each if-statement like this:

An empty permission is represented by a dash, -. The read permission is represented by r, write by w, and execute by x. Notice there are ten positions. The first character indicates the

file type. In a general sense, a directory can be considered a type of file. If the first character is a dash, a file is being listed. If it is d, information about a directory is being displayed. The next nine characters are arranged according to the different user categories. The first set of three characters is the owner's set of permissions for the file. The second set of three characters is the group's set of permissions for the file. The last set of three characters is the other users' set of permissions for the file. In Figure 12-5, the mydata file has the read and write permissions set for the owner category, the read permission only set for the group category, and the read permission set for the other users category. This means, although anyone in the group or any other user on the system can read the file, only the owner can modify it.

There are three if-statements where the sensory data is read twice This means that if the style above is used, the function rSense() would be called six times This is very wasteful and slow The sensory data is not changing between executing each of the ifstatements If we save the value returned by rSense() in a variable there is no need to call the function more than once In real life the rSense() function would be implemented by the use of electronics and some way of communicating between the electronics and the microprocessor The action of calling and interrogating the sensor is time consuming and battery utilization would be higher than needed In this manner of calling the sensor only once (since its data is not changing), we save time and battery life Notice that rSense() is used without specifying a color.