barcode printer vb.net 7: Working with Linux Users and Groups in Software

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7: Working with Linux Users and Groups
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We started this chapter discussing the need for a basic level of file system security on a Linux system We reviewed a scenario where multiple users on the same system were able to access each other s files and the problems that were encountered as a result The way to prevent these problems is to first control who can access the system and then control what they can access once they are in The first part is accomplished by using Linux user accounts With user accounts, users must supply a valid username and password (called credentials) before they are allowed to access the system This is called logging in We pointed out that the Linux operating system will customize the system environment based on the username of the user who logged on Users will have access to their own home directory and a customized desktop environment We pointed out that, by default, user home directories are located in /home, except for the root user, whose home directory is located in /root You can view specific information about a particular user using the finger username command at the shell prompt We also pointed out that each Linux user account has a unique ID number called the UID No two users on the system have the same UID SUSE Linux, by default, starts UIDs for standard users at 1000 Other distributions, such as Fedora, start UIDs at 500 No matter what distribution you re using, the root user s UID is always set to 0 You can view a user s UID (as well as group membership) using the id username command We then discussed the various locations where user accounts can be saved on a Linux system Using local authentication, user accounts are saved in /etc/passwd and /etc/shadow Using LDAP authentication, user accounts are saved in an LDAP-compliant directory service Using NIS authentication, user account files are distributed among several systems using the NIS service Using Windows Domain authentication, user accounts are stored in a central database on a Windows (or Linux Samba) domain controller We pointed out that, for the Linux+ exam, you only need to be familiar with the local authentication method In this configuration, the /etc/passwd file contains your user account information while the /shadow file contains your users encrypted passwords The /etc/passwd file stores user accounts in the following format:
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Username:Password:UID:GID:Full_Name:Home_Directory:Default_Shell
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We also pointed out that the /etc/passwd file contains both standard and system user accounts Standard user accounts are used for login System user accounts can t be used for login Instead, they are used by system services when they need to access the file system
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The /etc/shadow file stores user password information in the following format:
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Username:Password:Last_Modified:Min_Days:Max_Days:Days_Warn:Disabled_Days:Expire
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The /etc/passwd and /etc/shadow files must stay synchronized To do this, you should avoid editing these files directly with a text editor Instead you should use the various user and password management tools on your system To check your files, you can use the pwck command If you suspect the files are out of sync, you can use the pwconv command to add accounts from /etc/passwd to /etc/shadow We then discussed the different utilities you can use from the shell prompt to manage user accounts on your Linux system To add a user, you use the useradd utility If you don t supply any options when creating a user with useradd, the defaults contained in the /etc/default/useradd and /etc/logindefs files are used To set a user s password, you can use the passwd utility at the shell prompt To view the status of a user s account, you can use the S option with passwd To modify an existing user account, you use the usermod utility To remove a user account, you use the userdel utility By default, userdel will not remove a user s home directory when deleting an account If you want to remove the user s home directory, you can use the r option with userdel We then shifted gears and discussed the role of Linux groups Groups ease system administration by allowing you to group together users who need a common level of access to files and directories in the file system Linux groups are stored in /etc/group Group records in /etc/group are represented using the following syntax:
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