LPIC Level 1 101-500 LPI Study Notes

 The environment variable LD_LIBRARY_PATH is used to tell the dynamic linker to look in a specific directory for some of a program’s shared libraries. It is a colon-separated list of directories that are searched by the dynamic linker when looking for a shared library to load1. The directories are searched in the order they are mentioned in. For example, if we have a program that depends on a shared library libfoo.so that is located in /home/user/build/lib, we can run the program with:

LD_LIBRARY_PATH=/home/user/build/lib ./program

This will instruct the dynamic linker to search for libfoo.so in /home/user/build/lib before the default directories. The environment variable LD_LIBRARY_PATH can also be appended to an existing value with the += operator, for example:

LD_LIBRARY_PATH+=:/home/user/build/lib

This will add /home/user/build/lib to the end of the LD_LIBRARY_PATH list. The other options are not valid environment variables for the dynamic linker. LD_LOAD_PATH, LD_LIB_PATH, and LD_SHARE_PATH are not recognized by the dynamic linker. LD_RUN_PATH is a linker option that can be used to embed a librarysearch path in the executable at link time, but it is not an environment variable that can be set or modified at run time2. References:

• LPI Exam 101 Detailed Objectives, section 1.101.3
• ld-linux(8) — Linux manual page
• Setting the dynamic linker with an environment variable - narkive

The tr command is used to translate or delete characters from a given input. The -d option specifies that the characters in the second argument should be deleted from the input. In this case, the second argument is aieou, which are the vowels in the English alphabet. Therefore, the tr command will delete any vowel from the input, which is Hello World. The output will be the input without any vowels, which is Hll Wrld. References:

• [LPI Exam 101 Detailed Objectives], Topic 103: GNU and Unix Commands, Objective 103.7: Perform basic file management, Weight: 4, Key Knowledge Areas: Use of tr to perform basic text transformations.
• [LPI Linux Essentials Certification All-in-One Exam Guide], Chapter 5: Working with Files, Page 165, Using the tr Command.

 The output of the given command line is foo bar. The command line consists of three commands that are connected by pipes (|). A pipe is a symbol that redirects the standard output of one command to the standard input of another command. The echo command prints its argument to the standard output, which is foo bar in this case. The tee command reads from the standard input and writes to both the standard output and a file. The file name is given as an argument, which is bar in this case. The cat command reads from the standard input and writes to the standard output. Therefore, the command line does the following:

• The echo command prints foo bar to the standard output, which is redirected to the standard input of the tee command by the first pipe.
• The tee command reads foo bar from the standard input and writes it to both the standard output and the file bar. The standard output of the tee command is redirected to the standard input of the cat command by the second pipe.
• The cat command reads foo bar from the standard input and writes it to the standard output, which is displayed on the terminal.

Hence, the output of the command line is foo bar. The other options are either incorrect or not applicable. The cat, tee bar and bar are not printed by any of the commands. The foo is only part of the output, not the whole output. References:

• LPIC-1 Exam 101 Objectives, Topic 103: GNU and Unix Commands, 103.4 Use streams, pipes and redirects
• LPIC-1 Linux Administrator 101-500 Exam FAQ, LPIC-1 Exam 101 Objectives, GNU and Unix Commands (Total Weight: 25)

When starting a program with the nice command without any additional parameters, the nice level is set to 10 for the resulting process. This means that the process will have a lower priority than the default value of 0, and will be more willing to yield CPU time to other processes. The nice command can also take an optional argument -n followed by a number, which specifies the increment or decrement of the nice value from the default value of 0. For example, the command:

nice -n 5 /usr/bin/prog

will start the /usr/bin/prog process with a nice value of 5, which means a lower priority than the default. Similarly, the command:

nice -n -5 /usr/bin/prog

will start the /usr/bin/prog process with a nice value of -5, which means a higher priority than the default. If the -n argument is omitted, the nice command will assume a default increment of 10. For example, the command:

nice /usr/bin/prog

will start the /usr/bin/prog process with a nice value of 10, which means a very low priority. Note that only the root user can start a process with a negative nice value, as this requires special privileges.

References:

• [LPI Exam 101 Detailed Objectives], Topic 103: GNU and Unix Commands, Objective 103.6: Modify process execution priorities, Weight: 2, Key Knowledge Areas: Know the default priority of a job that is created.
• How to Set Process Priorities With nice and renice on Linux, Topic: The nice Command.