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The reserved space is a portion of the filesystem that is only accessible to the root user or users with sudo privileges. The reserved space serves two purposes: to prevent ordinary users from filling up the filesystem completely and crashing the system, and to reduce fragmentation by leaving some free space. The reserved space is set by default to 5% of the total filesystem size when the filesystem is created with the mkfs.ext4 command. This means that 5% of the blocks are reserved for the root user and are not counted as available space by the df command. The reserved space can be changed or disabled by using the tune2fs command with the -m option. For example, to change the reserved space to 1%, use the following command:

sudo tune2fs -m 1 /dev/sdXY

where /dev/sdXY is the device name of the ext4 filesystem.

To disable the reserved space completely, use 0% instead of 1%. However, this is not recommended, as it may affect the performance and stability of the system, especially if the filesystem is almost full.

The command that lists the dependencies of the RPM package file foo.rpm is rpm -qpR foo.rpm. The rpm command is the low-level tool for managing RPM packages on Linux systems. The -q or --query option is used to query information about installed or uninstalled packages. The -p or --package option is used to specify a package file instead of an installed package name. The -R or --requires option is used to list the capabilities that the package requires. The capabilities are usually the names of other packages, libraries, or files that the package depends on. The rpm command is part of the 101.1 Determine and configure hardware settings topic of the LPI Linux Essentials certification program .

The other options are either invalid or do not perform the desired task. The rpm -dep foo command does not exist, as -dep is not a valid option for rpm. The rpm -ld foo.rpm command does not list the dependencies of the package file, but the files in the package. The rpm -R foo.rpm command does not list the dependencies of the package file, but the capabilities that the package provides. The rpm -pD foo command does not exist, as -pD is not a valid option for rpm. References:

Linux Essentials - Linux Professional Institute Certification Programs

Exam 101 Objectives - Linux Professional Institute

rpm(8) - Linux manual page

RPM Package Manager - Wikipedia

This command will run dbmaint in the background and make it immune to hangup signals, which means it will continue to run even when the user logs out of the system. The nohup command prefixes the command with nohup, which intercepts the SIGHUP signal that is sent to the process when the terminal session ends. The output of the command is redirected to a file called nohup.out by default, unless specified otherwise. The & symbol puts the command in the background, allowing the user to run other commands in the same shell.

The other commands are incorrect for the following reasons:

A, job -b dbmaint: This command is invalid and will not work. There is no job command in Linux, and the -b option is not a valid option for any command. The command that is used to list the background jobs in the current shell session is jobs, and it does not have a -b option either.

B, dbmaint &>/dev/pts/null: This command will run dbmaint in the background, but it will not prevent it from being terminated when the user logs out of the system. The &> symbol redirects both the standard output and standard error of the command to a file or device, in this case /dev/pts/null. However, this device does not exist, and the redirection will fail. Even if the device was /dev/null, which is a special device that discards any output sent to it, the command would still be susceptible to hangup signals and would not survive the logout.

D, bg dbmaint: This command will not run dbmaint in the background, but it will try to resume a stopped background job with the name dbmaint. The bg command is used to move a stopped foreground process to the background and continue its execution. However, if there is no such process with the name dbmaint, the command will fail. Even if there was such a process, it would still be terminated when the user logs out of the system, unless it was disowned or prefixed with nohup.

E, wait dbmaint: This command will not run dbmaint in the background, but it will wait for a background process with the name dbmaint to finish and return its exit status. The wait command is used to pause the execution of the current shell until one or more background processes complete. However, if there is no such process with the name dbmaint, the command will fail. Even if there was such a process, it would still be terminated when the user logs out of the system, unless it was disowned or prefixed with nohup.

This command will display the path to the executable file that would be executed when the command foo is invoked. The syntax of the command is:

which [options] command

The which command is a utility that searches the directories listed in the PATH environment variable for the executable file that matches the given command. The options can modify the behavior of the which command, such as displaying all matches, ignoring aliases, or showing the version. The command is the name of the command to be located.

Therefore, the command which foo will search the PATH directories for the executable file named foo and print its full path on the standard output. If there are multiple matches, the command will print the first one found. If there is no match, the command will print nothing and return an exit status of 1.

The other commands are incorrect for the following reasons:

A, lsattr foo: This command will not display the path to the executable file, but it will display the file attributes of the file named foo in the current directory. The syntax of the command is:

lsattr [options] [file]

The lsattr command is a utility that lists the file attributes on a Linux second extended file system. The options can modify the behavior of the lsattr command, such as displaying the output in long format, recursing into subdirectories, or suppressing errors. The file is the name of the file whose attributes are to be listed. If no file is given, the command will list the attributes of all files in the current directory.

