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Question 181
A system administrator needs to modify the kernel parameter vm.swappiness temporarily without rebooting. Which command accomplishes this?
A) sysctl -w vm.swappiness=10
B) echo 10 > /proc/sys/vm/swappiness
C) set vm.swappiness=10
D) Both A and B
Answer: D
Explanation:
Both sysctl -w vm.swappiness=10 and echo 10 > /proc/sys/vm/swappiness can modify the kernel parameter vm.swappiness temporarily without requiring a system reboot. These commands provide different approaches to the same task of adjusting kernel parameters at runtime.
The sysctl command is the standard tool for viewing and modifying kernel parameters at runtime. The -w option writes a new value to a specified parameter. The syntax “sysctl -w vm.swappiness=10” sets the swappiness value to 10 immediately. This change affects system behavior instantly but does not persist across reboots unless also added to configuration files.
The /proc/sys/ directory tree provides a file-based interface to kernel parameters. Each parameter is represented as a file that can be read to view current values or written to modify settings. The command “echo 10 > /proc/sys/vm/swappiness” writes the value 10 to the swappiness parameter file, achieving the same result as the sysctl command. This direct file manipulation is the underlying mechanism that sysctl uses.
The vm.swappiness parameter controls how aggressively the kernel swaps memory pages to disk. Values range from 0 to 100, with higher values making the kernel more aggressive about swapping. Lower values like 10 make the system prefer keeping data in RAM rather than swapping to disk, which improves performance for systems with sufficient memory. The default value is typically 60, representing a balanced approach.
Understanding when to modify swappiness helps optimize system performance. Database servers often benefit from low swappiness values because database caching is more efficient than kernel page cache for database workloads. Desktop systems might use moderate values balancing application responsiveness with system stability. Systems with limited RAM might need higher values to prevent out-of-memory conditions.
Temporary versus permanent changes serve different purposes. Runtime changes using sysctl or /proc/sys/ allow testing parameter effects without commitment and take effect immediately without disruption. Permanent changes require editing configuration files to persist across reboots. The /etc/sysctl.conf file or files in /etc/sysctl.d/ directory contain parameter settings loaded at boot.
Making changes permanent involves adding entries to sysctl configuration files. Creating a file like /etc/sysctl.d/99-custom.conf with content “vm.swappiness=10” ensures the setting is applied at every boot. The sysctl -p command reloads configuration files, applying changes without reboot. This approach combines immediate effect with persistence.
Verification confirms changes took effect. The command “sysctl vm.swappiness” displays the current value. Reading the file directly with “cat /proc/sys/vm/swappiness” shows the same information. Monitoring system behavior after changes helps determine if adjustments improved performance or require further tuning.
Other important kernel parameters include vm.dirty_ratio controlling when dirty pages are written to disk, net.ipv4.ip_forward enabling packet forwarding for routing, kernel.shmmax setting maximum shared memory segment size, and fs.file-max defining maximum number of file handles. Each parameter affects specific aspects of system behavior and performance.
There is no “set” command for modifying kernel parameters in Linux. The sysctl command and /proc/sys/ file system are the appropriate mechanisms.
Since both sysctl -w and echoing to /proc/sys/ files accomplish the task correctly, option D indicating both A and B is accurate.
Question 182
Which command displays the routing table showing destination networks and gateway information?
A) route -n
B) ifconfig route
C) netstat -i
D) traceroute -s
Answer: A
Explanation:
The route command with the -n option displays the routing table showing destination networks, gateway information, and network interfaces used for packet forwarding. This command provides essential information for understanding and troubleshooting network connectivity and routing decisions.
The routing table is fundamental to network operation, determining how packets are forwarded to reach their destinations. Each entry in the table specifies a destination network or host, the gateway through which to reach that destination, the network interface to use, and various flags and metrics affecting routing decisions. The kernel consults this table for every outbound packet to determine the appropriate forwarding path.
The -n option displays addresses and networks numerically rather than attempting to resolve them to hostnames. This numeric display is faster because it avoids DNS lookups, provides consistent results regardless of name resolution availability, and shows exact IP addresses rather than potentially ambiguous hostnames. For routing analysis, numeric output is generally preferred for clarity and reliability.
The output format includes destination network addresses showing which traffic matches each route, gateway addresses indicating the next hop for packets or showing 0.0.0.0 or asterisk for directly connected networks, genmask (netmask) defining the network size, flags indicating route properties, metric values used for route selection when multiple routes exist, and interface names showing which network device transmits the packets.
Common flags in routing table output include U indicating the route is up and active, G indicating the route uses a gateway, H indicating a host-specific route rather than network route, and D indicating a route created by ICMP redirect. Understanding these flags helps interpret routing behavior and diagnose issues.
The default route is particularly important, typically shown as destination 0.0.0.0 with netmask 0.0.0.0. This catch-all route handles traffic to destinations not matching more specific routes, usually pointing to the default gateway provided by DHCP or static configuration. Without a proper default route, systems cannot reach destinations outside directly connected networks.
Static routes can be added manually using the route command. The syntax “route add -net 192.168.10.0 netmask 255.255.255.0 gw 192.168.1.1” adds a route for the 192.168.10.0/24 network via gateway 192.168.1.1. Deleting routes uses similar syntax with “del” instead of “add”. These manual routes supplement automatically configured routes from network interfaces and routing protocols.
Modern systems increasingly use the ip command as a replacement for route. The command “ip route show” or “ip route list” displays the routing table with similar information in different format. The ip command provides additional functionality including policy routing, multiple routing tables, and more detailed route attributes. While route remains available, ip is the preferred tool for current Linux distributions.
Dynamic routing protocols like RIP, OSPF, or BGP can populate routing tables automatically in complex network environments. These protocols exchange routing information between routers, automatically adapting to topology changes. However, most end-user systems and simple networks use static routing with manually configured or DHCP-provided routes.
Troubleshooting routing issues often begins with examining the routing table. Missing routes explain why certain destinations are unreachable. Incorrect gateway addresses cause packets to be sent to wrong next hops. Multiple routes to the same destination might indicate misconfigurations. The routing table provides visibility into how the system makes forwarding decisions.
