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Question 136:
Which command is used to display the current runlevel of a Linux system?
A) systemctl get-default
B) runlevel
C) init status
D) who -r
Answer: B
Explanation:
The runlevel command is the traditional and most straightforward way to display the current and previous runlevel of a Linux system. When executed without any arguments, it displays two values: the previous runlevel and the current runlevel. If there was no previous runlevel change since boot, it displays N (meaning none) followed by the current runlevel number.
Runlevels are predefined operating states in traditional SysV init systems. Each runlevel represents a different system configuration, ranging from 0 (halt) to 6 (reboot), with levels 1 through 5 representing different operational modes. Level 3 typically represents multi-user mode with networking, while level 5 includes the graphical user interface. Understanding runlevels is essential for system administration, particularly when troubleshooting boot issues or managing system services.
The command systemctl get-default is used in systemd-based systems to show the default target, which is the systemd equivalent of a default runlevel. While this shows what target the system boots into by default, it does not display the current operational state in the same way the runlevel command does. Systemd uses targets instead of runlevels, though it maintains backwards compatibility.
The init status command is not a valid command for checking runlevels. The init command is used to change runlevels or send signals to the init process, but it does not have a status option for displaying the current runlevel.
The who -r command can also display runlevel information and shows the current runlevel along with the time it was entered. While this is a valid alternative, the runlevel command remains the most commonly used and recommended method for checking the current runlevel in traditional init systems.
Question 137:
What is the purpose of the /etc/fstab file in Linux?
A) To store user passwords
B) To define filesystems to be mounted at boot time
C) To configure network interfaces
D) To list running processes
Answer: B
Explanation:
The /etc/fstab file (filesystem table) is a critical system configuration file that defines which filesystems should be automatically mounted at boot time and specifies how they should be mounted. Each line in this file represents a separate filesystem and contains six fields: the device or partition identifier, mount point, filesystem type, mount options, dump frequency, and fsck pass number. This file is read by the mount command and various system utilities during the boot process.
Understanding the structure of /etc/fstab is essential for system administrators. The first field identifies the device using methods like device name, UUID, or LABEL. The second field specifies the directory where the filesystem will be mounted. The third field indicates the filesystem type such as ext4, xfs, or nfs. The fourth field contains mount options like defaults, noatime, or ro (read-only). The fifth field is used by the dump utility to determine if the filesystem needs backing up, and the sixth field determines the order in which fsck checks filesystems at boot.
Storing user passwords is the function of /etc/shadow (or historically /etc/passwd), not /etc/fstab. The shadow file contains encrypted password hashes and password aging information for user accounts. This separation of password information from general user account information enhances system security.
Network interface configuration is handled by files in /etc/network/ (on Debian-based systems) or /etc/sysconfig/network-scripts/ (on Red Hat-based systems), or through systemd-networkd configuration files. These files define network parameters like IP addresses, netmasks, gateways, and DNS servers.
Listing running processes is done dynamically through the /proc filesystem or commands like ps and top, not through a static configuration file like /etc/fstab. The /proc filesystem is a virtual filesystem that provides real-time information about running processes and system state.
Question 138:
Which command displays the kernel ring buffer messages?
A) klog
B) dmesg
C) kernlog
D) syslog
Answer: B
Explanation:
The dmesg command displays the contents of the kernel ring buffer, which contains messages generated by the kernel during system boot and runtime. These messages include hardware detection, driver initialization, kernel module loading, and various system events. The kernel ring buffer is a circular buffer of fixed size, meaning older messages are overwritten when the buffer becomes full. This command is invaluable for troubleshooting hardware issues, driver problems, and understanding the boot sequence.
System administrators frequently use dmesg to diagnose hardware problems, verify that devices are recognized correctly, and identify kernel errors or warnings. The command can be used with various options such as dmesg -w to watch for new messages in real-time, dmesg -T to display human-readable timestamps, or dmesg -l err to filter messages by priority level. The output can be piped through grep to search for specific hardware or error messages.
The klog command is not a standard Linux command for viewing kernel messages. While some systems may have custom scripts or utilities with this name, it is not the standard tool provided by the Linux kernel for accessing the kernel ring buffer.
Similarly, kernlog is not a standard command in Linux systems. Kernel logging is handled through the dmesg command and the kernel logging facility, which can write messages to files through syslog or journald.
