Practice Free 102-500 Exam Online Questions
Question #31
Which of the following commands can be used to limit the amount of memory a user may use?
- A . umask
- B . usermod
- C . ulimit
- D . passwd
- E . chage
Correct Answer: C
C
Explanation:
The ulimit command can be used to limit the amount of memory a user may use. The ulimit command is a shell builtin that allows the user to view or modify the resource limits imposed by the operating system. The resource limits can affect the maximum size of files, the maximum number of processes, the maximum amount of CPU time, and the maximum amount of virtual memory a user can access.
To limit the amount of memory a user may use, the -v option can be used with the ulimit command. The -v option sets the maximum amount of virtual memory available to the current shell and its children in kilobytes. For example, the command ulimit -v 1000000 would limit the virtual memory to 1 GB. The -m option can also be used to set the maximum resident set size, which is the amount of physical memory used by a process, but this option is not supported by all systems.
The ulimit command can be used interactively in a shell session, or it can be placed in a shell initialization file, such as .bashrc or .profile, to apply the limits to all future shell sessions. The ulimit command can also be used in conjunction with the /etc/security/limits.conf file, which allows the system administrator to set global or per-user resource limits for all users and processes. The /etc/security/limits.conf file can specify hard and soft limits for each resource, as well as the scope of the limit, such as user, group, or domain.
The other commands listed are not related to limiting the amount of memory a user may use. The umask command sets the default file permissions for newly created files and directories. The usermod command modifies the user account information, such as the home directory, the login shell, or the password expiration date. The passwd command changes the user password. The chage command changes the password aging information, such as the minimum and maximum number of days between password changes, or the number of days before the password expires.
Reference: ulimit(1) – Linux manual page
How to limit user environment with ulimit Linux command Control server access using hosts.allow and hosts.deny files
C
Explanation:
The ulimit command can be used to limit the amount of memory a user may use. The ulimit command is a shell builtin that allows the user to view or modify the resource limits imposed by the operating system. The resource limits can affect the maximum size of files, the maximum number of processes, the maximum amount of CPU time, and the maximum amount of virtual memory a user can access.
To limit the amount of memory a user may use, the -v option can be used with the ulimit command. The -v option sets the maximum amount of virtual memory available to the current shell and its children in kilobytes. For example, the command ulimit -v 1000000 would limit the virtual memory to 1 GB. The -m option can also be used to set the maximum resident set size, which is the amount of physical memory used by a process, but this option is not supported by all systems.
The ulimit command can be used interactively in a shell session, or it can be placed in a shell initialization file, such as .bashrc or .profile, to apply the limits to all future shell sessions. The ulimit command can also be used in conjunction with the /etc/security/limits.conf file, which allows the system administrator to set global or per-user resource limits for all users and processes. The /etc/security/limits.conf file can specify hard and soft limits for each resource, as well as the scope of the limit, such as user, group, or domain.
The other commands listed are not related to limiting the amount of memory a user may use. The umask command sets the default file permissions for newly created files and directories. The usermod command modifies the user account information, such as the home directory, the login shell, or the password expiration date. The passwd command changes the user password. The chage command changes the password aging information, such as the minimum and maximum number of days between password changes, or the number of days before the password expires.
Reference: ulimit(1) – Linux manual page
How to limit user environment with ulimit Linux command Control server access using hosts.allow and hosts.deny files
Question #31
Which of the following commands can be used to limit the amount of memory a user may use?
- A . umask
- B . usermod
- C . ulimit
- D . passwd
- E . chage
Correct Answer: C
C
Explanation:
The ulimit command can be used to limit the amount of memory a user may use. The ulimit command is a shell builtin that allows the user to view or modify the resource limits imposed by the operating system. The resource limits can affect the maximum size of files, the maximum number of processes, the maximum amount of CPU time, and the maximum amount of virtual memory a user can access.
To limit the amount of memory a user may use, the -v option can be used with the ulimit command. The -v option sets the maximum amount of virtual memory available to the current shell and its children in kilobytes. For example, the command ulimit -v 1000000 would limit the virtual memory to 1 GB. The -m option can also be used to set the maximum resident set size, which is the amount of physical memory used by a process, but this option is not supported by all systems.