There is no “ifconfig route” command. The ifconfig command manages interface configuration but does not display routing tables.
The netstat -i command displays network interface statistics showing packet and error counts but not routing tables. The netstat -r command displays routing tables similarly to route.
The traceroute command traces packet paths through networks to reach destinations but does not display the local routing table. It shows the actual path taken, which may differ from what the local routing table suggests due to routing decisions by intermediate routers.
Question 183
A user wants to search for all lines in a file that do NOT contain the word “error”. Which grep command accomplishes this?
A) grep -v error filename
B) grep -i error filename
C) grep –exclude error filename
D) grep -n error filename
Answer: A
Explanation:
The grep command with the -v option inverts the match, displaying all lines that do NOT contain the specified pattern. The syntax “grep -v error filename” shows all lines in the file that do not contain the word “error”, which is exactly what is needed for this task.
The -v or –invert-match option reverses grep’s normal behavior of showing matching lines. Instead of displaying lines containing the pattern, it displays lines that do not match. This inversion is valuable for filtering out unwanted content, focusing on what remains after removing certain patterns, or identifying exceptions to rules.
Pattern matching in grep supports various complexities. Simple text strings match literally as in the example with “error”. Regular expressions enable sophisticated pattern matching with metacharacters like dot for any character, asterisk for zero or more repetitions, brackets for character classes, and anchors for line positions. The -E option enables extended regular expressions with additional operators.
Common use cases for inverted matching include filtering log files to exclude normal informational messages while retaining warnings and errors, removing comment lines from configuration files to see only active settings, excluding specific users or processes from command output, and identifying entries that lack expected patterns or keywords.
Combining grep options creates powerful filters. The command “grep -v -i error filename” performs case-insensitive inverted matching, excluding lines containing “error”, “ERROR”, “Error”, or any case variation. The -n option adds line numbers to output, useful for locating excluded content in the original file. Multiple options can be combined like “grep -vin error filename” for case-insensitive inverted match with line numbers.
Grep can process multiple files simultaneously. The command “grep -v error file1 file2 file3” searches all specified files, with filenames shown in output when multiple files are processed. Wildcards enable processing many files with patterns like “grep -v error *.log” to search all log files in the directory.
Pipeline usage enhances grep’s utility. Command output can be filtered through grep to extract or exclude specific information. For example, “ps aux | grep -v root” shows processes not running as root. The “command | grep pattern | grep -v exclude” chain applies multiple filters sequentially to refine results.
Regular expressions with inverted matching require careful construction. The pattern must match lines to exclude, and grep -v shows lines not matching that pattern. Complex patterns need testing to ensure they match intended content. The -E option helps with advanced patterns using alternation, grouping, and extended operators.
Performance considerations matter when processing large files. Grep is generally efficient, but complex regular expressions or processing extremely large files can be slow. The -F option treats patterns as fixed strings rather than regular expressions, significantly faster for simple literal matches. The –mmap option can improve performance on some systems.
The -i option performs case-insensitive matching but does not invert the match. It would show all lines containing “error” regardless of case, which is the opposite of what is needed.
There is no –exclude option for grep that excludes lines containing a pattern. The –exclude option relates to file exclusion when using grep recursively, not line exclusion.
The -n option adds line numbers to grep output but does not change which lines are selected. It would show matching lines with their line numbers, not non-matching lines.
Question 184
Which file system check and repair utility is used for ext4 file systems?
A)ext4
B) e2fsck
C) xfs_repair
D) Both A and B
Answer: D
Explanation:
Both fsck.ext4 and e2fsck are used for checking and repairing ext4 file systems. In fact, fsck.ext4 is typically a symbolic link to e2fsck, making them essentially the same utility accessed through different names.
The e2fsck utility is the primary file system consistency checker for ext2, ext3, and ext4 file systems. The name derives from “ext2 file system check” though it has been updated to handle ext3 and ext4 features. This tool scans file system metadata, identifies inconsistencies, and repairs corruption when possible, helping maintain file system integrity and recover from improper shutdowns or hardware failures.
File system checking occurs in multiple passes, each addressing different aspects of file system structure. Pass 1 checks inodes, blocks, and sizes. Pass 2 verifies directory structure. Pass 3 checks directory connectivity. Pass 4 verifies reference counts. Pass 5 checks group summary information. Each pass identifies and optionally repairs specific types of corruption or inconsistencies.
The fsck command serves as a front-end that invokes appropriate file system-specific checkers based on file system type. When run on an ext4 file system, fsck automatically calls fsck.ext4 or e2fsck. This abstraction allows administrators to use generic fsck commands without remembering file system-specific utilities, though directly invoking the specific checker is also valid.
Critical safety rules govern file system checking. The file system must be unmounted or mounted read-only before running checks to prevent corruption from simultaneous modifications. Attempting to check a mounted read-write file system can cause severe damage. The root file system cannot be unmounted while running, so it must be checked before mounting or from single-user mode during boot.
Common options control checking behavior. The -f option forces checking even if the file system appears clean, useful for thorough verification. The -n option performs non-interactive checking answering no to all repair prompts, safe for read-only assessment. The -y option automatically answers yes to repair prompts, appropriate when manual intervention is impractical. The -p option performs automatic safe repairs without prompting.
Scheduled automatic checks help maintain file system health. Systems can be configured to check file systems periodically based on mount count or time since last check, though modern journaling file systems require this less frequently than older systems. The tune2fs command configures these automatic check intervals with options like -c for mount count intervals and -i for time-based intervals.
Recovery from file system corruption involves several steps. Boot to single-user mode or from live media to unmount affected file systems. Run e2fsck with appropriate options, typically -f for forced check and -y for automatic repair or manual intervention for critical decisions. Review output for lost+found directory contents where orphaned files are placed. Restore data from backups if corruption is severe or repairs fail.
The lost+found directory serves as a repository for orphaned inodes recovered during file system checks. When e2fsck finds inodes that are allocated but not referenced by directories, it places them in lost+found with numeric names. Administrators can examine these recovered files and potentially restore them to proper locations or delete them if they represent corrupted data.