The syslog command or service refers to the system logging facility that collects messages from various sources including the kernel, system services, and applications. While syslog can receive and store kernel messages, it is not the direct command for viewing the kernel ring buffer. Modern systems often use rsyslog or syslog-ng as implementations of the syslog protocol, and systemd-based systems use journalctl to view logs managed by systemd-journal.
Question 139:
What does the chmod 755 filename command do?
A) Gives read, write, execute to owner; read and execute to group and others
B) Gives read and write to all users
C) Gives full permissions to all users
D) Removes all permissions from the file
Answer: A
Explanation:
The chmod 755 command sets specific permissions on a file or directory using octal notation. In this notation, each digit represents permissions for a different category of users: owner, group, and others. The number 7 translates to read (4) plus write (2) plus execute (1), giving the owner full permissions. The first 5 translates to read (4) plus execute (1), giving the group read and execute permissions. The second 5 gives the same read and execute permissions to all other users.
Understanding octal permission notation is fundamental to Linux file management. Each permission has a numerical value: read equals 4, write equals 2, and execute equals 1. These values are added together to create the permission digit for each user category. Therefore, 755 means the owner can read, write, and execute the file (4+2+1=7), while group members and other users can only read and execute it (4+1=5). This permission set is commonly used for executable files and directories that need to be accessible to all users but modifiable only by the owner.
Giving read and write to all users would require permission 666, where each digit is 6 (read 4 plus write 2). This permission set allows all users to read and modify the file but not execute it. Such permissions are rarely used for security reasons as they allow anyone to modify the file.
Full permissions to all users would be represented by 777, where each digit is 7 (read 4 plus write 2 plus execute 1). This grants everyone complete access to the file, including the ability to read, modify, and execute it. This permission set poses significant security risks and should be avoided except in very specific circumstances.
Removing all permissions would be accomplished with chmod 000, where each digit is 0. This makes the file inaccessible to all users except root, who can override file permissions. Even the file owner would not be able to read, write, or execute the file without first changing its permissions.
Question 140:
Which file contains the DNS server addresses used by the system?
A) /etc/hosts
B) /etc/resolv.conf
C) /etc/network
D) /etc/dns.conf
Answer: B
Explanation:
The /etc/resolv.conf file contains the DNS (Domain Name System) configuration for the system, including the IP addresses of DNS servers that the system should query to resolve domain names to IP addresses. This file typically contains nameserver entries, each followed by the IP address of a DNS server. The system queries these servers in the order they appear in the file. Additionally, the file can contain search domains and other DNS-related options that affect how domain name resolution is performed.
The resolv.conf file is read by the resolver library, which is responsible for translating domain names into IP addresses. A typical /etc/resolv.conf file might contain entries like nameserver 8.8.8.8 for Google’s public DNS server or nameserver 192.168.1.1 for a local network DNS server. It can also include a search directive that specifies domains to be searched for unqualified hostnames, and an options directive for various resolver options like timeout values and rotation of nameservers.
The /etc/hosts file serves a different purpose as a local hostname-to-IP address mapping file. Before querying DNS servers, the system checks this file to see if a hostname is defined locally. This file is useful for defining static mappings for frequently accessed hosts or for providing name resolution when DNS is unavailable. Each line contains an IP address followed by one or more hostnames.
The /etc/network directory (primarily on Debian-based systems) contains network interface configuration files but does not directly store DNS server information in the same way as resolv.conf. While network configuration files might specify DNS servers that get written to resolv.conf, the actual DNS resolver configuration resides in resolv.conf.
The /etc/dns.conf file is not a standard Linux configuration file for DNS settings. While some applications might use custom configuration files with similar names, the system-wide DNS configuration is standardized in /etc/resolv.conf across all Linux distributions.
Question 141:
Which command is used to change the ownership of a file?
A) chown
B) chmod
C) chgrp
D) usermod
Answer: A
Explanation:
The chown command (change owner) is used to change the ownership of files and directories in Linux. This command can modify both the user owner and the group owner of a file. The basic syntax is chown user:group filename, though you can change just the user with chown user filename or just the group with chown :group filename. Only the root user or a user with appropriate sudo privileges can change file ownership, as this is a privileged operation that affects system security and file access control.