The ulimit command can be used interactively in a shell session, or it can be placed in a shell initialization file, such as .bashrc or .profile, to apply the limits to all future shell sessions. The ulimit command can also be used in conjunction with the /etc/security/limits.conf file, which allows the system administrator to set global or per-user resource limits for all users and processes. The /etc/security/limits.conf file can specify hard and soft limits for each resource, as well as the scope of the limit, such as user, group, or domain.
The other commands listed are not related to limiting the amount of memory a user may use. The umask command sets the default file permissions for newly created files and directories. The usermod command modifies the user account information, such as the home directory, the login shell, or the password expiration date. The passwd command changes the user password. The chage command changes the password aging information, such as the minimum and maximum number of days between password changes, or the number of days before the password expires.
Reference: ulimit(1) – Linux manual page
How to limit user environment with ulimit Linux command Control server access using hosts.allow and hosts.deny files
C
Explanation:
The ulimit command can be used to limit the amount of memory a user may use. The ulimit command is a shell builtin that allows the user to view or modify the resource limits imposed by the operating system. The resource limits can affect the maximum size of files, the maximum number of processes, the maximum amount of CPU time, and the maximum amount of virtual memory a user can access.
To limit the amount of memory a user may use, the -v option can be used with the ulimit command. The -v option sets the maximum amount of virtual memory available to the current shell and its children in kilobytes. For example, the command ulimit -v 1000000 would limit the virtual memory to 1 GB. The -m option can also be used to set the maximum resident set size, which is the amount of physical memory used by a process, but this option is not supported by all systems.
The ulimit command can be used interactively in a shell session, or it can be placed in a shell initialization file, such as .bashrc or .profile, to apply the limits to all future shell sessions. The ulimit command can also be used in conjunction with the /etc/security/limits.conf file, which allows the system administrator to set global or per-user resource limits for all users and processes. The /etc/security/limits.conf file can specify hard and soft limits for each resource, as well as the scope of the limit, such as user, group, or domain.
The other commands listed are not related to limiting the amount of memory a user may use. The umask command sets the default file permissions for newly created files and directories. The usermod command modifies the user account information, such as the home directory, the login shell, or the password expiration date. The passwd command changes the user password. The chage command changes the password aging information, such as the minimum and maximum number of days between password changes, or the number of days before the password expires.
Reference: ulimit(1) – Linux manual page
How to limit user environment with ulimit Linux command Control server access using hosts.allow and hosts.deny files
Question #33
Which of the following lines are valid in the file /etc/hosts? (Choose TWO correct answers.)
- A . 2001:db8::15 www.example.com www
- B . www.example.com www 203.0.13.15
- C . 203.0.113.15 www.example.com www
- D . www.example.com,www 203.0.13.15,2001:db8::15
- E . 2003.0.113.15,2001:db8::15 www.example.com www
Correct Answer: A, C
A, C
Explanation:
The valid lines in the file /etc/hosts are A and
C. The format of the /etc/hosts file is as follows12:
IP_address canonical_hostname [aliases…]
where IP_address is the IPv4 or IPv6 address of the host, canonical_hostname is the official name of the host, and aliases are optional alternative names for the host. Each field is separated by whitespace (spaces or tabs). The # character indicates the beginning of a comment, and the rest of the line is ignored.
The lines B, D, and E are invalid because they do not follow the format of the /etc/hosts file. Line B has the hostname and aliases before the IP address, which is incorrect. Line D has multiple IP addresses and hostnames separated by commas, which is also incorrect. Line E has two IP addresses for the same host, which is not supported by the /etc/hosts file. If a host has more than one IP address, it should have a separate line for each address3.
Reference: 1: hosts(5) – Linux manual page – man7.org
2: Format of /etc/hosts on Linux (different from Windows?)
3: hosts File Format for TCP/IP – IBM
A, C
Explanation:
The valid lines in the file /etc/hosts are A and
C. The format of the /etc/hosts file is as follows12:
IP_address canonical_hostname [aliases…]
where IP_address is the IPv4 or IPv6 address of the host, canonical_hostname is the official name of the host, and aliases are optional alternative names for the host. Each field is separated by whitespace (spaces or tabs). The # character indicates the beginning of a comment, and the rest of the line is ignored.
The lines B, D, and E are invalid because they do not follow the format of the /etc/hosts file. Line B has the hostname and aliases before the IP address, which is incorrect. Line D has multiple IP addresses and hostnames separated by commas, which is also incorrect. Line E has two IP addresses for the same host, which is not supported by the /etc/hosts file. If a host has more than one IP address, it should have a separate line for each address3.
