PDF 102-500 Study Guide

 The /etc/passwd file assigns a user to its primary group by specifying the group ID (GID) of the primary group in the fourth field of each line. The /etc/passwd file contains the basic information for each user account on a Linux system, such as the user name, user ID (UID), group ID (GID), home directory, login shell, etc. The format of each line is:

username:password:UID:GID:comment:home:shell

For example, the following line assigns the user bob to the primary group bob, which has the GID of 1001:

bob:x:1001:1001::/home/bob:/bin/sh

The /etc/passwd file can be viewed and modified by the root user or by using the useradd, usermod, or userdel commands123. The other files listed in the options do not assign a user to its primary group. The /etc/pgroup file does not exist by default on most Linux systems, and it is not related to the primary group. The /etc/shadow file contains the encrypted passwords and other information for each user account, but not the primary group4. The /etc/group file contains the information for each group on the system, such as the group name, group password, group ID, and group members, but not the primary group of each user5. The /etc/gshadow file contains the encrypted passwords for group accounts. References: 12345

The TZ environment variable is used to change the time zone for the commands run from within the environment variable’s scope. It specifies the name of a time zone as defined in the /usr/share/zoneinfo directory or a custom time zone in the POSIX format12. The TZ variable can be set either globally in a shell profile file or locally in a shell session. For example, to set the time zone to America/New_York for the current shell session, one can use the following command:

export TZ=America/New_York

To verify the change, one can use the date command to display the current date and time according to the TZ variable. The TZ variable can also be used to run a single command with a different time zone without affecting the system’s time zone. For example, to run the date command with the Asia/Tokyo time zone, one can use the following syntax:

TZ=Asia/Tokyo date

The TZ variable is useful for testing how applications behave in different time zones or for displaying the time in different locations34. References:

How to Set or Change the Time Zone in Linux | Linuxize

Linux / UNIX: TZ Environment Variable - nixCraft

Get Current System Time Zone in Linux | Baeldung on Linux

Setting the TZ Environment Variable on Linux | InterSystems Developer

 The /etc/crontab file is the system-wide crontab file that can be edited only by root. It has a different format from the normal crontab files that can be edited by individual users using the crontab command. The differences are:

The /etc/crontab file can specify a year field as the sixth field in a cron entry. This allows for scheduling jobs that run only in specific years. The normal crontab files do not have a year field and assume the current year for all entries.

The /etc/crontab file has a user field as the seventh field in a cron entry. This allows for running commands as different users from the crontab owner (root). The normal crontab files do not have a user field and run commands as the crontab owner.

The /etc/crontab file does not need to be installed with the crontab command. It is read by the cron daemon automatically. The normal crontab files need to be installed with the crontab command to be recognized by the cron daemon.

The /etc/crontab file and the normal crontab files both allow for environment variable substitution. However, the /etc/crontab file sets some default environment variables such as SHELL, PATH, MAILTO, and HOME, which can be overridden by entries in the file. The normal crontab files inherit the environment variables from the cron daemon, which are usually minimal.

References:

crontab(5) - Linux manual page

Linux Crontab Format

How to schedule a task using Linux crontab (/etc/crontab) file

/etc/crontab - Linux Bash Shell Scripting Tutorial Wiki

The commands that can be used to change a user’s account aging information are:

usermod: this command can modify various user account properties, including the password expiration date, the account expiration date, the minimum and maximum password age, the password warning period, and the password inactivity period. To use this command, you need to specify the option and the value for the property you want to change, followed by the username. For example, to set the password expiration date for the user test to February 11, 2022, you can run:

usermod -e 2022-02-11 test

To view the current account aging information for a user, you can use the -l option with the usermod command. For example, to view the information for the user test, you can run:

usermod -l test

passwd: this command can change the password of a user account, as well as some password aging options. To use this command, you need to specify the username and the option for the property you want to change. For example, to change the password of the user test, you can run:

passwd test

To set the maximum password age for the user test to 90 days, you can run:

passwd -x 90 test

To view the current password aging information for a user, you can use the -S option with the passwd command. For example, to view the information for the user test, you can run:

passwd -S test

chage: this command can change the user password expiry and aging information, such as the password expiration date, the account expiration date, the minimum and maximum password age, the password warning period, and the password inactivity period. To use this command, you need to specify the option and the value for the property you want to change, followed by the username. For example, to set the account expiration date for the user test to February 11, 2022, you can run:

chage -E 2022-02-11 test

To view the current account aging information for a user, you can use the -l option with the chage command. For example, to view the information for the user test, you can run:

chage -l test

The other options are incorrect because:

chattr: this command can change the file attributes on a Linux file system, such as making a file immutable, append-only, or undeletable. It has nothing to do with user account aging information.

chsh: this command can change the login shell of a user account, such as bash, zsh, or ksh. It has nothing to do with user account aging information.

References:

How to Manage User Password Expiration and Aging in Linux - Tecmint

Use the Chage Command in Linux

How to set user password expirations on Linux | Enable Sysadmin

How to change password and account expiry options on Linux using chage - Linux Tutorials - Learn Linux Configuration

3 ways to change user password expiration date in Linux - howtouselinux