Modern ext4 file systems with journaling are more resilient than older file systems. The journal records pending changes allowing rapid recovery after improper shutdown without full file system checks. However, hardware failures, kernel bugs, or severe corruption can still necessitate thorough checking and repair.
The xfs_repair utility is specific to XFS file systems, not ext4. While XFS is another advanced journaling file system used in Linux, it requires different tools than ext family file systems.
Since both fsck.ext4 and e2fsck correctly identify the file system checker for ext4, option D indicating both A and B is accurate.
Question 185
A system administrator needs to find all SUID (Set User ID) files on the system. Which find command accomplishes this?
A) find / -perm -4000
B) find / -perm 4000
C) find / -user root -perm 755
D) find / -type s
Answer: A
Explanation:
The find command with “find / -perm -4000” searches for all files with the SUID bit set throughout the entire file system. The SUID permission is a special file mode that allows users to execute a file with the permissions of the file owner, commonly used for programs that need elevated privileges.
The SUID bit is represented by the octal value 4000 in the permission mode. When set on an executable file, the process runs with the effective user ID of the file owner rather than the user who executed it. This mechanism enables regular users to perform operations requiring elevated privileges through carefully designed programs, such as the passwd command which needs to modify protected password files.
The -perm option specifies permission modes for matching files. The minus sign before the mode (-4000) means “at least these permission bits are set” allowing files to have additional permission bits beyond those specified. This is important because SUID files also have regular permission bits (read, write, execute) so the search must find files where the SUID bit is set regardless of other permissions.
Without the minus sign, -perm 4000 would find only files with exactly those permissions set and no others, which would miss most SUID files since they typically have additional permissions like execute. The minus sign ensures the search finds all files with SUID set whether they have permissions like 4755, 4711, or any other combination including the SUID bit.
Security implications make finding SUID files important. SUID programs are potential security risks because they run with elevated privileges, making them attractive targets for attackers. Unauthorized SUID files, especially SUID root files, might indicate system compromise. Regular audits of SUID files help identify unauthorized additions or modifications.
Common legitimate SUID programs include /usr/bin/passwd for changing passwords, /bin/su for switching users, /usr/bin/sudo for executing commands with elevated privileges, and various system utilities requiring root access. These are expected and necessary, but unexpected SUID files warrant investigation.
Additional find options enhance the search. Adding -user root filters to SUID files owned by root, which are the most security-sensitive. The -ls option produces detailed output showing permissions, ownership, size, and path. For example, “find / -perm -4000 -user root -ls 2>/dev/null” finds all SUID root files with detailed information while suppressing permission denied errors.
SGID (Set Group ID) files use a similar mechanism but execute with the group’s privileges instead of the owner’s. The octal value for SGID is 2000, and the command “find / -perm -2000” finds SGID files. Both SUID and SGID can be searched simultaneously with “find / -perm -6000” finding files with either or both bits set.
The sticky bit is another special permission using octal value 1000, commonly set on directories like /tmp to prevent users from deleting files they do not own even if directory permissions would normally allow it. The command “find / -perm -1000 -type d” finds directories with the sticky bit set.
Output can be extensive when searching the entire file system. Redirecting stderr to /dev/null with “2>/dev/null” suppresses permission denied errors from directories the user cannot read. Limiting the search to specific directories like “find /usr /bin /sbin -perm -4000” reduces scope and runtime when full system scans are unnecessary.
Using exact permission 4000 without the minus sign would rarely find files because it requires exact match with no other permission bits set, which is extremely uncommon for executable SUID files.
The option “-user root -perm 755” searches for files owned by root with specific permissions but does not check for the SUID bit, missing the actual requirement.
The -type s option searches for socket files, not SUID files. Socket files are a different file type used for inter-process communication, unrelated to SUID permissions.
Question 186
Which command displays the manual page for the passwd command’s configuration file rather than the command itself?
A) man passwd
B) man 5 passwd
C) man -k passwd
D) info passwd
Answer: B
Explanation:
The command “man 5 passwd” displays the manual page for the passwd configuration file rather than the passwd command. The number 5 specifies the manual section containing file format documentation, distinguishing it from section 1 which contains user command documentation.
Manual pages are organized into numbered sections representing different categories of documentation. Section 1 contains user commands and executable programs. Section 2 documents system calls. Section 3 covers library functions. Section 4 describes special files like devices. Section 5 contains file formats and conventions. Section 6 is for games. Section 7 provides miscellaneous information including protocols and conventions. Section 8 documents system administration commands.
The passwd name appears in multiple sections. Section 1 contains the passwd command for changing user passwords. Section 5 contains the passwd file format describing the structure of /etc/passwd. Without specifying a section, man displays the first match found, typically from section 1 for common commands. Specifying the section number ensures the desired manual page is displayed.
The syntax “man [section] name” accesses specific sections. The section number comes before the name. Alternative syntax “man -S section name” achieves the same result. When multiple pages exist with the same name, viewing all of them uses “man -a name” which displays each sequentially, allowing users to page through all relevant documentation.
Searching manual pages helps locate documentation when the exact name is unknown. The apropos command or “man -k keyword” searches manual page names and descriptions for keywords. For example, “man -k password” finds all manual pages related to passwords. The whatis command or “man -f name” displays brief descriptions of manual pages matching the exact name.
Manual page sections follow consistent structure including NAME providing the name and brief description, SYNOPSIS showing command syntax or function prototypes, DESCRIPTION explaining detailed functionality, OPTIONS listing available flags and parameters, FILES identifying related configuration and data files, SEE ALSO referencing related manual pages, and AUTHOR or BUGS providing additional information.
Navigation within manual pages uses standard less pager controls. Space or Page Down advances one screen. The b key moves backward one screen. Slash followed by text searches forward. The n key repeats the last search. The q key quits the manual page. These controls make exploring lengthy documentation efficient.
The MANPATH environment variable determines where man searches for manual pages. Multiple directories can be specified separated by colons, similar to PATH. The /etc/manpath.config file configures default manual page locations. Local documentation or third-party software might install manual pages in non-standard locations requiring MANPATH adjustments.
