RedHat RH033 - Red Hat Linux Essentials

Using the -f option causes tail to continue to display the file in real time, showing added lines to the end of the file as they occur.

What is the tail -f command?

The tail command is used to display the last few lines of a text file or piped data. It has a special command line option –f (follow) that allows a file to be monitored. Instead of displaying the last few lines and exiting, tail displays the lines and then monitors the file. As new lines are added to the file by another process, tail updates the display.

This is particularly useful for monitoring log files. The following command will display the last 10 lines of messages and append new lines to the display as new lines are added to messages: tail -f /var/adm/messages

Answer option C is incorrect. The cat command simply shows the content of the file when it was last opened, cat does not continue to show updates of the file.

Answer option D is incorrect. The tail -n 3 /var/adm/messages command is used to show most recent entries in log file but it will not continue to show updates of the log file.

Answer option B is incorrect. The tail command will show the last few lines of the log file, but it will not continue to show updates of the log file.

All these information are stored in the /etc/passwd file.

What is the /etc/passwd file?

In Linux operating systems, the /etc/passwd file lists information about each of the users that may login to the system. The file is so named because originally it also contained the data used to verify passwords. However, on

modern Unix systems, the security-sensitive password information is instead often stored in a different file using shadow passwords.

The /etc/passwd file typically has world-readable permissions, though it may only be edited by the

superuser or by using a few special purpose commands.

The first field is the user name. Each record in the file must have a unique user name field.

The second field stores information used to confirm a user's password; however, in most modern uses, this field is usually set to "x" (or some other indicator) with the actual password information being stored in a separate shadow password file.

The third field is the user identifier, the number that the operating system uses for internal purposes. It does not have to be unique.

The fourth field is the group identifier. This number identifies the primary group of the user; all files that are created by this user will initially belong to this group.

Reserved gecos ID (Typically, this is a set of comma-separated values including the user's full name

and contact details).

The sixth field is the path to the user's home directory.

The seventh field is the shell program that is started every time the user logs into the system.

Answer option C is incorrect. The shadow password is stored in the /etc/shadow file.

What is /etc/shadow file?

The /etc/shadow file stores the account information in the following format:

The usermod -d -m command is used to change the home directory of a user as well as move all of its contents to the new home directory.

pgrep looks through the currently running processes and lists the process IDs that match the selection criteria to stdout. All the criteria have to match. pkill is a tool that looks up or signals processes based on name and other attributes. Pkill will send the specified signal (by default SIGTERM) to each process instead of listing them on stdout.

Answer option A is incorrect. The top command is used to produce a frequently-updated list of processes. By default, the processes are ordered by percentage of CPU usage, with only the "top" CPU consumers shown. This command shows how much processing power and memory are being used, as well as other information about the running processes. The following figure demonstrates the output of the top command:

Answer option C is incorrect. The sort command is used to print the lines of its input or concatenation of all files listed in its argument list in sorted order. Sorting is done based on one or more sort keys extracted from each line of input. By default, the entire input is taken as the sort key. The blank space is taken used as the default field separator in the output of the sort command.

The -r flag, however, reverses the sort order. For example, the ls -s | sort -nr command will sort the directories of the root directory in reverse order of size.

The cp -u command copies only when the source file is newer than the destination file or when the destination file is missing.

Answer option C is incorrect. The cp -p command copies the files from source to destination with preserving file attributes, such as timestamps, mode, and ownership.

Answer option B is incorrect. The cp -l command links files from source to destination instead of copying.

Answer option D is incorrect. The cp -s command makes symbolic links instead of copying.

The evince utility is used to view PDF files on the Linux operating system.

What is the evince command?

The evince command is a document viewer capable of displaying multiple and single page document formats such as PDF and Postscript.

Syntax:

evince [Options] [Filename]

Answer options B and D are incorrect. These commands are used to convert a text to PostScript and send it to a file or the print queue.

What is PostScript?

PostScript is a language developed by Adobe Systems for printing documents on laser printers. It can also be adapted to produce images on other types of devices. It is the standard for desktop publishing. Every PostScript printer contains a built-in interpreter that executes PostScript instructions. Level 1, Level 2, and PostScript 3 are three basic versions of PostScript language.

Answer option A is incorrect. The ps2pdf utility is used to create PDF files.

The date command with the %M specifier prints the current time (Minutes). Since the output is redirected towards the date.txt file, the current time (Minutes) will be printed in the date.txt file.

The concatenate (cat) command is used to display or print the contents of a file.

Syntax:

cat filename

For example, the following command will display the contents of the /var/log/dmesg file:

cat /var/log/dmesg

Note: The more command is used in conjunction with the cat command to prevent scrolling of the screen while displaying the contents of a file.

In order to accomplish the task, you should create a recurring CRON job to delete the temporary files on the server.

You can view all running processes with the help of the ps -aux command. Hence, any of the ps –aux commands (i.e., ps -aux | grep cc, ps -aux

| more, or ps -aux) can be used to check whether the cc process is running on the computer.

Answer option B is incorrect. The tail command is used to display the last few lines of a text file or piped data. It has a special command line option -f (follow) that allows a file to be monitored. Instead of displaying the last few lines and exiting, tail displays the lines and then monitors the file. As new lines are added to the file by another process, tail updates the display. This is particularly useful for monitoring log files. The following command will display the last 10 lines of messages and append new lines to the display as new lines are added to messages: tail -f /var/adm/messages