Unix command to see file permissions

If a permission is denied, then its value is always zero. In the example above, all permissions have been granted.

For each category of user owner, group member, or other these three permission values potentially add up to seven. If we deny one or more type of permission, then that value 4, 2, or 1 is subtracted from the value for that category of user. These changes are shown in the array below:. The total value is now rather than Note that whatever combination of permissions we create, the numbers will always be a unique representation of that combination, as shown in the following chart:.

Just as each column designates a specific combination of permissions, so the total value represents a specific combination of permissions associated with user types since the order is always given as: owner group other. Thus, from any three digit total value, you can deduce each of the nine possible permissions.

Remember that this total value is always given in the order: owner group others. When you wish to set the mode of a file set the permissions you use the UNIX command chmod at the system prompt. As you become familiar with the chmod command, try using the -v option for a verbose response as in the following example:.

This command designates that the file named myfile. Setting permissions for a directory follows exactly the same procedure; you would simply substitute the directory name for the file name. You can also use the letters r, w, and x to set read, write, and execute permissions and the letters u, g, o, and a to specify user, group, other or all:. Here are some examples that if done in the following order would set the permissions as shown:.

The chmod command specifies read-write-execute permissions for the user, and read-execute permissions for group and other. Classes are formed by combining one or more of the following letters:. The modes specify which permissions are to be added to or removed from the specified classes. There are three primary values which correspond to the basic permissions, and two less frequently-used values that are useful in specific circumstances:.

Using the above definitions, the previous octal notation example can be done symbolically:. Every user on a Unix system is a member of one or more Unix groups, including their primary or default group.

Every file or directory on the system has an owner and an associated group. When a user creates a file, the file's associated group will be the user's default group. The user owner has the ability to change the associated group to any of the groups to which the user belongs. Unix groups can be defined that allow users to share data with other users who belong to the same group. Group names are limited to eight characters. A user's default group is the same as their username.

NERSC users usually belong to several other groups, including groups associated with specific research projects. Associated with this the repo is the Unix group "bigsci". The user elvis would then be a member of two file groups, elvis and bigsci. Because a NERSC user can be a member of more than one research project, such a user would be a member of more than one repo-associated Unix groups. Continuing with the example above, if user elvis wants to collaborate with another user "jimi", but does not want other members of bigsci to be able to see the data, the PI for Big Science could create a new group for example, "ejdata", for elvis and jimi's data.

The PI would then add elvis and jimi to the ejdata group. Those two users could then use "group permissions" on directories and files to share data with one another. A user is the owner of the file.

By default, the person who created a file becomes its owner. Hence, a user is also sometimes called an owner. A user- group can contain multiple users. All users belonging to a group will have the same Linux group permissions access to the file. Suppose you have a project where a number of people require access to a file. Instead of manually assigning permissions to each user, you could add all users to a group, and assign group permission to file such that only this group members and no one else can read or modify the files.

Any other user who has access to a file. This person has neither created the file, nor he belongs to a usergroup who could own the file. Practically, it means everybody else. Hence, when you set the permission for others, it is also referred as set permissions for the world. It is like you do not want your colleague, who works on your Linux computer , to view your images. This is where Permissions set in, and they define user behavior. The third part is for the world which means any user.

This means the user can only:.