Different distributions sometimes vary in manual page content or organization, though core documentation remains consistent. Documentation formats beyond traditional man pages include info pages providing hyperlinked documentation with navigation commands, HTML documentation accessible through web browsers, and package-specific documentation in /usr/share/doc providing README files and examples.
Keeping manual pages current is important. Package updates typically include updated documentation. The mandb command rebuilds the manual page index database used by apropos and whatis, necessary after installing new software or manual pages to ensure searches return complete results.
The command “man passwd” without section number displays the section 1 manual page for the passwd command, not the file format documentation.
The -k option searches for keywords in manual page descriptions but does not specify a particular manual page to display.
The info command accesses GNU info documentation which is a different documentation system than manual pages, though some topics have both man and info documentation.
Question 187
A user needs to create a symbolic link named link.txt pointing to the file original.txt. Which command accomplishes this?
A) ln -s original.txt link.txt
B) ln link.txt original.txt
C) cp -s original.txt link.txt
D) link original.txt link.txt
Answer: A
Explanation:
The command “ln -s original.txt link.txt” creates a symbolic link named link.txt that points to original.txt. The ln command with the -s option is the standard method for creating symbolic links in Linux systems.
Symbolic links, also called soft links or symlinks, are special files that contain a pathname reference to another file or directory. Unlike hard links which point directly to inode data, symbolic links store the path as text and are resolved when accessed. This indirection provides flexibility but creates dependencies on the target’s existence and location.
The syntax for creating symbolic links is “ln -s target linkname” where target is the file or directory being linked to and linkname is the name of the symbolic link being created. The -s option is essential as it specifies symbolic link creation rather than hard link. The target should be specified first, followed by the link name, though this order sometimes confuses users familiar with copy commands.
Absolute versus relative paths in symbolic links have important implications. If the target is specified as an absolute path like “/home/user/original.txt”, the link works from any location. If specified as a relative path like “original.txt”, the link is resolved relative to the link’s location, which can break if the link is moved. Choosing appropriate path types depends on use case and portability requirements.
Symbolic links provide numerous benefits including creating convenient aliases for frequently accessed files in easier-to-reach locations, maintaining compatibility when moving files by leaving links at old locations, organizing files logically without duplicating data, and enabling multiple names or access paths for the same content. These capabilities make symlinks valuable for system organization and administration.
Broken symbolic links occur when targets are deleted, renamed, or moved. The link persists but points to nonexistent targets, causing errors when accessed. The find command can locate broken links with “find / -xtype l” which finds symbolic links where the target does not exist. Cleaning up broken links prevents confusion and potential issues.
Permissions on symbolic links are generally irrelevant because the target file’s permissions control access. When accessing through a symlink, the system follows the link to the target and applies the target’s permissions. However, link ownership can matter for deletion rights in directories with sticky bits.
Directory symbolic links enable organizing file system hierarchies flexibly. A symlink can point to a directory, and accessing it behaves like accessing the target directory. This is useful for compatibility when moving directories, providing convenient mount points, or creating logical structures. However, some commands like rm -r and du must handle directory symlinks carefully to avoid following links unexpectedly.
The ls command shows symbolic links distinctly. Using “ls -l” displays symlinks with special notation showing “link.txt -> original.txt” indicating the link and its target. The first character of permissions shows “l” identifying symbolic links. Colors in ls output often show symlinks distinctly, with broken links in different colors.
Removing symbolic links uses rm or unlink commands targeting the link itself, not the target. The command “rm link.txt” removes the link without affecting original.txt. Care must be taken with directory symlinks because “rm -r link/” might follow the link and delete target contents rather than just removing the link.
The command “ln link.txt original.txt” without -s creates a hard link rather than symbolic link, and the argument order is also reversed from what’s needed.
The cp command copies files and does not have a -s option for creating symbolic links. Copying creates independent duplicates, a fundamentally different operation than linking.
There is no standard “link” command with this syntax for creating symbolic links. The ln command is the standard utility for both hard and symbolic link creation.
Question 188
Which command shows the last 15 commands executed in the current shell session?
A) history 15
B) last -15
C) tail -15 ~/.bash_history
D) show history 15
Answer: A
Explanation:
The history command with argument 15 displays the last 15 commands executed in the current shell session. The history feature maintains a list of previously executed commands, providing convenient recall and re-execution capabilities that enhance productivity and reduce typing.
The bash shell maintains command history in memory during the session and typically writes it to ~/.bash_history when the session ends. The history command accesses this list, displaying commands with sequential numbers. Without arguments, history shows all remembered commands up to the limit defined by HISTSIZE. Specifying a number like “history 15” limits output to the most recent 15 entries.
Command numbers displayed by history enable convenient re-execution. The syntax “!n” where n is a command number executes that specific command. For example, “!523” executes command number 523 from history. The syntax “!!” executes the previous command, equivalent to “!-1”. The syntax “!-n” executes the command n positions back, so “!-3” runs the third-most-recent command.
String-based recall provides additional flexibility. The syntax “!string” executes the most recent command starting with string. For example, “!grep” executes the most recent grep command. The syntax “!?string?” executes the most recent command containing string anywhere, not just at the beginning. These shortcuts save time when repeating similar commands.
History substitution enables modifying previous commands. The syntax “^old^new^” replaces the first occurrence of old with new in the previous command and executes it. For example, if the previous command was “cat file1.txt”, executing “^file1^file2^” runs “cat file2.txt”. More complex substitutions use “:s/old/new/” syntax with command recall.
The HISTSIZE environment variable controls how many commands are retained in memory. The HISTFILESIZE variable determines how many lines are kept in ~/.bash_history. These can be set in shell configuration files like ~/.bashrc. Larger values provide more extensive history but consume more memory and disk space.
History behavior can be customized through variables and settings. HISTCONTROL affects what gets saved, with values like ignoredups preventing consecutive duplicate commands and ignorespace omitting commands starting with spaces. HISTIGNORE specifies patterns for commands to exclude from history. HISTTIMEFORMAT adds timestamps to history entries showing when commands were executed.