Therefore, the command lsattr foo will list the file attributes of the file named foo in the current directory, if it exists. If it does not exist, the command will report an error and return an exit status of 1.

B, apropos foo: This command will not display the path to the executable file, but it will display a list of manual page names and descriptions that contain the keyword foo. The syntax of the command is:

apropos [options] keyword

The apropos command is a utility that searches the whatis database for the keyword and prints the manual page names and descriptions that match. The whatis database is a set of files containing short descriptions of system commands and programs. The options can modify the behavior of the apropos command, such as using regular expressions, ignoring case, or displaying the section numbers. The keyword is the word to be searched in the whatis database.

Therefore, the command apropos foo will search the whatis database for the word foo and print the manual page names and descriptions that contain it. If there are no matches, the command will print nothing and return an exit status of 1.

C, locate foo: This command will not display the path to the executable file, but it will display a list of file names that contain the string foo. The syntax of the command is:

locate [options] pattern

The locate command is a utility that searches a database of file names and prints the file names that match the given pattern. The database is updated periodically by the updatedb command and may not reflect the current state of the file system. The options can modify the behavior of the locate command, such as using regular expressions, ignoring case, or limiting the number of results. The pattern is the string to be matched in the file names.

Therefore, the command locate foo will search the database of file names and print the file names that contain the string foo. If there are no matches, the command will print nothing and return an exit status of 1.

D, whatis foo: This command will not display the path to the executable file, but it will display a short description of the command or program named foo. The syntax of the command is:

whatis [options] name

The whatis command is a utility that searches the whatis database for the name and prints the manual page name and description that match. The whatis database is a set of files containing short descriptions of system commands and programs. The options can modify the behavior of the whatis command, such as displaying the section numbers, using wildcards, or searching in a specific section. The name is the name of the command or program to be described.

Therefore, the command whatis foo will search the whatis database for the name foo and print the manual page name and description that match. If there are no matches, the command will print nothing and return an exit status of 1.

The command that prints a list of usernames (first column) and their primary group (fourth column) from the /etc/passwd file is cut -d : -f 1,4 /etc/passwd. The cut command is used to extract selected fields or characters from each line of a file or standard input. The -d or --delimiter option specifies the character that separates the fields, which in this case is a colon (:). The -f or --fields option specifies the fields to be printed, which in this case are the first and the fourth fields. The /etc/passwd file is a text file that contains information about the user accounts on the system, such as the username, password, user ID, group ID, home directory, and login shell. The first column of the file is the username and the fourth column is the group ID, which corresponds to the primary group of the user. The cut command will print these two columns separated by a colon for each line of the file. For example, running cut -d : -f 1,4 /etc/passwd will produce an output like this:

root:0 daemon:1 bin:1 sys:3 sync:4 games:5 man:12 lp:7 mail:8 news:9 uucp:10 proxy:13 www-data:33 backup:34 list:38 irc:39 gnats:41 nobody:65534 systemd-network:100 systemd-resolve:101 syslog:102 messagebus:103 _apt:104 lxd:105 uuidd:106 dnsmasq:107 sshd:108 pollinate:109 vboxadd:999 ubuntu:1000

The other commands are either invalid or do not perform the desired task. The fmt command is used to reformat paragraphs of text, but it does not have a -f option. The sort command is used to sort lines of text, but it does not have a -t option. The paste command is used to merge lines of files, but it does not have a -f option. The split command is used to split a file into pieces, but it does not have a -c option. References:

Linux Essentials - Linux Professional Institute Certification Programs

Exam 101 Objectives - Linux Professional Institute

cut(1) - Linux manual page

/etc/passwd file - Linux.com

The command that uninstalls a package but keeps its configuration files in case the package is re-installed is dpkg -r pkgname. The dpkg command is the low-level tool for installing, building, removing, and managing Debian packages. The -r or --remove option removes an installed package from the system, but it does not delete the configuration files and other data that belong to the package. This way, if the package is re-installed later, the previous settings are preserved. The dpkg command is part of the 101.1 Determine and configure hardware settings topic of the LPI Linux Essentials certification program12.

The other options are either invalid or do not perform the desired task. The dpkg -s pkgname command shows the status of an installed package, but it does not uninstall it. The dpkg -L pkgname command lists the files that belong to an installed package, but it does not uninstall it. The dpkg -P pkgname command purges an installed or removed package, which means it deletes the configuration files and other data that belong to the package. The dpkg -v pkgname command shows the version of an installed package, but it does not uninstall it.