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Linux is a multi-user OS that is based on the Unix concepts of file ownership and permissions to provide security at the file system level. If you are planning to improve your Linux skills, it is essential that you have a decent understanding of how ownership and permissions work.
There are many intricacies when dealing with file ownership and permissions, but we will try our best to distill the concepts down to the details that are necessary for a foundational understanding of how they work. In this tutorial, we will cover how to view and understand Linux ownership and permissions. Access to a Linux server is not strictly necessary to follow this tutorial, but having one to use will let you get some first-hand experience. If you want to set one up, check out this link for help. As mentioned in the introduction, Linux is a multi-user system.
We must understand the basics of Linux users and groups before we can talk about ownership and permissions, because they are the entities that the ownership and permissions apply to. In Linux, there are two types of users: system users and regular users. Traditionally, system users are used to run non-interactive or background processes on a system, while regular users used for logging in and running processes interactively.
When you first log in to a Linux system, you may notice that it starts out with many system users that run the services that the OS depends on—this is completely normal. Each line in this file contains information about a single user, starting with its user name the name before the first :. Print the passwd file with this command:. In addition to the two user types, there is the superuser , or root user, that has the ability to override any file ownership and permission restrictions.
In practice, this means that the superuser has the rights to access anything on its own server. This user is used to make system-wide changes, and must be kept secure. In fact, creating a normal user that has sudo privileges for system administration tasks is considered to be best practice.walivofa.tk
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Groups are collections of zero or more users. A user belongs to a default group, and can also be a member of any of the other groups on a server. In Linux, each and every file is owned by a single user and a single group, and has its own access permissions. The most common way to view the permissions of a file is to use ls with the long listing option, e.
If you want to view the permissions of all of the files in your current directory, run the command without an argument, like this:.
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Here is an example screenshot of what the output might look like, with labels of each column of output:. Aside from the Mode column, this listing is fairly easy to understand. To help explain what all the groupings and letters mean, take a look at this closeup of the mode of the first file in the example above:. In Linux, there are two basic types of files: normal and special. The file type is indicated by the first character of the mode of a file—in this guide, we refer to this as the file type field. Normal files can be identified by files with a hyphen - in their file type fields.
Normal files are just plain files that can contain data.
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They are called normal, or regular, files to distinguish them from special files. Special files can be identified by files that have a non-hyphen character, such as a letter, in their file type fields, and are handled by the OS differently than normal files. The character that appears in the file type field indicates the kind of special file a particular file is.
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For example, a directory, which is the most common kind of special file, is identified by the d character that appears in its file type field like in the previous screenshot. There are several other kinds of special files but they are not essential what we are learning here. From the diagram, we know that Mode column indicates the file type, followed by three triads, or classes, of permissions: user owner , group, and other.
The order of the classes is consistent across all Linux distributions. The next thing to pay attention to are the sets of three characters, or triads, as they denote the permissions, in symbolic form, that each class has for a given file. A hyphen - in the place of one of these characters indicates that the respective permission is not available for the respective class.
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Now that you know how to read which permissions of a file, you probably want to know what each of the permissions actually allow users to do. We will explain each permission individually, but keep in mind that they are often used in combination with each other to allow for meaningful access to files and directories. For a directory, write permission allows a user to delete the directory, modify its contents create, delete, and rename files in it , and modify the contents of files that the user can read.
For a normal file, execute permission allows a user to execute a file the user must also have read permission. As such, execute permissions must be set for executable programs and shell scripts before a user can run them. For a directory, execute permission allows a user to access, or traverse, into i. Now that know how to read the mode of a file, and understand the meaning of each permission, we will present a few examples of common modes, with brief explanations, to bring the concepts together. As you may have noticed, the owner of a file usually enjoys the most permissions, when compared to the other two classes.
This makes sense because files should only be accessible to users who need access to them for a particular reason. Another thing to note is that even though many permissions combinations are possible, only certain ones make sense in most situations.
To keep this tutorial simple, we will not cover how to modify file ownership and permissions here. You should now have a good understanding of how ownership and permissions work in Linux. If you would like to learn more about Linux basics, it is highly recommended that you read the next tutorial in this series:. Learning how to manage users effectively is an essential skill for any Linux system administrator. In this guide, we will discuss how to add and delete users and assign sudo privileges on an Ubuntu Quotas are used to limit the amount of disk space a user or group can use on a filesystem.
I presume the discrepancy in the week of the year is to do with when the week turns - as you can see I ran the command on a Monday. Some systems have the turn of the week being Monday, others have it on Sunday. The -UFormat option is a really nice thing to have. The file has to exist. As well as write-output there are a couple of options for use in Powershell scripts and functions:. In bash, echo -n echoes back the string without printing a newline, so if you do this:.
The best PowerShell equivalent to egrep or grep is select-string :. The -context switch allows you to see a specified number of lines either side of the matching one. This is an interesting one, in that it points up a conceptual difference between PowerShell and Bash. The egrep is searching across the whole line. In PowerShell there are two set of environment variables: - windows-level variables and - Powershell-level variable.
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It shows system error and log messages. The simplest Powershell equivalent of the bash find is simply to stick a -recurse on the end of a dir command. If you want Powersehll to give you output that looks more like the Unix find then you can pipe into select fullname. There is no direct equivalent of the shell functionality you get with set -o vi sadly. You can up- and down- arrow by default, but if you want to search through your history then you need to do something like this.
First, it shows the similarity between PowerShell and Bash, which I think is encouraging for anyone reading this e-book. Second it allows me make this brief point about using aliases. I had to look it up. It works really well. The Powershell equivalent of ls -a is get-childitem -force. There are two much-used options: -full and -examples. To give some idea of scale, on my laptop get-help get-process currently returns just over a screenful of information, whereas get-help -get-process -full returns 9 screenfuls.