Linux with Operating System Concepts

Permission
Explanation
Usefulness
000
No one has any type of access to the item
Not useful other than system files accessible only 
by root
200
Write only
Not useful, need to be able to read the file before 
writing back to it
400
Read only for owner
Useful if file contains confidential read-only 
information, not common
644
Access to owner, read-only for all others
Very common
664
Access to owner and group, read-only 
for world
Common for file sharing with group members
660
Access to owner and group, no access for 
world
Common if file should not be accessed outside of 
group
646
World can write but group members 
cannot
Not useful as we would expect group to have equal 
or greater access than world
755
Adding execute access to 644, used for 
directories, script files, and executable 
programs
Very common
745
Execute privilege added to owner and 
world
Often used instead of 755 for script files
775
Adds write access to group
Less common than 755 but useful if group needs 
write access to directory or script file, or 
members of group might compile the program
711
Group members and world can execute 
the file only
Often used for web server directories
666/777
Read and write access to everyone
Dangerous

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Navigating the Linux File System
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103
which discusses several different 3-digit combinations and why they might be useful or 
not useful.
3.5.3 Altering Permissions from the GUI
You can also modify a file’s permissions through the GUI. From the Places menu (in Gnome), 
select a location. This opens a file browser. You can double click on any directory to open it 
up into a new window. Files appear in a variety of different icon shapes. Figure 3.7 illustrates 
some of the types you can find. Here, we have a text file, a text file storing a script, an execut-
able file, an archive or package, and a directory, respectively from left to right.
Right clicking on a file brings up a pop-up menu. Select Properties and the file’s (direc-
tory’s) Properties window will appear (this is similar to a file’s properties window found 
in Windows operating systems). There are several tabs including Basic to describe the file, 
Open With so that you can select software to open the file into, and Permissions. You will 
see the file’s owner and group owner, and the access currently provided for owner (u), 
group (g), and world (o). Drop down menus allow you to change any of these along with 
the group’s owner and the SELinux context. You also see the last changed date. Figure 3.8 
demonstrates a file’s Properties window, in this case, a file named foo.
For files, permissions available in the drop down menus are limited to “Read only” and 
“Read and write” for the owner and “None,” “Read only,” and “Read and write” for group 
FIGURE 3.8 
Properties window for the file foo.
FIGURE 3.7 
Different file icons.

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104
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Linux with Operating System Concepts
and others. Notice that “executable” is not an option even if the file is an executable pro-
gram or script. For directories, permissions available are “List files only,” “Access files,” 
“Create and delete files,” “---,” and “None” for group and world. You also have the ability 
to alter file access for all files within the directory, selecting one of “Read-only,” “Read and 
write,” “---,” and “None” for group and world. Given the limitations available through the 
GUI, the chmod command is a more expressive way to alter permissions and can be much 
easier if you want to change permissions on multiple files.
3.5.4 Advanced Permissions
There are three other types of permissions to cover. One of these, SELinux, utilizes a far 
more complex scheme than the ugo/rwx mechanism described here. We introduce SELinux 
in Chapter 8 although as a Linux user and even system administrator, you would probably 
not change the SELinux settings from their default. Another form of permission deals 
with the user ID bit and the group ID bit. This influences how a process will execute. This 
is reserved for executable files only and will be discussed in Chapter 4. The last remaining 
topic is known as the