Fedora Linux

? All users in the smmsp group can read the file.

? No one else on the system can read, write, or execute the file.

The permissions on the directories that contain the file also come into play when determining what a user can do with a file. If he does not have execute permission on all of the directories in the path from the root (/) to the file, then he will not be able to access the file, regardless of the permissions on the file itself. Likewise, if he has execute permission on all of those directories, plus write permission on the directory containing the file, then he can delete the file (destroying all the data), even if he can't write to itand then create a new file with the same name.

4.8.1.2. Viewing the current user, group, and mode graphically

GNOME's Nautilus file manager normally displays files and directories as icons. To change the display to a list resembling the output of ls -l , select the menu option View>View as List. The default display shows the file name, size, type, and date modified.

You can add the permissions, owner, and group to the display by selecting Edit>Preferences, which presents the File Management Preferences window shown in Figure 4-11. Click on the List Columns tab, and then click on the checkboxes for permissions, owner, and group to include them on the display. You can also use the Move Up and Move Down buttons to change the displayed order of the fields. Click Close when the display is configured to your liking.

Figure 4-11. Nautilus File Management Preferences window

KDE's Konqueror application provides a similar display when you select View>View Mode>Detailed List View.

4.8.1.3. Changing permissions graphically

Right-clicking on a file in Nautilus or Konqueror will bring up the file Properties window shown in Figure 4 -12 . The Permissions tab within that window contains checkboxes for each of the three permissions in each of the three communitiesnine checkboxes total, plus three for the special permissions (to view the checkboxes in Konqueror, use the Advanced Permissions button).

Figure 4-12. Nautilus File Properties window

To change the permissions, simply toggle checkmarks in the appropriate boxes using your mouse. When you're done, click Close.

4.8.1.4. Changing permissions from the command line

The chmod (change-mode) command is used to change permissions from a shell prompt. The permissions can be specified using either of two different syntaxes: relative symbolic notation or octal notation.

Relative symbolic notation uses any combination of the three community letters ( u , g , or o ) or the letter a to indicate all three communities; an operation symbol, which is + to add a permission and - to remove it, or = to exactly set a permission; and finally, one or more permission letters ( r , w , or x ). Table 4-14 shows some examples of relative symbolic notation; note that multiple operations can be specified using commas as separators (no spaces).

Table 4-14. Relative symbolic notation used by chmod

Notation	Description
u+w	Adds write permission for the user.
o-rwx	Removes read, write, and execute permission for others.
ug+r,o-r	Adds read permission for the user and the group; removes read permission for others.
a-x, ugo-x	Removes execute permission for all users.
u=rw,go=r	Sets exactly read and write permission for the user, and only read permission for group and others. The difference between = and + is that = will disable other permissions (such as execute for the user in this example), while + will leave other permissions at their previous value.

Special permissions are specified based on their appearance in ls -l output:

? SUID is specified as u+s .

? SGUID is specified as g+s .

? Sticky is specified as o+t .

Octal notation uses a multidigit number, where each digit represents one community (in u g o order). The digit is the sum of values representing each enabled permission:

? 4 for read permission

? 2 for write permission

? 1 for execute permission

Therefore, the octal permission 764 represents read/write/execute for the user (4+2+1=7), read/write for the group (4+2=6), and read for others (4): rwxrw-r-- .

When using octal notation, special permissions are given as a fourth digit placed in front of the others; the value of this fourth digit is the sum of 4 for SUID, 2 for SGID, and 1 for Sticky. Octal permission 2770 represents