Reference: 1: hosts(5) – Linux manual page – man7.org
2: Format of /etc/hosts on Linux (different from Windows?)
3: hosts File Format for TCP/IP – IBM
Question #34
Which file contains a set of services and hosts that will be allowed to connect to the server by going through a TCP Wrapper program such as tcpd? (Specify the full name of the file, including path.) https://lh3.googleusercontent.com/-5cd-clmKnbk/AAAAAAAAAAI/AAAAAAAAADM/-SXesH19Ido/s46-c-k-no/photo.jpg
Correct Answer: /etc/hosts.allow
Explanation:
The /etc/hosts.allow file contains a set of rules that specify which services and hosts are allowed to connect to the server by going through a TCP Wrapper program such as tcpd. TCP Wrappers are a security mechanism that can filter incoming requests based on the source address, destination address, and service name. TCP Wrappers can also perform logging, redirection, and execution of commands based on the rules.
The /etc/hosts.allow file has the following format:
service_list : host_list [ : option_list ]
The service_list is a comma-separated list of service names, such as sshd, telnet, or ftp. The host_list is a comma-separated list of host names, IP addresses, or network masks that are allowed to access the services. The option_list is an optional list of keywords that can modify the behavior of the rule, such as twist, spawn, deny, or allow.
For example, the following rule in /etc/hosts.allow allows ssh access from any host in the 192.168.1.0/24 network, and logs the connection attempt:
sshd : 192.168.1.0/255.255.255.0 : spawn /bin/echo %a from %h attempted to access %d >>
/var/log/sshd.log
The /etc/hosts.allow file is processed before the /etc/hosts.deny file, which contains the rules for denying access to the server. If a request matches a rule in /etc/hosts.allow, it is granted access and the processing stops. If it does not match any rule in /etc/hosts.allow, it is checked against the rules in /etc/hosts.deny. If it matches a rule in /etc/hosts.deny, it is denied access and the processing stops. If it does not match any rule in either file, it is granted access by default.
Reference: LPI 102-500 Exam Objectives, Topic 110.3: Implement host security
LPI 102-500 Study Guide, Chapter 10: Securing Your System, Section 10.3: TCP Wrappers hosts.allow man page
Explanation:
The /etc/hosts.allow file contains a set of rules that specify which services and hosts are allowed to connect to the server by going through a TCP Wrapper program such as tcpd. TCP Wrappers are a security mechanism that can filter incoming requests based on the source address, destination address, and service name. TCP Wrappers can also perform logging, redirection, and execution of commands based on the rules.
The /etc/hosts.allow file has the following format:
service_list : host_list [ : option_list ]
The service_list is a comma-separated list of service names, such as sshd, telnet, or ftp. The host_list is a comma-separated list of host names, IP addresses, or network masks that are allowed to access the services. The option_list is an optional list of keywords that can modify the behavior of the rule, such as twist, spawn, deny, or allow.
For example, the following rule in /etc/hosts.allow allows ssh access from any host in the 192.168.1.0/24 network, and logs the connection attempt:
sshd : 192.168.1.0/255.255.255.0 : spawn /bin/echo %a from %h attempted to access %d >>
/var/log/sshd.log
The /etc/hosts.allow file is processed before the /etc/hosts.deny file, which contains the rules for denying access to the server. If a request matches a rule in /etc/hosts.allow, it is granted access and the processing stops. If it does not match any rule in /etc/hosts.allow, it is checked against the rules in /etc/hosts.deny. If it matches a rule in /etc/hosts.deny, it is denied access and the processing stops. If it does not match any rule in either file, it is granted access by default.
Reference: LPI 102-500 Exam Objectives, Topic 110.3: Implement host security
LPI 102-500 Study Guide, Chapter 10: Securing Your System, Section 10.3: TCP Wrappers hosts.allow man page
Question #35
Which of the following nmcli subcommands exist? (Choose two.)
- A . nmcli ethernet
- B . nmcli device
- C . nmcli wifi
- D . nmcli address
- E . nmcli connection
Correct Answer: B,E
B,E
Explanation:
The nmcli command is a command-line interface for NetworkManager, which is a tool for configuring and managing network settings on Linux systems. The nmcli command consists of different subcommands that correspond to different aspects of network configuration and management.