Understanding file ownership is crucial for Linux system administration and security. Every file and directory has an associated user owner and group owner. The user owner is typically the person who created the file, and the group owner is usually the primary group of that user. File permissions are then applied based on these ownership attributes, determining who can read, write, or execute the file. The chown command is frequently used when transferring files between users, setting up shared directories, or configuring system services that need specific ownership for security reasons.
The chmod command serves a different purpose as it changes file permissions (read, write, execute) rather than ownership. While both commands are essential for file access control, chmod modifies what users can do with a file, whereas chown determines which users and groups own the file. These commands work together to implement Linux’s file security model.
The chgrp command changes only the group ownership of a file, not the user ownership. While chown can change both user and group ownership, chgrp is specialized for changing just the group. The command syntax is chgrp groupname filename. This command is useful when you only need to modify group ownership without affecting user ownership.
The usermod command is used to modify user account properties such as home directory, login shell, group memberships, and account expiration dates. It does not change file ownership. However, usermod can add users to groups, which indirectly affects their access to files owned by those groups.
Question 142:
What is the purpose of the grep command?
A) To copy files
B) To search for patterns in text
C) To compress files
D) To list directory contents
Answer: B
Explanation:
The grep command (Global Regular Expression Print) is one of the most powerful and frequently used text searching utilities in Linux. It searches through files or standard input for lines that match a specified pattern and prints those matching lines to standard output. The pattern can be a simple string or a complex regular expression, making grep extremely versatile for text processing tasks. System administrators and users rely on grep daily for log file analysis, configuration file searches, and data extraction from command output.
The power of grep lies in its support for regular expressions, which allow for sophisticated pattern matching. Basic usage includes commands like grep error /var/log/syslog to find all lines containing the word error in the system log. Advanced usage involves regular expressions such as grep -E ‘^[0-9]{3}-[0-9]{2}-[0-9]{4}$’ to match social security number patterns. The command supports numerous options including -i for case-insensitive matching, -r for recursive directory searching, -v for inverse matching (showing lines that do not match), and -n for displaying line numbers.
Copying files is accomplished with the cp command, not grep. The cp command duplicates files or directories from one location to another. While grep can be used in combination with other commands to selectively copy files (such as finding files with grep and then copying them), grep itself does not perform file copying operations.
File compression is handled by commands like gzip, bzip2, or xz, not grep. These compression utilities reduce file size for storage efficiency or faster transmission. However, zgrep and bzgrep are variants that can search compressed files without manually decompressing them first, combining the functionality of decompression and pattern searching.
Listing directory contents is the function of the ls command, which displays files and directories with various formatting options. While you might pipe ls output through grep to filter directory listings, the core functionality of grep is pattern matching, not directory listing.
Question 143:
Which command displays active network connections and listening ports?
A) ifconfig
B) netstat
C) route
D) ping
Answer: B
Explanation:
The netstat command (network statistics) is a powerful networking tool that displays various network-related information including active network connections, listening ports, routing tables, interface statistics, and network protocol statistics. When used with appropriate options, netstat provides detailed information about TCP and UDP connections, the processes using them, and the state of each connection. Common usage includes netstat -tuln to show all TCP and UDP listening ports with numeric addresses, and netstat -tunaP to include the process ID and name of programs using each connection.
Network administrators use netstat extensively for troubleshooting connectivity issues, identifying which services are listening on which ports, detecting unauthorized network connections, and monitoring network performance. The command displays connection states such as ESTABLISHED for active connections, LISTENING for ports waiting for connections, TIME_WAIT for connections in the closing process, and others. Understanding these states helps diagnose network problems and security issues. Modern Linux distributions are transitioning to the ss command as a replacement for netstat, as ss is faster and provides more detailed information, though netstat remains widely used and understood.
The ifconfig command (interface configuration) is used to configure and display network interface parameters such as IP addresses, netmasks, broadcast addresses, and interface status. While it shows information about network interfaces, it does not display active connections or listening ports. The command has largely been replaced by the ip command in modern Linux distributions, though it remains available for compatibility.
The route command displays and modifies the IP routing table, showing how network packets are directed to their destinations. It shows which gateway or interface is used to reach specific networks. While routing information is crucial for network connectivity, the route command does not show active connections or listening ports.
