Archive for the ‘Linux’ Category
The Top Ten Concepts for Beginners to Linux – Number 3, Users
September 2nd, 2010 | 
Author: Unlike Windows, Linux users must have an account. This account is accorded permissions such as reading and writing files and executing programs. Accounts are combined into groups to simplify their management. Users who are members of a given group may be assigned additional permissions that smooth the process of accessing common files. Let’s take a closer look at how Linux handles these users. A future article examines permissions and groups.
Linux provides two categories of users, ordinary users essentially restricted to working with files of their own creation, and the superuser, or root, who administers the system and is allowed to do almost anything. The real situation is of course somewhat more complex. For example, ordinary users may be accorded restricted access to additional files deemed to be of interest to everyone. The superuser can assign some privileges to deputies reducing an otherwise crushing workload.
The root user is the only one authorized to run the adduser or the more complicated useradd command. If you have downloaded Damn Small Linux you can test these commands on your Windows computer. In response to the adduser command Linux requests the user name, the password, and then a few optional fields that may be skipped. An organization with many users has or should have a policy for composing user names.
The password is a key element in protecting the computer’s security. We could easily write an article discussing the password and password selection. Remember that Linux distinguishes between lower-case and upper-case letters. If you’re new to computing you may try working with easily remembered passwords at first. But protecting your account and account information depends on your unguessable passwords.
When Linux creates your account it also creates a directory whose name is easy to remember. If your account name is lucy then your working directory is /home/lucy . I’m leaving it up to you to find out the location of lucy’s working directory for Windows systems.
Damn Small Linux shares an interesting feature with many other Linux systems. The system administrator can provide new users with copies of some directories and folders by placing them in the /etc/skel directory before launching the adduser command.
How is the system administrator supposed to handle creating dozens, hundreds, or even thousands of new users, for example at the beginning of the semester? He or she won’t have the time to undertake this backbreaking task. And there is no way that the root password should be revealed to the employee who is assigned this task. The answer is simple: write a program to create these new accounts. This program can even spiff up the account creation interface and perhaps grab some necessary information from sources such as the student registration file.
Of course we don’t want to create users without the possibility of removing them. The live process is fairly complicated and includes removing the user’s password from the /etc/passwd directory, removing all files from the user directory, and other activities such as making backups of essential information. The next article discusses the related concepts of permissions and groups.
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The Top Ten Concepts for Linux Beginners – Number 5, Inodes
Linux functionality is often found, albeit in a different form in Windows. Inodes, the subject of this article, are an exception to this rule. Because of their practicality, Windows now includes an imitation of this functionality. But if you want to take full advantage of this somewhat complicated concept, you’ll have to go to Linux or Unix.
So what are inodes and why would anyone want to use them? Inodes are the internal description of a file. As we will see below, the specific inode contents for a given file are different in memory and on disk. But the key to the inode is that a single file may have different names. Why would anybody want to do that? One very important reason is that a user may access a shared file by an intuitive name. Accounting department users could give a certain file an accounting-style name, while users from the marketing department could access this same file by a name that makes sense to them. This functionality alone makes inodes worth while.
Furthermore, let’s say that by accident the “accounting” file was deleted. Accounting users could still access that file if they knew the “marketing” name. Remember, we are talking about one single file that has different names. People don’t have to know about these complications to be able to access the file. The extra processing is carried out behind the scenes.
Now let’s examine inodes more closely. The inode on disk contains the following information: file owner identifier, file type, file access permissions, file access information, number of links, list of data addresses, and file size. The file owner identifier specifies the file owner and the group owner as discussed in our article on permissions and groups. The file type indicates whether we are talking about a regular file, a directory, or something else. The file access permissions denote the permissions, also discussed in a previous article. A given file may have different permissions for different users, for example accounting users may have permission to read and modify the file while marketing users only have permission to read it.
File access information specifies when the file was last accessed, last modified, and when the associated inode was last modified. The number of links indicates the number of names that the file has. In our example, the file has two links. If either accounting or marketing delete the file (assuming that they have such permission) the number of links is reduced to 1. But the file is still there and the number of links could be increased.
The inode on disk provides a list of data addresses; the single file may be scattered across the disk. The final value is the size of the file in bytes.
The inode in memory contains all of the above information plus additional information, for example, whether the file is available for processing (someone else may be using it) and whether someone is waiting to process the file. Remember, several people may be using Unix or Linux simultaneously. If there were no file usage control two different people in accounting could pay a bill at the same time. Windows doesn’t have this problem; it’s not a multi-user system.
The ln command is used to create a link to a given file. For example, ln acct1 mark1 links the mark1 file to the acct1 file; it makes the name mark1 available for the acct1 file. It’s the same file, but with a new name. The ls ‘i command provides information about the file including the number of links (the number of names for the file.)
The next article in this series discusses the Linux kernel and processes.
So what are inodes and why would anyone want to use them? Inodes are the internal description of a file. As we will see below, the specific inode contents for a given file are different in memory and on disk. But the key to the inode is that a single file may have different names. Why would anybody want to do that? One very important reason is that a user may access a shared file by an intuitive name. Accounting department users could give a certain file an accounting-style name, while users from the marketing department could access this same file by a name that makes sense to them. This functionality alone makes inodes worth while.
Furthermore, let’s say that by accident the “accounting” file was deleted. Accounting users could still access that file if they knew the “marketing” name. Remember, we are talking about one single file that has different names. People don’t have to know about these complications to be able to access the file. The extra processing is carried out behind the scenes.
Now let’s examine inodes more closely. The inode on disk contains the following information: file owner identifier, file type, file access permissions, file access information, number of links, list of data addresses, and file size. The file owner identifier specifies the file owner and the group owner as discussed in our article on permissions and groups. The file type indicates whether we are talking about a regular file, a directory, or something else. The file access permissions denote the permissions, also discussed in a previous article. A given file may have different permissions for different users, for example accounting users may have permission to read and modify the file while marketing users only have permission to read it.
File access information specifies when the file was last accessed, last modified, and when the associated inode was last modified. The number of links indicates the number of names that the file has. In our example, the file has two links. If either accounting or marketing delete the file (assuming that they have such permission) the number of links is reduced to 1. But the file is still there and the number of links could be increased.
The inode on disk provides a list of data addresses; the single file may be scattered across the disk. The final value is the size of the file in bytes.
The inode in memory contains all of the above information plus additional information, for example, whether the file is available for processing (someone else may be using it) and whether someone is waiting to process the file. Remember, several people may be using Unix or Linux simultaneously. If there were no file usage control two different people in accounting could pay a bill at the same time. Windows doesn’t have this problem; it’s not a multi-user system.
The ln command is used to create a link to a given file. For example, ln acct1 mark1 links the mark1 file to the acct1 file; it makes the name mark1 available for the acct1 file. It’s the same file, but with a new name. The ls ‘i command provides information about the file including the number of links (the number of names for the file.)
The next article in this series discusses the Linux kernel and processes.