LPI 102-500 - LPIC-1 Exam 102, Part 2 of 2, version 5.0

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/localtime file is used to configure the system-wide timezone of the local system that is used by applications for presentation to the user. It should be either a symlink to or a copy of a timezone information file that contains the binary data for the configured timezone. The timezone information files are located under /usr/share/zoneinfo/ and are named after the geographic regions and cities, such as Europe/Berlin or Etc/UTC. The timezone identifier is extracted from the symlink target name of /etc/localtime, so it is recommended to use a symlink rather than a copy. The timezone can be changed by using the timedatectl command or by creating a new symlink to the desired timezone file123. References:

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

localtime(5) - Linux manual page - man7.org

localtime(5) — Arch manual pages

The /etc/cron.allow file is a file that contains a list of users who are allowed to use the cron scheduling system. The cron scheduling system is a way of running commands or scripts at specified times or intervals. Users can create their own cron jobs by using the crontab command, which edits a file called crontab that stores the user’s scheduled tasks. However, not all users may have access to the crontab command or the cron system. The access is controlled by two files: /etc/cron.allow and /etc/cron.deny. If the /etc/cron.allow file exists, then only the users listed in this file can use the crontab command and the cron system. The file should have one user name per line. If the /etc/cron.allow file does not exist, then the /etc/cron.deny file is checked. If this file exists, then the users listed in this file are denied access to the crontab command and the cron system. If neither file exists, then the access depends on the configuration of the cron daemon, which is the program that runs the cron jobs. By default, only the root user can use the cron system if no files exist. The root user can always use the cron system regardless of the existence or content of these files. To create or edit the /etc/cron.allow file, the root user needs to use a text editor such as vi, nano, or emacs. For example, to allow the users alice and bob to use the cron system, the root user can use the following command:

sudo vi /etc/cron.allow

And then add the following lines to the file:

alice bob

And then save and exit the file. References:

How cron.allow and cron.deny can be used to limit access to crontab for …

/etc/cron.allow - Linux Bash Shell Scripting Tutorial Wiki

Linux / UNIX Restrict at / cron Usage To Authorized Users

 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

 systemd-journald is a system service that collects and stores logging data from various sources, such as kernel, user-mode programs, and services1. It creates and maintains structured, indexed journals that include metadata and binary data where necessary1. The journal format is secure and unfakeable1. systemd-journald is not incompatible with syslog and can coexist with it. It can forward log messages to a syslog daemon for further processing, filtering, or storage2. This can be enabled by setting the ForwardToSyslog option to yes in the /etc/systemd/journald.conf file2. systemd-journald does not only process messages of systemd, but also messages of any other tools that use the standard logging interfaces, such as syslog(3), sd_journal_print(3), or systemd-cat(1)1. systemd-journald also supports syslog facilities, such as kern, user, and auth, which are used to specify the type of program that is logging the message3. These facilities can be used to filter the journal entries by using the -p or --priority option of the journalctl command4. For example, to show only kernel messages, we can use journalctl -p kern4. References:

systemd-journald.service

Introduction to the Systemd journal

systemd/Journal

journalctl: Query the systemd Journal

The ulimit command is used to set or display the limitations on the system resources available to the current shell and its descendants. One of the resources that can be controlled by ulimit is the maximum size of a core file that is created when a program crashes. A core file is a snapshot of the memory and registers of a process at the time of termination, which can be used for debugging purposes. By default, the core file size limit is zero, which means no core file will be generated. To change the core file size limit, the option -c can be used with ulimit, followed by a number that represents the maximum number of blocks (usually 512 bytes) that can be written to a core file. For example, the command ulimit -c 1000 will set the core file size limit to 512000 bytes. To remove the core file size limit, the option -c can be used with ulimit, followed by unlimited. For example, the command ulimit -c unlimited will allow core files of any size to be created. References:

LPIC-1 Exam 102 Objectives, Topic 103: Linux Installation and Package Management, Subtopic 103.3: Manage shared libraries, Weight: 1, Key Knowledge Areas: Identify the location and purpose of important file and directories as defined in the FHS, Objective: Use the ulimit command to set or display limitations on the system resources available to the current shell and its descendants.

LPIC-1 Exam 102 Learning Materials, Topic 103: Linux Installation and Package Management, Subtopic 103.3: Manage shared libraries, Section 103.3.2: ulimit, Page 14-15.

The file that can be used to enable and disable network services running on this host is /etc/xinetd.conf. This file is the main configuration file for the xinetd daemon, which is a super-server that can start other network services on demand. The /etc/xinetd.conf file contains global settings and a list of services that xinetd can manage. Each service has its own configuration section, which can include the disable keyword to enable or disable the service. For example, to disable the telnet service, the configuration section would look like this12:

service telnet

{

disable = yes

socket_type = stream

protocol = tcp

wait = no

user = root

server = /usr/sbin/in.telnetd

log_on_failure += USERID

}

The other files are not related to network services. The /etc/profile file is a global configuration file for the Bash shell, which sets environment variables and aliases for all users3. The /etc/ports file does not exist by default in Linux, and it is not a standard file for network configuration. The /et/host.conf file is a typo, and it should be /etc/host.conf, which is a file that controls the behavior of the resolver library, which is used to look up host names and IP addresses4. The /etc/host.conf file is not used to enable or disable network services, but to specify the order of host name resolution methods5. References:

1: How to enable or disable services with xinetd - LinuxConfig.org

2: xinetd.conf(5) - Linux manual page - man7.org

3: What is /etc/profile file in Linux? - LinuxForDevices

4: host.conf(5) - Linux manual page - man7.org

5: Linux host.conf file - Computer Notes