Searching history interactively enhances productivity. The Ctrl+R key combination activates reverse incremental search, allowing users to type search terms and see matching commands from history. Pressing Ctrl+R repeatedly cycles through matches. This interactive search is often faster than viewing full history output.
Managing history involves several useful commands. The “history -c” command clears all history from memory. The “history -d offset” deletes a specific entry. The “history -w” writes current history to file immediately rather than waiting for session end. The “history -r” reads history file into current session, useful after modifications.
Security considerations apply to command history. Sensitive information like passwords entered in commands is stored in history, potentially exposing credentials. Using HISTCONTROL=ignorespace and prefixing sensitive commands with a space prevents them from being saved. Alternatively, sensitive operations should avoid including credentials in command lines, instead prompting for them or reading from secure sources.
The last command shows login history and system reboot information, not shell command history. The -15 option would show the last 15 login records, a completely different purpose.
While tailing ~/.bash_history shows previously saved commands, it does not reflect the current session’s complete history since history is written to file primarily at session end. This approach misses commands from the current session.
There is no standard “show history” command in bash. The history command itself is the proper utility for this purpose.
Question 189
A system administrator needs to change the password expiration warning period to 14 days for a user. Which command accomplishes this?
A) chage -W 14 username
B) passwd -w 14 username
C) usermod -W 14 username
D) pwage -w 14 username
Answer: A
Explanation:
The chage command with option -W 14 sets the password expiration warning period to 14 days for the specified user. The chage utility, short for “change age”, manages password aging policies including expiration, warning periods, and account locking parameters.
Password aging policies enhance security by requiring periodic password changes, preventing indefinite use of potentially compromised credentials. The warning period specifically controls how many days before password expiration the user receives warnings at login. A 14-day warning period means users see expiration warnings starting 14 days before their password expires, providing ample time to change passwords before forced expiration.
The chage command manages various password aging parameters. The -W option sets the warning period. The -M option sets maximum password age in days. The -m option sets minimum days between password changes. The -I option sets account inactivity period after password expiration before account locks. The -E option sets absolute account expiration date. These options provide comprehensive password policy control.
Interactive mode provides an alternative to command-line options. Running “chage username” without options starts interactive mode, prompting for each aging parameter and displaying current values. This mode helps administrators unfamiliar with options or who need to review and modify multiple settings simultaneously.
Password aging information is stored in /etc/shadow with specific fields for each parameter. The fields include days since epoch of last password change, minimum password age, maximum password age, warning period, inactivity period, and account expiration date. The chage command modifies these fields, providing a higher-level interface than direct file editing.
Displaying current password aging information uses “chage -l username” which lists all aging parameters in readable format including last password change date, password expiration date, account expiration date, and current settings for minimum, maximum, warning, and inactivity periods. This information helps administrators verify policy application and plan account maintenance.
Default password aging policies can be configured system-wide in /etc/login.defs. Variables like PASS_MAX_DAYS, PASS_MIN_DAYS, and PASS_WARN_AGE set defaults applied to new accounts. Existing accounts retain their current settings unless explicitly modified. The /etc/default/useradd file also contains defaults for new user creation.
Security best practices recommend appropriate password aging policies balanced against usability. Very short maximum ages annoy users and may encourage weak passwords written down for memory. Moderate maximum ages like 90 days balance security and usability. Warning periods should provide sufficient notice, typically 7-14 days, allowing users to change passwords conveniently before expiration.
Account management considerations include planning for service accounts and automated systems. Automated accounts often need non-expiring passwords since no human monitors warnings and changes passwords. Setting maximum age to -1 or 99999 effectively disables expiration for such accounts, though this should be documented and reviewed for security compliance.
Special cases require attention. Setting minimum age prevents users from immediately changing back to old passwords, enforcing password history policies. Inactivity periods lock accounts after passwords expire if not changed, providing additional security for abandoned accounts. Account expiration dates handle temporary accounts or contractors with known end dates.
The passwd command manages passwords themselves but does not have a -w option for setting warning periods. Passwd focuses on setting, locking, and unlocking passwords rather than aging policy configuration.
The usermod command modifies user account properties like groups, home directory, and shell but does not manage password aging parameters. Password aging is the domain of chage.
There is no standard “pwage” command in Linux systems. The chage command is the established utility for password aging management.
Question 190
Which environment variable defines the search path for executable commands in Linux?
A) HOME
B) PATH
C) SHELL
D) USER
Answer: B
Explanation:
The PATH environment variable defines the search path for executable commands in Linux systems. This colon-separated list of directories determines where the shell looks for commands when users execute them without specifying absolute or relative paths.
When a user enters a command name like “ls” or “grep” without a path, the shell searches directories listed in PATH in the order they appear. The first match found is executed. This automatic searching provides convenience, allowing users to run commands by name rather than typing full paths like “/bin/ls” or “/usr/bin/grep”.
The typical PATH includes directories like /usr/local/bin for locally compiled or administrator-installed software, /usr/bin for user commands from distributions, /bin for essential system binaries, /usr/local/sbin and /usr/sbin for system administration commands, and sometimes user-specific directories like ~/bin for personal scripts and programs.
Order matters in PATH because the shell uses the first match found. If two directories contain programs with the same name, the one in the earlier directory is executed. This ordering allows administrators to override system commands with custom versions, or can cause confusion when unexpected versions of programs run.
Viewing the current PATH uses the echo command like “echo $PATH”. The output shows all directories separated by colons, for example “/usr/local/bin:/usr/bin:/bin:/usr/games”. Understanding the current PATH helps troubleshoot command-not-found errors and verify that expected directories are included.
Modifying PATH is common for adding custom directories. The syntax “PATH=PATH:/new/directory”appendsadirectorytotheexistingPATH.Prependinguses”PATH=/new/directory:PATH:/new/directory” appends a directory to the existing PATH. Prepending uses “PATH=/new/directory: PATH:/new/directory”appendsadirectorytotheexistingPATH.Prependinguses”PATH=/new/directory:PATH” which gives the new directory higher priority. These changes affect only the current shell unless made permanent in configuration files.