The subcommands are:
nmcli general: shows status and permissions of NetworkManager, as well as system hostname and logging level and domains.
nmcli connection: enables you to create, modify, activate, deactivate, delete, and show network connections.
nmcli device: enables you to show, modify, and control network devices, such as interfaces, bonds, teams, bridges, etc.
nmcli monitor: monitors activity of NetworkManager and watches for changes in the state of connectivity and devices.
nmcli networking: enables or disables overall networking.
nmcli radio: enables or disables radio transmitters for Wi-Fi, Bluetooth, and WWAN devices.
nmcli agent: registers as a secret agent that provides and caches network credentials.
The other options listed are not valid nmcli subcommands. There is no nmcli ethernet, nmcli wifi, or nmcli address subcommand. However, nmcli device and nmcli connection can be used to configure and manage Ethernet and Wi-Fi connections and addresses.
Reference: NetworkManager configuration and usage | SLE Micro 5.3 nmcli: NetworkManager Reference Manual – GNOME nmcli: command not found C The Geek Diary
B,E
Explanation:
The nmcli command is a command-line interface for NetworkManager, which is a tool for configuring and managing network settings on Linux systems. The nmcli command consists of different subcommands that correspond to different aspects of network configuration and management.
The subcommands are:
nmcli general: shows status and permissions of NetworkManager, as well as system hostname and logging level and domains.
nmcli connection: enables you to create, modify, activate, deactivate, delete, and show network connections.
nmcli device: enables you to show, modify, and control network devices, such as interfaces, bonds, teams, bridges, etc.
nmcli monitor: monitors activity of NetworkManager and watches for changes in the state of connectivity and devices.
nmcli networking: enables or disables overall networking.
nmcli radio: enables or disables radio transmitters for Wi-Fi, Bluetooth, and WWAN devices.
nmcli agent: registers as a secret agent that provides and caches network credentials.
The other options listed are not valid nmcli subcommands. There is no nmcli ethernet, nmcli wifi, or nmcli address subcommand. However, nmcli device and nmcli connection can be used to configure and manage Ethernet and Wi-Fi connections and addresses.
Reference: NetworkManager configuration and usage | SLE Micro 5.3 nmcli: NetworkManager Reference Manual – GNOME nmcli: command not found C The Geek Diary
Question #36
Which of the following files, when existing, affect the behavior of the Bash shell? (Choose TWO correct answers.)
- A . ~/.bashconf
- B . ~/.bashrc
- C . ~/.bashdefaults
- D . ~/.bash_etc
- E . ~/.bash_profile
Correct Answer: B, E
B, E
Explanation:
The Bash shell can be configured by various files that affect its behavior, such as setting environment variables, aliases, functions, options, and prompts. Some of these files are global, meaning they apply to all users of the system, and some are local, meaning they apply to individual users. The global files are usually located in the /etc directory, while the local files are usually located in the user’s home directory, which is denoted by the tilde (~) symbol1. The local files that affect the Bash shell are:
~/.bash_profile: This file is executed when a user logs in to the system. It is used to set up the user’s environment, such as the PATH, the default editor, the umask, and other variables. It can also run commands that are needed only once per login session, such as ssh-agent or fortune. This file can also source other files, such as ~/.bashrc, to inherit their settings12.
~/.bashrc: This file is executed when a user starts a new interactive shell, such as opening a terminal window or running a script with the shebang #!/bin/bash. It is used to set up the user’s shell preferences, such as aliases, functions, options, and prompts. It can also source other files, such as /etc/bashrc, to inherit their settings12.
~/.bash_logout: This file is executed when a user logs out of the system. It is used to perform any cleanup tasks, such as clearing the screen, deleting temporary files, or printing a farewell message1. The other files listed in the question are not valid Bash configuration files and do not affect the behavior of the shell. Therefore, the correct answer is
B. ~/.bashrc and
E. ~/.bash_profile.
Reference: 1: Bash Shell Configuration Files – Land of Linux 2: Bash Startup Files – GNU Project
B, E
Explanation:
The Bash shell can be configured by various files that affect its behavior, such as setting environment variables, aliases, functions, options, and prompts. Some of these files are global, meaning they apply to all users of the system, and some are local, meaning they apply to individual users. The global files are usually located in the /etc directory, while the local files are usually located in the user’s home directory, which is denoted by the tilde (~) symbol1. The local files that affect the Bash shell are:
~/.bash_profile: This file is executed when a user logs in to the system. It is used to set up the user’s environment, such as the PATH, the default editor, the umask, and other variables. It can also run commands that are needed only once per login session, such as ssh-agent or fortune. This file can also source other files, such as ~/.bashrc, to inherit their settings12.
