vb.net barcode component Figure 14-2. The file permissions part of a file listing can be broken down into four in Java

Encoding Code 39 Extended in Java Figure 14-2. The file permissions part of a file listing can be broken down into four

Figure 14-2. The file permissions part of a file listing can be broken down into four
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The four groups are as follows: Type of file: This character represents the file type. A standard data file is indicated with a dash (-). Most files on your system fall into this category. A d shows that the entry is not a file but a directory. Table 14-2 lists the file type codes. User permissions: Next come the permissions of the person who owns the file. The three characters indicate what the person who owns the file can do with it. The owner of a file is usually the user who created it, although it s also possible to change the owner later on. In this example, you see rw-. This means that the owner of the file can read (r) and write (w) the file. In other words, he can look at it and also save changes to it. However, there s a dash afterward, and this indicates that the user cannot execute the file. If this were possible, there would be an x in this spot instead.
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CHAPTER 14 UN DERS TANDING LINUX FILES AN D US ERS
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Group permissions: After the owner s permissions are the permissions given to the specified group. This is indicated by another three characters in the same style as those for user permissions. In the example, the group s permission is r--, which means that the members of the specified group can read the file but don t have permission to write to it, since there s a dash where the w would normally appear. In other words, as far as they re concerned, the file is read-only. Everyone else s permissions: The last set of permissions indicates the permissions of everyone else on the system (other users in other groups). In the example, they can only read the file (r); the two dashes afterward indicate that they cannot write to the file nor execute it.
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Table 14-2. File Type Codes
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Standard file Standard directory Symbolic link (a shortcut to another file) Named pipe (a file that acts as a conduit for data between two programs) Socket (a file designed to send and receive data over a network) Character device (a hardware device driver, usually found in /dev) Block device (a hardware device driver, usually found in /dev)
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As you might remember from Windows, programs are stored as files on your hard disk, just like standard data files. On Linux, program files need to be explicitly marked as being executable. This is indicated in the permission listing by an x. Therefore, if there s no x in a file s permissions, it s a good bet that the file in question isn t a program or script (although this isn t always true for various technical reasons). To make matters a little more confusing, if the entry in the list of files is a directory (indicated by a d), then the rules are different. In this case, an x indicates that the user can access that directory. If there s no x, then the user s attempts to browse to that directory will be met with an access denied message. File permissions can be difficult to understand, so let s look at a few real-world examples. These examples assume that you re logged in to Linux as the user keir.
CHAPTER 14 UNDERS TA NDIN G LINUX FILES AN D US ERS
LESS COMMON FILE PERMISSIONS
Instead of the x or dash in the list of permissions for a directory, you might sometimes see a t. This is referred to as the sticky bit and means that the only people who can delete or alter a file in that directory are the users who created the file in the first place. This is a useful option to have in some circumstances. It s used with the /tmp (temporary) folder, for example, to ensure that one user can t delete another user s temporary files but is able to delete his own temporary files. To set the sticky bit for a directory, type chmod +t directoryname. You might sometimes see a set of permissions like rws. The s stands for set user id and is often referred to as the suid bit . Like x, it indicates that the file is executable, except in this case, it means that the file will be run with the permissions of the person who owns it, rather than the user who is executing it. In other words, if user frank tries to run a program owned by keir that has the execute permission set as s, that program will be run as if keir were running it. This is very useful, because it can make programs that require root powers usable by ordinary users, although this brings with it obvious security risks. To set the suid bit, type chmod +s filename. However, it s very unlikely you ll ever need to use this command.
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