Permanent PATH modifications belong in shell initialization files. For bash, ~/.bashrc for non-login shells and ~/.bash_profile or ~/.profile for login shells are appropriate locations. System-wide changes can go in /etc/profile or files in /etc/profile.d/. Care must be taken to preserve existing PATH content when modifying these files.
Security implications require attention when managing PATH. Including the current directory (.) in PATH creates security risks because running commands in untrusted directories might execute malicious programs. For example, if “.” is in PATH and an attacker places a malicious “ls” program in a directory, running “ls” executes the malicious version rather than the system command.
Command resolution follows specific precedence. Shell built-in commands execute first regardless of PATH. Aliases defined by users override commands from PATH. Functions defined in the shell come next. Finally, external commands are searched in PATH directories. The type command like “type ls” shows how a command name is resolved, whether as alias, function, built-in, or external program.
The which command locates the executable file that would run for a given command name based on current PATH. For example, “which python” shows the full path to the python executable, like “/usr/bin/python”. This helps verify which version of a program runs when multiple versions are installed.
The whereis command finds binaries, source code, and manual pages for commands, searching standard locations rather than limiting to PATH directories. The locate command searches a database of all files on the system but does not consider PATH in its operation.
Common issues with PATH include commands not being found because their directories are not in PATH, wrong versions of commands running due to PATH ordering, and errors after PATH modifications in configuration files. Troubleshooting often involves checking PATH contents, verifying file locations with which, and temporarily modifying PATH for testing.
The HOME environment variable contains the path to the user’s home directory, not the executable search path. HOME affects where ~ expands and where some programs look for user-specific configuration and data.
The SHELL environment variable contains the path to the user’s login shell, indicating which shell program the system uses but not defining command search paths.
The USER environment variable contains the current username, providing identification information but not affecting command location or execution.
Question 191
A user wants to extract a specific file named document.txt from an archive named backup.tar.gz. Which command accomplishes this?
A) tar -xzf backup.tar.gz document.txt
B) tar -czf backup.tar.gz document.txt
C) untar document.txt backup.tar.gz
D) extract -f backup.tar.gz document.txt
Answer: A
Explanation:
The command “tar -xzf backup.tar.gz document.txt” extracts only the specific file document.txt from the compressed archive backup.tar.gz. This selective extraction capability saves time and disk space when only specific files from large archives are needed.
The options work together to accomplish extraction. The -x option means extract files from an archive. The -z option handles gzip compression, decompressing during extraction. The -f option specifies the filename of the archive to extract from, and must be followed by the archive name. Specifying document.txt as an additional argument limits extraction to that file only.
Path specifications in archives matter for selective extraction. If the archive contains document.txt in a subdirectory like docs/document.txt, the extraction command must specify the full path as stored in the archive: “tar -xzf backup.tar.gz docs/document.txt”. The -t option lists archive contents showing exact paths, helping identify correct specifications.
Multiple files can be extracted by listing them all. The command “tar -xzf backup.tar.gz file1.txt file2.txt file3.txt” extracts three specific files. Wildcards work with –wildcards option, enabling pattern matching like “tar -xzf backup.tar.gz –wildcards ‘*.txt'” to extract all text files.
Extraction preserves file attributes including permissions, ownership, and timestamps when possible. Running as root or with sudo preserves ownership; regular users see files owned by themselves. The extracted files recreate original directory structure relative to the current directory, so document.txt might appear in ./docs/document.txt if that was its archive path.
The -C option changes extraction directory. The command “tar -xzf backup.tar.gz -C /restore/location document.txt” extracts to /restore/location instead of the current directory. This option must appear before filenames to extract, affecting where they are placed.
Verification before extraction helps avoid unexpected results. The -t option lists contents without extracting, showing all files and their paths. Combined with grep, this helps find files: “tar -tzf backup.tar.gz | grep document” searches for files matching document in archive listings.
Error handling includes checking for file existence in the archive. If the specified file is not present or path is incorrect, tar reports an error. Archive corruption or incomplete downloads cause extraction failures. Testing archives before relying on them verifies integrity.
Compression types require appropriate options. While -z handles gzip compression (.tar.gz or .tgz), -j handles bzip2 (.tar.bz2), and -J handles xz compression (.tar.xz). Modern tar versions often auto-detect compression, but explicit options ensure compatibility and clarity.
Complete extraction without filename arguments extracts all archive contents. The command “tar -xzf backup.tar.gz” without specifying files extracts everything. Selective extraction is valuable for large archives where only portions are needed, saving time and disk space.
Security considerations include being cautious about archive sources and extraction locations. Malicious archives might contain files with absolute paths like /etc/passwd or relative paths with .. components attempting to write outside intended directories. Inspecting archives with -t before extraction reveals suspicious paths.
The -c option creates archives rather than extracting them, so “tar -czf” would attempt to create an archive, the opposite of the intended operation.
There is no standard “untar” command in Linux systems. The tar command handles both creation and extraction through different options.
There is no “extract” command with this syntax. The tar utility is the standard tool for working with tar archives.
Question 192
Which command displays disk usage for a directory and all its subdirectories in human-readable format?
A) du -h directory
B) df -h directory
C) ls -lh directory
D) stat -h directory
Answer: A
Explanation:
The du command with -h option displays disk usage for a directory and all its subdirectories in human-readable format. The du utility, short for “disk usage”, calculates actual space consumed by files and directories, providing essential information for storage management and capacity planning.
The -h option converts raw byte or kilobyte counts into human-readable units like megabytes (M), gigabytes (G), or terabytes (T) based on the size magnitude. This formatting makes output much easier to interpret quickly compared to large numbers in bytes or kilobytes. For example, “15G” is more readable than “15728640” kilobytes.
The du command operates recursively by default, examining the specified directory and all subdirectories. It reports usage for each subdirectory and finally provides a total for the entire tree. This comprehensive view helps identify which subdirectories consume the most space, guiding cleanup efforts and storage optimization.
