Flowcontrol and Redirects

Question 1

What are environmental

variables?

Answer 1

Each process has a defined environment in which it is executed. This environment contains certain variables that affect a process’ behaviour. Every variable contains certain values, which can be modified before calling an application.

Question 2

What does the PATH environmental variable contain?

Answer 2

Contains a list of search paths for programs

Question 3

What does the HOME environmental variable contain?

Answer 3

Contains the path to the user’s home directory

Question 4

What does the USER environmental variable contain?

Answer 4

Contains the current user name

Question 5

How can the content of a variable be listed?

Answer 5

Using a single command, the content of a variable can be listed if its name is known. If the name is unknown, all defined variables can be listed. For printing the variable’s content, the name must be prefixed with a $ sign.

Question 6

What is a File Descriptor?

Answer 6

A File Descriptor is a low-level reference to a file, used by the kernel to access the file. Usually, it is a simple integer.

Question 7

Why can you refer to the three standard streams with a file descriptor?

Answer 7

On Unix (and Linux), everything is a file or represented as a file. The same is true for the three standard streams.

Question 8

What are the three standard streams?

Answer 8

Standard Input (stdin) = 0, Standard Output (stdout) = 1 as well as Standard Error (stderr) =2. Each program has at least those three file descriptors, represented by the numbers

These three standard streams allow for basic input/output operations of a program. In Java, you can access these streams as System.in, System.out and System.err.

Question 9

What is Standard Input?

Answer 9

Standard Input is where the program receives its inputs from. In the case of a command line application, this can be the keyboard.

Question 10

What is Standard Output?

Answer 10

Standard Output, on the other hand, is where the program prints its output to. Usually, this is the display in the case of a command line application.

Question 11

What is Standard Error?

Answer 11

Standard Error is where all error messages of a program go to. This can be either configured as a log file or the screen, or both.

Question 12

What for do you need Redirectors?

Answer 12

As it is not always desirable or practical to read from the keyboard when you want your inputs defined in a file, these standard streams can be redirected to other sources/destinations. For instance a log file, the printer, another program or the nirvana (/dev/null) if no output is desired.

Question 13

How can you redirect a file?

Answer 13

< file Redirect STDIN to read from file
> file Redirect STDOUT to write to file
» file Redirect STOUT to append to file

Question 14

What does the command “2> file” do?

Answer 14

Redirect STDERR to write to file

Question 15

What does the command “2» file” do?

Answer 15

Redirect STDERR to append to file

Question 16

What does the command “2>&1” do?

Answer 16

Redirect STDERR to STDOUT

Question 17

What does the command “> file 2>&1” do?

Answer 17

Redirect STDOUT to file and STDERR to STDOUT (=file)

Question 18

What is included by the saying “on Unix/Linux, everything is treated as a file”?

Answer 18

This includes all devices on your sytem, like harddisks, sound cards, scanners, printers or even the USB port.

Question 19

What is the special file “/dev/null” needed for?

Answer 19

Is necessary for sending output to the nirvana

Question 20

What is the special file “/dev/zero” needed for?

Answer 20

Is a special file that always reports NULL

Question 21

What is the special file “/dev/random” needed for?

Answer 21

Reports random characters

Question 22

What is the special file “/dev/urandom” needed for?

Answer 22

Reports random characters, but is less secure than /dev/random

Question 23

What does “cat” do on Linux?

Answer 23

The name derives from “catenate” wich means “concatenate” (appending one thing to another thing).
“cat” is used to read from one or more files and output their content to standard output. Obviously, the output can be redirected to files. Sometimes, the usage of this command is useless if the destination command already supports file names.

Question 24

What can the | Pipe character do?

Answer 24

The Pipe character “|” combines stdout and stdin. It accepts the standard output of one command and passes it to the standard input of another command, specified after the pipe. Longer pipes are also possible by adding another “|” and another program. Think of it as a stream of fluid (generated by the first program on its standard output) that passes through all specified commands (which eventually modify the stream) and finally end up on the standard output. The pipe character “|” holds them all together.

Question 25

How and why can you combine cat and pipe?

Answer 25

“cat” is used to read the content of a very long file that can’t fit to the screen. The content is then piped to “less”, which displays a nice reading environment.

Question 26

What does grep stand for?

Answer 26

Grep is an acronym meaning Globally search for regular expression and print

Question 27

What is grep used for?

Answer 27

It is used to search for a text or a special pattern in either standard input or a file.

Question 28

What does the tail command do?

Answer 28

The “tail”-command prints, as the name suggests, the last few lines of a file or standard input to its standard output.

Question 29

What does the head command do?

Answer 29

Again, the name of the command is nearly self-explanatory. Contrary to the “tail”-command, “head” outputs the first few lines of a file or standard intput.

Question 30

What does sed stand for?

Answer 30

Streaming editor

Question 31

How does sed work?

Answer 31

It operates on streams, which can bei either standard input/standard output or files. Its main purpose is to search and replace in files, although it could do about everything a graphical editor can perform.

“sed” can take regular expressions for its operations, thus enabling very powerful search/replace operations.

Question 32

What are Regular Expressions?

Answer 32

A language for defining patterns for matching operations. RegEx, as they are commonly abbreviated, are mainly used for search/replace operations. The language is very powerful, but longer expressions can become very complex.

Question 33

What matches the following RegEx:

.at

Answer 33

. Matches any single character, exactly one occurrence

e.g. matches “cat”, “hat”, “bat”, …

Question 34

What matches the following RegEx:

[hc]at

Answer 34

[ ] Matches a single character contained in the brackets

e.g. matches only “cat” and “hat”

Question 35

What matches the following RegEx:

[^b]at

Answer 35

[^] Matches a single character NOT contained in the brackets

e.g. matches anything from .at except “bat”

Question 36

What matches the following RegEx:

^[hc]at

Answer 36

^ Matches the start of a new line

e.g. matches “cat” and “hat” only at the beginning of a new line

Question 37

What matches the following RegEx:

[hc]at$

Answer 37

$ Matches the end of a line

e.g. matches “cat” and “hat” only at the end of a line

Question 38

What matches the following RegEx:

s.*

Answer 38

• Matches the preceding character zero or more times

e. g. matches any string starting with “s”, e.g. “saw” or “seed”