~/.bashrc: This file is executed when a user starts a new interactive shell, such as opening a terminal window or running a script with the shebang #!/bin/bash. It is used to set up the user’s shell preferences, such as aliases, functions, options, and prompts. It can also source other files, such as /etc/bashrc, to inherit their settings12.
~/.bash_logout: This file is executed when a user logs out of the system. It is used to perform any cleanup tasks, such as clearing the screen, deleting temporary files, or printing a farewell message1. The other files listed in the question are not valid Bash configuration files and do not affect the behavior of the shell. Therefore, the correct answer is
B. ~/.bashrc and
E. ~/.bash_profile.
Reference: 1: Bash Shell Configuration Files – Land of Linux 2: Bash Startup Files – GNU Project
Question #37
Which command is used to set the hostname of the local system? (Specify ONLY the command without any path or parameters.)
Correct Answer: hostname
Explanation:
The hostname command is used to set the hostname of the local system. The hostname command can take a single argument, which is the new hostname to be assigned to the system. For example, to set the hostname to linux, one can run:
hostname linux
The hostname command can also be used without any arguments to display the current hostname of the system. For example, to show the current hostname, one can run: hostname
The hostname command only changes the hostname temporarily, meaning that the original hostname will be restored after a reboot. To change the hostname permanently, one has to edit the configuration files that store the hostname information, such as /etc/hostname, /etc/hosts, /etc/sysconfig/network, etc. The exact files and commands may vary depending on the Linux distribution and the system initialization process. For more details, please refer to the web search results1 or the question answering results2.
Reference:
Explanation:
The hostname command is used to set the hostname of the local system. The hostname command can take a single argument, which is the new hostname to be assigned to the system. For example, to set the hostname to linux, one can run:
hostname linux
The hostname command can also be used without any arguments to display the current hostname of the system. For example, to show the current hostname, one can run: hostname
The hostname command only changes the hostname temporarily, meaning that the original hostname will be restored after a reboot. To change the hostname permanently, one has to edit the configuration files that store the hostname information, such as /etc/hostname, /etc/hosts, /etc/sysconfig/network, etc. The exact files and commands may vary depending on the Linux distribution and the system initialization process. For more details, please refer to the web search results1 or the question answering results2.
Reference:
Question #38
Of the ways listed, which is the best way to temporarily suspend a single user’s ability to interactively login?
- A . Add the user name to /etc/nologin.
- B . Change the user’s password.
- C . Change the user name in /etc/passwd.
- D . Use change to expire the user account.
- E . Place the command logout in the user’s profile.
Correct Answer: D
D
Explanation:
The best way to temporarily suspend a single user’s ability to interactively login is to use the chage command to expire the user account. The chage command can modify the expiration date of a user account, which is stored in the /etc/shadow file. By setting the expiration date to a past date, the user account will be locked and the user will not be able to login. This method is temporary because the expiration date can be changed again to a future date or removed to unlock the user account. For example, to expire the user account linuxconfig, we can use the following command: # chage -E 0 linuxconfig
This will set the expiration date to January 1, 1970, which is the epoch date.
To check the expiration date of a user account, we can use the -l option:
To remove the expiration date of a user account, we can use the -E option with an empty argument:
# chage -E "" linuxconfig
The other options are either invalid or not recommended. Adding the user name to /etc/nologin will not work, because /etc/nologin is a file that contains a message to be displayed to users who try to login when the system is down for maintenance. Changing the user’s password is not a good idea, because it will affect the user’s authentication and may cause security issues. Changing the user name in /etc/passwd will also affect the user’s authentication and may cause inconsistencies with other files and services. Placing the command logout in the user’s profile will not prevent the user from logging in, but only log them out immediately after login, which is not very elegant or secure.