Additional options enhance du functionality. The -s option produces a summary showing only the total for specified directories without listing subdirectories, useful for quick checks. The -c option adds a grand total line when multiple directories are specified. The –max-depth=N option limits recursion depth, showing only N levels of subdirectories to reduce output volume.
Sorting du output reveals largest space consumers. Piping to sort enables various arrangements: “du -h directory | sort -h” sorts by size in human-readable format. The -h option to sort properly handles human-readable sizes like “15G” and “2T”. Combining with head shows top consumers: “du -h directory | sort -hr | head -10” displays the ten largest items.
The du command counts actual disk usage, which may differ from file sizes shown by ls. Files smaller than the file system block size still consume full blocks. Sparse files that contain large regions of zeros may report smaller actual usage than their apparent size. Hard links are counted once even though multiple directory entries reference the same data.
Excluding files or directories from calculations uses the –exclude option. The command “du -h –exclude=’*.tmp’ directory” skips temporary files. Multiple exclusions can be specified with multiple –exclude options. This filtering helps focus on relevant data and avoid skewing results with temporary or cache files.
Performance considerations matter when scanning large directory trees. Du must traverse the entire tree and stat every file, which can be slow for millions of files or slow storage. Running du during low-activity periods minimizes impact. The –time option shows modification times, though it increases overhead.
Common use cases include identifying directories consuming excessive space before cleanup, monitoring growth trends over time by running periodic reports, verifying backup sizes before archiving, investigating quota violations when users exceed limits, and capacity planning by understanding current usage patterns.
Differences from df are important to understand. The du command shows space used by files in specific directories, while df shows file system capacity and utilization. Du answers “how much space do these files use”, while df answers “how much space is available on this file system”. Both provide valuable but different perspectives on storage.
The df command displays file system disk space usage showing total capacity, used space, and available space for mounted file systems. While valuable for overall capacity monitoring, it does not provide per-directory breakdowns like du.
The ls -lh command lists directory contents with human-readable file sizes but shows only direct contents, not subdirectories recursively, and does not sum total usage.
The stat command displays detailed information about individual files including size, permissions, and timestamps, but does not calculate directory usage or work recursively through directory trees.
Question 193
A system administrator needs to kill all processes with the name httpd. Which command accomplishes this?
A) killall httpd
B) kill httpd
C) pkill -f httpd
D) Both A and C
Answer: D
Explanation:
Both killall httpd and pkill -f httpd can terminate all processes with the name httpd, though they work slightly differently. These commands provide convenient ways to signal multiple processes matching specific criteria without manually identifying individual process IDs.
The killall command sends signals to all processes matching a specified name exactly. The syntax “killall httpd” sends SIGTERM (the default signal) to all processes named exactly “httpd”. This command is straightforward for terminating all instances of a known process name, commonly used for stopping multiple instances of daemons or services.
Killall matches process names strictly, comparing against the process name shown in ps output which is typically the command name without arguments. This strict matching ensures only intended processes are affected. Different signals can be sent using options like “killall -9 httpd” for SIGKILL or “killall -HUP httpd” for SIGHUP to reload configurations.
The pkill command provides more flexible pattern matching. The -f option matches against the full command line including arguments, not just the process name. While “pkill httpd” matches process names like killall, “pkill -f httpd” matches any process whose full command line contains “httpd”, potentially matching more processes.
Pattern matching in pkill uses regular expressions by default, providing powerful selection capabilities. The command “pkill ‘^http'” matches processes whose names start with http, including httpd, httpdns, or httputil. This flexibility enables sophisticated process selection but requires care to avoid unintended matches.
Additional pkill options refine selection. The -u option limits to processes owned by specific users like “pkill -u apache httpd”. The -t option targets processes on specific terminals. The -P option matches children of specified parent processes. These filters enable precise process targeting in complex environments.
Safety considerations are critical when killing multiple processes simultaneously. Testing selection criteria using pgrep (which lists matching processes without killing them) helps verify correct matching before executing kills. The command “pgrep -f httpd” shows which processes pkill -f httpd would affect, allowing verification before committing to termination.
Signal selection affects process termination behavior. The default SIGTERM allows graceful shutdown with cleanup. SIGKILL forcefully terminates processes without cleanup, used when SIGTERM fails. SIGHUP reloads configurations for many daemons. Choosing appropriate signals ensures desired outcomes while minimizing disruption.
Differences between killall and pkill implementations exist across systems. Some Unix variants’ killall commands have different behaviors, potentially affecting all user processes or working differently than Linux versions. Understanding system-specific behavior prevents accidents. Pkill is more consistent across platforms following similar patterns.
Service management alternatives often provide better approaches than directly killing processes. The systemctl command “systemctl stop httpd” or service command “service httpd stop” properly stops services using init system mechanisms, ensuring clean shutdown and proper status tracking. These methods should be preferred when available.
Process groups and sessions provide additional management capabilities. Killing a process group terminates all related processes. Session leaders and controlling terminals affect how signals propagate. Understanding these relationships helps manage complex process hierarchies effectively.
Output and feedback vary between commands. Killall reports errors for non-existent processes. Pkill silently succeeds even if no matches are found. Checking return codes or using pgrep for verification confirms expected outcomes.
The kill command requires specific process IDs as arguments and cannot accept process names directly. It must be combined with process ID lookup through ps or pidof for name-based killing.
Since both killall and pkill -f can accomplish the task of killing all httpd processes, option D indicating both A and C is correct.
Question 194
Which file contains configuration for DNS resolver, specifying nameserver IP addresses?
A) /etc/hosts
B) /etc/resolv.conf
C) /etc/hostname
D) /etc/nsswitch.conf
Answer: B
Explanation:
The /etc/resolv.conf file contains configuration for the DNS resolver, specifying nameserver IP addresses that the system uses for domain name resolution. This file is central to network name resolution, determining how the system translates hostnames to IP addresses for network communication.
The resolv.conf file consists of simple keyword-value pairs defining resolver behavior. The nameserver directive specifies DNS server IP addresses, with one address per line. Multiple nameserver entries provide redundancy, with queries attempting servers in order until successful or all fail. Most systems support up to three nameservers effectively.
