Linux (UNIX) File Permissions
Linux and UNIX have a fairly simple but flexible OS-level file access permission scheme. The permissions (attributes) which can be set on a file are read, write and execute. Read permission allows accessing or looking at the contents of the file. Write permission allows changing the file, by changing existing data, deleting data, or adding new data. Execute permission allows the file to be run as a program. Even if a file internally looks like a program file, if it does not have execute permissions, Linux (UNIX) will not execute it. Each of these permissions can be set for the user which owns the file, all users in the same group as the group of the file, and others not in either of the first two groups. You can allow a permission for one of those classes of users, without allowing it for the other two.
For example, the owner of the file could remove write permission for himself on a file, if its important data that he doesnt want to accidentally overwrite.
Or, the owner could enable read permission for himself, the users in the files group, and all others on the system, if he wanted everyone to be able to read that file.
Directories (folders) are a special case, and the permissions mean slightly different things for directories than for normal files. Read allows you to view the names of the files in that directory. Write allows creating files in that directory. Execute allows you to access files that you know are in that directory, according to the permissions on the files, but it does not allow you to view the names of the files in that directory. Thus, you can only access a file in a directory that you do not have Execute permission for, if you already know its name.
Either the owner of the file, or the UNIX root user can set file permissions.
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