Reference: 1: How to disable user login with Linux nologin – LinuxConfig.org 2: Disable a user’s login without disabling the account – Unix & Linux Stack Exchange 3: How to Block or Disable Normal User Logins in Linux? – GeeksforGeeks 4: How to Disable User Logins on Linux | Baeldung on Linux 5: How to Disable a User in Linux – Linux Nightly 6: How to deactivate or disable a user account in Ubuntu 20.04 LTS – Vitux 7: chage(1) – Linux manual page
D
Explanation:
The best way to temporarily suspend a single user’s ability to interactively login is to use the chage command to expire the user account. The chage command can modify the expiration date of a user account, which is stored in the /etc/shadow file. By setting the expiration date to a past date, the user account will be locked and the user will not be able to login. This method is temporary because the expiration date can be changed again to a future date or removed to unlock the user account. For example, to expire the user account linuxconfig, we can use the following command: # chage -E 0 linuxconfig
This will set the expiration date to January 1, 1970, which is the epoch date.
To check the expiration date of a user account, we can use the -l option:
To remove the expiration date of a user account, we can use the -E option with an empty argument:
# chage -E "" linuxconfig
The other options are either invalid or not recommended. Adding the user name to /etc/nologin will not work, because /etc/nologin is a file that contains a message to be displayed to users who try to login when the system is down for maintenance. Changing the user’s password is not a good idea, because it will affect the user’s authentication and may cause security issues. Changing the user name in /etc/passwd will also affect the user’s authentication and may cause inconsistencies with other files and services. Placing the command logout in the user’s profile will not prevent the user from logging in, but only log them out immediately after login, which is not very elegant or secure.
Reference: 1: How to disable user login with Linux nologin – LinuxConfig.org 2: Disable a user’s login without disabling the account – Unix & Linux Stack Exchange 3: How to Block or Disable Normal User Logins in Linux? – GeeksforGeeks 4: How to Disable User Logins on Linux | Baeldung on Linux 5: How to Disable a User in Linux – Linux Nightly 6: How to deactivate or disable a user account in Ubuntu 20.04 LTS – Vitux 7: chage(1) – Linux manual page
Question #39
Which file contains the date of the last change of a user’s password?
- A . /etc/gshadow
- B . /etc/passwd
- C . /etc/pwdlog
- D . /etc/shadow
- E . /var/log/shadow
Correct Answer: D
D
Explanation:
The /etc/shadow file contains the encrypted passwords and other information for each user account on a Linux system. The third field in each line of this file is the date of the last password change, expressed as the number of days since Jan 1, 1970. This information is used by the system to determine when a user must change their password, based on the password aging policy. The /etc/shadow file can be viewed and modified by the root user or by using the chage command123. The other files listed in the options do not store the date of the last password change. The /etc/gshadow file contains the encrypted passwords for group accounts4. The /etc/passwd file contains the basic information for each user account, such as the user name, user ID, group ID, home directory, login shell, etc., but not the password5. The /etc/pwdlog file does not exist by default on most Linux systems, and it is not related to the password change date. The /var/log/shadow file also does not exist by default on most Linux systems, and it is not related to the password change
date.
Reference:
https://www.redhat.com/sysadmin/password-changes-chage-command
https://www.golinuxcloud.com/check-last-password-change-expiration-linux/
D
Explanation:
The /etc/shadow file contains the encrypted passwords and other information for each user account on a Linux system. The third field in each line of this file is the date of the last password change, expressed as the number of days since Jan 1, 1970. This information is used by the system to determine when a user must change their password, based on the password aging policy. The /etc/shadow file can be viewed and modified by the root user or by using the chage command123. The other files listed in the options do not store the date of the last password change. The /etc/gshadow file contains the encrypted passwords for group accounts4. The /etc/passwd file contains the basic information for each user account, such as the user name, user ID, group ID, home directory, login shell, etc., but not the password5. The /etc/pwdlog file does not exist by default on most Linux systems, and it is not related to the password change date. The /var/log/shadow file also does not exist by default on most Linux systems, and it is not related to the password change
date.
Reference:
https://www.redhat.com/sysadmin/password-changes-chage-command
https://www.golinuxcloud.com/check-last-password-change-expiration-linux/
Question #40
Which of the following crontab entries will execute myscript at 30 minutes past every hour on Sundays?
- A . 0 * * * 30 myscript
- B . 30 * * * 6 myscript
- C . 30 0 * * 0 myscript
- D . 30 0-23 * * 0 myscript
- E . 0 0-23 * * 30 myscript
Correct Answer: D
D
Explanation:
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
D
Explanation:
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