Common configuration includes nameserver lines like “nameserver 8.8.8.8” for Google’s public DNS or “nameserver 192.168.1.1” for local network DNS servers. The domain directive sets the local domain name appended to unqualified hostnames during lookups. The search directive lists domains for hostname search paths, allowing shorthand names to be expanded.
The search directive affects how short hostnames are resolved. If “search example.com test.com” is configured and a user queries “server”, the resolver tries “server.example.com” then “server.test.com” before trying “server” alone. This convenience allows users to omit domain suffixes for local resources.
Options in resolv.conf tune resolver behavior. The timeout option sets query timeout in seconds. The attempts option specifies how many times to query each nameserver. The rotate option distributes load among nameservers by rotating the order used. These tuning parameters optimize performance and reliability.
Dynamic configuration has become common with modern networking. DHCP clients typically receive DNS server addresses and automatically update resolv.conf. Network Manager and systemd-resolved manage resolv.conf dynamically, sometimes making it a symbolic link to dynamically generated content. Manual modifications may be overwritten, requiring careful management approaches.
Protecting manual configurations from automatic overwrites requires system-specific approaches. Some systems use the chattr +i command to make the file immutable. Others provide hooks or configuration files that inject custom content into generated resolv.conf. Network Manager has per-connection DNS settings that persist through updates.
Testing DNS resolution helps verify resolv.conf configuration. The nslookup command queries DNS servers for specific hostnames. The dig command provides detailed DNS query information. The host command offers simpler output for basic lookups. These tools help troubleshoot resolution problems and confirm correct server usage.
IPv6 nameservers can be specified using IPv6 addresses in nameserver directives. Systems with dual-stack networking might configure both IPv4 and IPv6 DNS servers. The resolver attempts appropriate address families based on network connectivity and application requirements.
Split DNS configurations use different nameservers for different domains, implemented through various mechanisms. VPN connections might inject specific nameservers for corporate domains while maintaining default servers for internet domains. Systemd-resolved provides sophisticated split DNS capabilities through its configuration.
Security considerations include DNS privacy and security. Traditional DNS queries are unencrypted and visible to network observers. DNS over TLS (DoT) and DNS over HTTPS (DoH) encrypt queries for privacy. Some systems support these protocols through resolv.conf extensions or through systemd-resolved configuration.
Common issues include incorrect nameserver addresses causing all lookups to fail, lack of nameservers when resolv.conf is empty or missing, slow resolution from distant or overloaded nameservers, and automatic overwrites of manual configurations. Troubleshooting involves verifying file contents, testing with dig or nslookup, checking network connectivity to nameservers, and understanding how the system manages resolv.conf.
The /etc/hosts file provides static hostname-to-IP mappings bypassing DNS for specified entries, but does not configure DNS nameservers.
The /etc/hostname file contains the system’s hostname but does not configure DNS resolution or nameservers.
The /etc/nsswitch.conf file configures the Name Service Switch, determining the order of lookup methods (files, DNS, LDAP, etc.) but does not specify DNS server addresses.
Question 195
A user wants to display the first 25 lines of a file named log.txt. Which command accomplishes this?
A) head -25 log.txt
B) head -n 25 log.txt
C) tail -25 log.txt
D) Both A and B
Answer: D
Explanation:
Both head -25 log.txt and head -n 25 log.txt display the first 25 lines of log.txt. These syntaxes are equivalent, with modern head implementations accepting both the traditional numeric option and the explicit -n format.
The head command displays the beginning portion of files, defaulting to the first 10 lines when no count is specified. Specifying a line count with either -25 or -n 25 overrides this default. The -n option explicitly names the lines parameter, while the simpler -25 shorthand is widely supported and commonly used.
Multiple files can be processed simultaneously. The command “head -25 file1.txt file2.txt file3.txt” displays the first 25 lines of each file, with headers showing filenames to distinguish output. The -q option suppresses headers for cleaner output when processing multiple files.
Byte-based output provides an alternative to line counting. The -c option specifies bytes instead of lines. For example, “head -c 1024 log.txt” displays the first 1024 bytes regardless of line breaks. This is useful for binary files or when exact byte counts matter more than line counts.
Negative counts supported by GNU head enable showing all but the last N lines. The syntax “head -n -10 log.txt” displays everything except the last 10 lines. This inverted behavior complements tail’s negative count option, providing flexible content selection from both ends of files.
Pipeline usage enhances head’s utility. Commands can pipe output through head to limit results: “ls -l | head -20” shows only the first 20 entries. Combining with other tools creates powerful filters: “cat large.log | head -1000 | grep ERROR” searches only the first 1000 lines for errors.
The head command efficiently handles large files by reading only the required portion. When displaying 25 lines from a gigabyte log file, head stops after reading 25 lines rather than processing the entire file. This efficiency makes head suitable for examining large files quickly.
Common use cases include previewing file contents before processing, extracting headers from data files, sampling log files to check format or recent entries, and limiting output from commands generating extensive results. Head provides quick insight into file contents without overwhelming displays.
Differences from tail are complementary. While head shows the beginning, tail shows the end. Both commands support similar options for line counts, byte counts, and multiple file processing. Together they enable flexible content extraction from any part of files.
Text versus binary files affect head behavior. For text files, head operates on logical lines determined by newline characters. For binary files without meaningful line structure, byte-based output with -c makes more sense. Head handles both but is most commonly used with text files.
Character encoding considerations matter for non-ASCII files. Multi-byte characters in UTF-8 or other encodings are handled correctly by line-based output. Byte-based output might split multi-byte characters, producing invalid output for some encodings.
Standardization across Unix-like systems makes head behavior consistent. POSIX specifies head functionality ensuring portable scripts work across different systems. Minor variations exist in extended options but core functionality remains consistent.
The tail command displays the end of files rather than the beginning. While “tail -25 log.txt” shows the last 25 lines, the question asks for the first 25 lines, making tail inappropriate for this task.
Since both -25 and -n 25 syntaxes correctly display the first 25 lines with the head command, option D indicating both A and B is accurate.