Permission Denied on a Linux Profile
When you are logged into a Linux system under a particular profile or account, you may see the message "Permission Denied" pop up either on the command line or under the X Windows system when you are trying to access a file or install an application. If you are an administrator you can use the root user or the sudo command to get around this message.
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File Permissions
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The security of Unix based systems, such as Linux, comes from the way permissions to access or modify files are highly compartmentalized. Configuration files that are central to the system's function can only be modified by the system's root user, also known as the super user. Regular users can only access files that either they created, or files whose owners have given them permission to access. This system of differing permissions protects both the system itself, as well as the security of individual users on the system. The only exception to restrictions on access to the files of individual users is the root user, who can access and modify any file on the entire system.
Root
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There is only a single root user account on any Linux system, and it is created during the system's installation process. Because of the root user's system wide permissions, it is usually only used for creating new user accounts, modifying file permissions to allow other users to access specific files, or system maintenance such as installations or upgrades. Usually system administrators do not log into a system as the root user, but open a command line from within their normal user account and start a session as the root user by typing "su" followed by the root password.
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Sudo
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A common security convention is that users should avoid assuming full root privileges unless they are performing extensive system maintenance. Rather, convention says that system administrators should use the command "sudo" to give individual actions root privileges. Any account that the root user has added to the "wheel" user group and added to the "sudoers" list can use sudo. Using this command will let you access any file on the system, and is also commonly used for installing programs or system updates.
Restricted Access
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Just as other operating systems can have multiple administrator accounts, a Linux system can have multiple user accounts that have permission to use the "sudo" command. If a user whose account does not have sudo permissions receives a "permission denied" message, it simply means that he was trying to do something he was not supposed to. If a user does not have administrator privileges then he is not meant to access system files, the files of other users, or install new software on the system.
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References
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