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This slide presentation was created by B6Plus. The audio accompanying this presentation is AI-generated.

List file system permissions on files and directories, and interpret the effect of those permissions on access by users and groups.

Every file on a Linux system has an owner and belongs to a group. File permissions control who can access which files.

The effect of permissions on directories is different than that of ordinary files. Read permission means the contents of the directory can be listed. Write permission allows files to be created or deleted in the directory. The execute permission allows use of the cd command to make the directory the current directory.

The -l option to the ls command displays file permissions and file ownership.

The first character indicates the file type. The most common are a - (dash) for an ordinary file while d indicates a directory and l a symbolic link.

The next field shows permissions for the owner of the file, the group the file belongs to, and finally for others on the system. The possible permissions are read, write, and execute. A dash in this field indicates the lack of that permission.

SELinux is a mandatory access control mechanism. A period here indicates that permissions are controlled by SELinux.

A file will have at least one link but you can create additional links either hard or symbolic with the ln command.

This is the account that owns the file.

This is the group that the file belongs to. Regular users will have a private group.

This field shows the file size in bytes. Adding the h option to the ls command makes the size easier to read.

This is the date the file was created or last modified.

The last field is the name of the file.

After reading this section in the textbook complete the quiz in Canvas.

Change the permissions and ownership of files with command-line tools.

The chmod or "change mode" command is used to change file permissions. The command can be used to change permissions by the symbolic method or the octal method.

Often using the octal or "numeric" method to set permissions is easier. In this example the ssh command will not use a private key that is accessible by others.

The number can be added together to get the desired permission. For example, 4+2=6 which is read plus write permissions.

The chmod command used here fixes the issue with the SSH private key shown previously. The order of permissions is special permissions (0), User permissions (read plus write), and no permissions (0) for group or others.

Every file (directories are files, too.) on a Linux system belongs to a group and has an owner.

A new file is owned by the user that creates it and belongs to that owner’s primary group. Only root can change the owner of a file and a user can change the group to which a file belongs to another group which the user is a member of. Notice the colon preceding consultant.

Use file system permissions to create a directory in which all members of a particular group can add and delete files.

When a new file is created it inherits the default permissions set on the system. These permissions are set in the umask section of the /etc/login.defs file.

Sometimes a program needs to be run with the permissions of the owner of the program rather than the user who runs the program. The passwd command is an example. The /etc/shadow file must be updated when a user sets a new password. Root privileges are required to change that file. Running with normal unprivileged permissions would be impossible.

The sticky bit assigned to a directory such as /tmp prevents files from being changed or deleted except by the owner of the file.

When a new file is created it inherits the default permissions set on the system. These permissions are set in the umask section of the /etc/login.defs file. The executable bit (1) must always be explicitly set.

The umask command without an arguments displays the current value of the umask.

Regular users can temporarily change the umask. Given the umask in the previous slide (0022) what would be the resulting umask after student runs this command?

Use the Vim text editor to create this script in the /etc/profile.d directory which changes the default umask for regular users.

Control the permissions on files that are created in a directory by using umask settings and the setgid permission.

Lab Notes

This is a good lab to practice using Esc+. to insert the previous command's argument into the current command. See Chapter 2: Execute Commands with the Bash Shell.

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