The ping command is a diagnostic tool that tests connectivity to remote hosts by sending ICMP echo request packets and waiting for replies. It is used to verify that a remote host is reachable and to measure round-trip time and packet loss. However, ping does not display connection information or listening ports.
Question 144:
What does the tar command do?
A) Compresses files only
B) Creates archives and extracts archived files
C) Deletes files permanently
D) Encrypts files
Answer: B
Explanation:
The tar command (tape archive) is a fundamental Linux utility for creating archives and extracting files from archives. It bundles multiple files and directories into a single archive file, preserving file attributes such as permissions, ownership, and timestamps. While tar itself does not compress files, it is commonly used in combination with compression utilities like gzip or bzip2 to create compressed archives. The command supports numerous operations including creating archives with -c, extracting with -x, listing contents with -t, and appending files with -r.
Common tar usage patterns include tar -czf archive.tar.gz directory/ to create a gzip-compressed archive, tar -xzf archive.tar.gz to extract a gzip-compressed archive, and tar -tf archive.tar to list the contents without extracting. The command’s options can be combined, with -c for create, -x for extract, -z for gzip compression, -j for bzip2 compression, -v for verbose output, and -f to specify the archive filename. Understanding tar is essential for system backups, software distribution, and file management. Modern usage often includes the -p option to preserve permissions and the -C option to change to a specific directory before extracting.
While tar is frequently used with compression utilities, it does not perform compression by itself. The archive created by tar alone (a .tar file) simply concatenates files together without reducing their size. Compression is achieved by piping tar’s output through gzip, bzip2, or xz, or by using tar’s built-in compression options like -z for gzip or -j for bzip2.
Deleting files permanently is done with the rm command, not tar. The rm command removes files and directories from the filesystem. While tar can be used to back up files before deletion, it does not perform deletion operations itself. The tar command is designed for archiving and preservation, not destruction of data.
File encryption is handled by commands like gpg (GNU Privacy Guard) or openssl, not tar. These encryption tools secure file contents by converting them into an unreadable format that requires a password or key to decrypt. While you can encrypt tar archives using separate encryption tools, tar itself does not provide encryption functionality.
Question 145:
Which directory contains system log files in Linux?
A) /etc/logs
B) /var/log
C) /usr/log
D) /tmp/log
Answer: B
Explanation:
The /var/log directory is the standard location for system log files in Linux systems, following the Filesystem Hierarchy Standard. This directory contains logs generated by the kernel, system services, applications, and various daemons. Important log files include /var/log/syslog or /var/log/messages for general system messages, /var/log/auth.log for authentication and authorization events, /var/log/kern.log for kernel messages, and application-specific logs like /var/log/apache2/ for web server logs. Understanding the log file structure is crucial for system administration, troubleshooting, and security monitoring.
System administrators regularly examine log files to diagnose problems, monitor system health, investigate security incidents, and track application behavior. The /var/log directory may also contain subdirectories for specific services, such as /var/log/cups/ for printing services or /var/log/mysql/ for database logs. On systemd-based systems, logs may also be stored in binary format in the systemd journal, accessible through the journalctl command, though traditional text logs in /var/log are often maintained for compatibility. Log rotation is typically configured through logrotate to prevent log files from consuming excessive disk space, with old logs being compressed or deleted according to retention policies.
The /etc/logs directory is not a standard location for log files in Linux systems. The /etc directory is reserved for system configuration files, not log files. While some applications might create logs in non-standard locations, the established convention places logs in /var/log to separate variable data from static configuration.
The /usr/log directory is also not a standard location for system logs. The /usr directory hierarchy contains user programs, libraries, documentation, and other files that do not change during normal system operation. Log files, which are constantly changing, do not belong in /usr according to the Filesystem Hierarchy Standard.
The /tmp/log directory is not a standard or recommended location for system logs. The /tmp directory is for temporary files that may be deleted at any time, particularly during system reboots. Storing important system logs in /tmp would risk losing critical diagnostic and audit information. Temporary files created during logging operations might appear in /tmp, but permanent log storage belongs in /var/log.
Question 146:
What is the function of the ps command?
A) To power off the system
B) To display currently running processes
C) To schedule tasks
D) To partition disks
Answer: B
Explanation:
The ps command (process status) displays information about currently running processes on the system. It shows details such as process ID (PID), terminal associated with the process, CPU time consumed, and the command that started the process. The command offers numerous options for customizing the output, with ps aux being one of the most common combinations that shows all processes for all users with detailed information. The ps command is essential for system monitoring, process management, and troubleshooting performance issues or identifying problematic applications.
