Get Certified: Proven Methods to Pass the LPI 102-500 Exam

The correct crontab entry for executing myscript at 30 minutes past every hour on Sundays is D. 30 0-23 * * 0 myscript. This is because the crontab format consists of six fields: minute, hour, day of month, month, day of week, and command. The values for each field can be:

• A single number, such as 5 or 10.
• A range of numbers, such as 1-5 or 10-15.
• A list of numbers separated by commas, such as 1,3,5 or 10,12,14.
• An asterisk (*), which means all possible values for that field.
• A step value, which means every nth value for that field, such as */5 or 10-20/2.

The day of week field can be either a number from 0 to 6, where 0 and 7 are Sunday, or a three-letter abbreviation, such as SUN or MON. The month field can be either a number from 1 to 12, or a three-letter abbreviation, such as JAN or FEB.

In this case, the crontab entry D. 30 0-23 * * 0 myscript means:

• 30: Execute the command at the 30th minute of every hour.
• 0-23: Execute the command for every hour from 0 (midnight) to 23 (11 PM).
• *: Execute the command for every day of the month, regardless of the month.
• *: Execute the command for every month, regardless of the year.
• 0: Execute the command only on Sundays.

The other options are either incorrect or do not match the requirement. For example, option A. 0 * * * 30 myscript means:

• 0: Execute the command at the 0th minute of every hour.
• *: Execute the command for every hour of the day.
• *: Execute the command for every day of the month, regardless of the month.
• *: Execute the command for every month, regardless of the year.
• 30: Execute the command only on the 30th day of the week, which is invalid.

References:

• Crontab Explained in Linux [With Examples]
• ‘crontab’ in Linux with Examples - GeeksforGeeks
• Crontab Syntax on Linux + Useful Examples - Hostinger

 A crontab is a file that contains commands to be executed by the cron daemon at specified intervals. The crontab command allows users to create, edit, list, or remove their own crontab files. Each user can have their own crontab file, which is stored in /var/spool/cron/crontabs directory. The cron daemon checks the crontab files every minute for any changes and reloads them automatically when necessary. There is no need to restart or reload the cron daemon manually after changing a crontab file.

The other statements are false because:

• Changing a crontab does not require a reload/restart of the cron daemon. The cron daemon will detect the changes and reload the crontab files automatically.
• hourly is not the same as “0 * * * *”. The hourly keyword is a shortcut for running a command every hour on the hour, which is equivalent to “@hourly” or “0 * * * *”. The “0 * * * *” syntax means running a command at the zeroth minute of every hour of every day of every month of every year.
• A cron daemon does not need to run for each existing crontab. There is only one cron daemon that runs on the system and reads all the crontab files of all the users and the system.

References:

• Crontab Explained in Linux [With Examples]
• Linux Crontab Command Help and Examples
• Crontab Syntax on Linux + Useful Examples
• Scheduling Cron Jobs with Crontab

The /etc/at.deny file is a file that contains a list of users who are not allowed to use the at command to schedule jobs. If the file exists, any user who is not in the /etc/at.allow file and is in the /etc/at.deny file will be denied access to the at command. To prevent a specific user from scheduling tasks with at, the administrator can simply add the user’s name to the /etc/at.deny file. For example, to prevent the user bob from using the at command, the administrator can use the following command:

echo “bob” | sudo tee -a /etc/at.deny

The other options are not correct. The /etc/at.allow file is a file that contains a list of users who are allowed to use the at command. Adding a user to this file will not prevent them from scheduling tasks with at. The /etc/atd.conf file is a configuration file for the at daemon, which does not have a [deny] section. Adding a user to this file will not affect their access to the at command. The nojobs group is not a predefined group in Linux, and adding a user to this group will not prevent them from scheduling tasks with at. The atd command does not have a --deny option, and running this command will not prevent a user from scheduling tasks with at. References:

• at Command in Linux with Examples - GeeksforGeeks
• How to Use the Linux at Command {9 Examples} - phoenixNAP
• at(1) - Linux man page

The password field of /etc/passwd is used to store the user’s encrypted password or a special character that indicates how the password is stored. In older Linux systems, the user’s encrypted password was stored in the /etc/passwd file. On most modern systems, this field is set to x, and the user password is stored in the /etc/shadow file12. The /etc/shadow file is more secure than the /etc/passwd file because it is readable only by the root user and not by regular users1. The other options are not valid characters for the password field of /etc/passwd. References:

• Understanding the /etc/passwd File | Linuxize
• Understanding the /etc/passwd File - GeeksforGeeks