Different variations of ps provide different views of process information. The ps -ef command shows all processes in full format, including parent process IDs, while ps -eLf includes thread information. The command can be combined with grep to search for specific processes, such as ps aux | grep apache to find all Apache web server processes. Understanding the output columns is important: USER shows the process owner, PID is the unique process identifier, %CPU and %MEM show resource usage, VSZ and RSS indicate memory consumption, STAT shows process state, START shows when the process began, TIME shows CPU time consumed, and COMMAND shows the executable and arguments.
Powering off the system is accomplished with commands like shutdown, poweroff, or init 0, not ps. These commands initiate the system shutdown sequence, stopping services, unmounting filesystems, and halting the system. The ps command is purely informational and does not affect system power state or perform any system control operations.
Scheduling tasks is handled by cron for recurring tasks or at for one-time scheduled tasks, not ps. The cron daemon executes commands at specified times and intervals based on crontab configurations. While ps can show that cron is running, it does not provide task scheduling functionality.
Disk partitioning is performed using commands like fdisk, parted, or gdisk, not ps. These partitioning tools create, modify, and delete disk partitions. The ps command only displays process information and has no capability to modify disk structures or filesystems.
Question 147:
Which command is used to find files in a directory hierarchy?
A) search
B) locate
C) find
D) where
Answer: C
Explanation:
The find command is a powerful utility for searching through directory hierarchies to locate files and directories based on various criteria. It can search by filename, file type, size, modification time, permissions, ownership, and many other attributes. The basic syntax is find /path/to/search -name filename, but find supports complex search expressions combining multiple criteria. Common examples include find /home -name “*.txt” to find all text files, find / -type f -size +100M to find files larger than 100 megabytes, and find /var/log -mtime -7 to find files modified within the last seven days.
The find command’s versatility makes it indispensable for system administration tasks. It can execute commands on found files using the -exec option, such as find /tmp -type f -mtime +30 -exec rm {} ; to delete files older than 30 days. The command searches in real-time by traversing the directory structure, ensuring results reflect the current filesystem state. Options like -user to find files by owner, -perm to search by permissions, and -regex for regular expression matching provide extensive search capabilities. Unlike database-driven search tools, find always provides current results but may be slower for large directory trees.
The search command is not a standard Linux command for finding files. While some desktop environments or custom scripts might use this name, it is not a built-in system utility for file searching in Linux.
The locate command is a legitimate file-finding tool that searches a pre-built database of filenames, making it much faster than find for simple filename searches. However, locate uses a database updated periodically (usually daily) by updatedb, so it may not reflect recent filesystem changes. The locate command is best for quickly finding files by name, while find is better for complex searches requiring real-time results or searches based on attributes other than filename.
The where command is not a standard Linux file-searching utility. The which command, which is different, locates executable files in the user’s PATH, but this is different from searching arbitrary directory hierarchies. The whereis command finds binary, source, and manual page files for commands, but again, this is specialized and not a general-purpose file finder.
Question 148:
What does the df command display?
A) Directory files
B) Disk space usage of filesystems
C) Default file permissions
D) Deleted files
Answer: B
Explanation:
The df command (disk free) displays information about disk space usage for all mounted filesystems. It shows the total size, used space, available space, and percentage of space used for each filesystem. The default output displays values in 1-kilobyte blocks, but the -h option provides human-readable output with sizes in gigabytes, megabytes, or kilobytes as appropriate. Common usage includes df -h to view filesystem usage in human-readable format, df -i to show inode usage instead of block usage, and df -T to include filesystem type in the output.
Understanding disk space usage is critical for system administration to prevent filesystems from filling up, which can cause system instability, application failures, and data loss. The df command helps identify which filesystems are approaching capacity, allowing administrators to take corrective action such as deleting old files, archiving data, or expanding storage. The command shows each mounted filesystem with its mount point, making it easy to identify which directories are on which physical or logical volumes. For example, a server running low on space in /var/log would be immediately visible with df -h, prompting log rotation or cleanup activities.
The df command does not list directory files or directory contents. Listing files and directories is the function of the ls command, which displays files, directories, and their attributes within a specified directory. While df shows mount points which are directories, it does not show the contents of those directories.