The /etc/passwd file stores local accounts of the system. It is a readable text file and uses colons (:) to separate the fields. This file helps with converting user IDs to names (and back). It is fine that all users can read this file, but they should not be able to change fields. Therefore, the most correct permissions for /etc/passwd are -rw-r–r–, which means that only the owner (root) can write to the file, and everyone can read it. The /etc/shadow file contains information about the system’s users’ passwords. It is owned by user root and group shadow, and has 640 permissions. The password is stored as a long string of characters, which is a combination of the hashing algorithm, optional salt applied, and the hashed password itself. Other users are not allowed to read the file directly, to prevent them from gathering hashed passwords of others. Therefore, the most correct permissions for /etc/shadow are -r--------, which means that only the owner (root) can read the file, and no one else can read or write to it. References:

 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 crontab entry that could be used to set the system time at regular intervals is the one that uses the ntpdate command to synchronize the system clock with a Network Time Protocol (NTP) server. The ntpdate command takes one or more NTP server names or IP addresses as arguments and adjusts the system clock accordingly12. The crontab entry B specifies that the ntpdate command should be executed at the first minute of the zeroth hour (i.e., 00:01) of every day of every month of every weekday, using the NTP server ntp1.digex.net34. This will ensure that the system time is updated daily with a reliable source.

The other crontab entries are either invalid or ineffective for setting the system time at regular intervals. The date command can be used to display or set the system date and time, but it requires a specific format for the argument, not an NTP server name5. The runcron and settime commands are not standard Linux commands and their functionality is unknown. The $d, $t, and $24 variables are also undefined and meaningless in this context.

References: 1: Linux At, Batch, Atq, Atrm Command Help and Examples - Computer Hope 2: How to set a cron job to run at a exact time? - Stack Overflow 3: 107.2 Lesson 1 - Linux Professional Institute Certification Programs 4: How to setup a crontab to execute at specific time - Stack Overflow 5: Writing a specific format of time in a text file every minute using … - Ask Ubuntu

 The correct crontab entry to execute the script chklog three times per month between 3 p.m. and 5 p.m. is:

3 15,16,17 * * * chklog

The crontab entry has five fields that specify the time and frequency of the job, followed by the command or script to be executed. The fields are:

• Minute: the minute of the hour when the job should run, from 0 to 59
• Hour: the hour of the day when the job should run, from 0 to 23 (in 24-hour format)
• Day of month: the day of the month when the job should run, from 1 to 31
• Month: the month of the year when the job should run, from 1 to 12
• Day of week: the day of the week when the job should run, from 0 to 6 (where 0 and 7 are Sunday)

The asterisk (*) means any value, and the comma (,) means a list of values. Therefore, the crontab entry above means:

• Run the job at the 3rd minute of the hour
• Run the job at the 15th, 16th, and 17th hour of the day (which are 3 p.m., 4 p.m., and 5 p.m.)
• Run the job on any day of the month
• Run the job on any month of the year
• Run the job on any day of the week

This will execute the script chklog three times per day, every day of the month, and every month of the year, which is equivalent to three times per month.

The other options are incorrect because:

• A. This will run the job at any minute of the hour, but only at the 3rd, 4th, and 5th hour of the day (which are 3 a.m., 4 a.m., and 5 a.m.), and only on the 1st day of the month.
• B. This will run the job at the 3rd minute of the hour, but only at the 3rd, 4th, and 5th hour of the day (which are 3 a.m., 4 a.m., and 5 a.m.), and only on the 1st day of the month.
• D. This will run the job at the 0th minute of the hour (which is the top of the hour), but only at the 15th, 16th, and 17th hour of the day (which are 3 p.m., 4 p.m., and 5 p.m.), and only on the 1st day of the month.
• E. This will run the job at any minute of the hour, but only at the 15th, 16th, and 17th hour of the day (which are 3 p.m., 4 p.m., and 5 p.m.), and only on the 1st day of the month.

References:

• Crontab Explained in Linux [With Examples]
• ‘crontab’ in Linux with Examples - GeeksforGeeks
• Linux Crontab Command Help and Examples - Computer Hope
• Crontab in Linux with 20 Useful Examples to Schedule Jobs - TecAdmin
• Linux crontab tutorial with Examples - Linux Tutorials - Learn Linux …

Each entry in a crontab file consists of six fields, specifying in the following order: minute, hour, day, month, weekday, and command1. Any of these fields can be set to an asterisk (*), which stands for “first through last.” So, for example, to run a job every hour, put * in the hour field1. Each entry in a crontab file must end with a newline character (\n), which indicates the end of a line2. A newline character is created by pressing the Enter key on the keyboard. The other options are not valid characters for ending a crontab entry. A tab or a space is used to separate each field, and a backslash is used to escape special characters or continue a long command to the next line2. References:

• How to Use the Cron Job Format to Schedule Task in Linux
• Syntax of crontab File Entries - Oracle.