Default file permissions are determined by the umask setting, not displayed by df. The umask command shows or sets the default permission mask that determines initial permissions for newly created files and directories. File permissions are displayed by ls -l, and the default umask can be shown with the umask command alone.
Recovering or displaying deleted files is not a function of df. Deleted file recovery requires specialized tools like extundelete for ext filesystems, debugfs, or photorec for data recovery. Once files are deleted and their inodes released, standard commands like df cannot access them. The df command only shows current filesystem space usage, not historical data about deleted files.
Question 149:
Which command changes the current working directory?
A) chdir
B) cd
C) pwd
D) ls
Answer: B
Explanation:
The cd command (change directory) is a shell built-in command used to change the current working directory in Linux. It allows users to navigate through the filesystem hierarchy by specifying either absolute paths (starting from root, like /home/user/documents) or relative paths (relative to the current location, like ../downloads). Common variations include cd alone or cd ~ to move to the home directory, cd – to return to the previous directory, cd .. to move up one directory level, and cd / to go to the root directory. Understanding directory navigation is fundamental to working efficiently in the Linux command-line environment.
The cd command is essential for filesystem navigation and is one of the first commands new Linux users learn. Since it is a shell built-in rather than an external program, it executes very quickly and can modify the shell’s environment directly. The command works in conjunction with environment variables like HOME and PWD, which store the home directory path and present working directory respectively. Users can also use tab completion with cd to quickly navigate to directories without typing full names, improving efficiency. Mastering cd usage, including understanding absolute versus relative paths and using shortcuts like ~ and .., significantly enhances command-line productivity.
The chdir command is primarily used in Windows command prompt environments and some programming languages, not as a standard Linux command-line utility. While some Linux systems might recognize chdir as an alias for cd, the standard and universally recognized command in Linux shells is cd.
The pwd command (print working directory) displays the current working directory path but does not change it. This command is useful for confirming your current location in the filesystem, especially when working with complex directory structures or when your shell prompt does not display the full path. The pwd command simply outputs the absolute path of the current directory.
The ls command lists the contents of directories but does not change the current working directory. It shows files and subdirectories within the specified directory or the current directory if no argument is provided. While ls is often used in combination with cd to explore the filesystem, it does not perform directory navigation.
Question 150:
What is the purpose of the kill command?
A) To terminate processes
B) To delete files
C) To stop network connections
D) To shutdown the system
Answer: A
Explanation:
The kill command is used to send signals to processes, most commonly to terminate them. Despite its name suggesting termination, kill actually sends various signals to processes, with termination being just one possible outcome. The most common usage is kill PID to send the default TERM signal (signal 15) to a process, requesting graceful termination. If a process does not respond to TERM, the stronger KILL signal (signal 9) can be sent with kill -9 PID or kill -KILL PID, which forces immediate termination without allowing the process to clean up. Other useful signals include HUP (hangup) to reload configuration, STOP to pause a process, and CONT to resume a stopped process.
Understanding signal handling is important for effective process management. The kill command requires the process ID (PID) which can be obtained using ps or pidof commands. For example, to terminate a hanging Firefox browser, you might use ps aux | grep firefox to find its PID, then kill that PID number. The killall command provides an alternative by allowing you to specify a process name instead of PID, such as killall firefox. The pkill command offers even more flexibility by allowing pattern matching and other criteria. Only the process owner or root can send signals to a process, providing a security mechanism. The kill -l command lists all available signals, showing that there are many signals beyond just termination signals.
Deleting files is accomplished with the rm command, not kill. The rm command removes files and directories from the filesystem. While the name kill might suggest destruction, it operates on processes in memory, not files on disk. Confusing these commands could lead to attempting to use kill on file operations, which would not work.
Stopping network connections is typically done through commands like ifdown to bring down network interfaces, or by stopping network services with systemctl or service commands. While you might kill a process that is maintaining network connections (which would close those connections as a side effect), the kill command itself does not directly operate on network connections.
Shutting down the system is done with commands like shutdown, halt, poweroff, or reboot, not kill. While you could theoretically kill the init or systemd process to crash the system, this would be improper system administration and could cause data corruption. The proper shutdown commands ensure services stop cleanly, filesystems are unmounted properly, and the system shuts down gracefully.