Chapter 3

Question 1

What are the three purposes of operating systems?

Answer 1

Provide reliable and efficient concurrent execution of multiple
processes.

Provide mechanisms for inter-process communication

Simplify application development

Question 2

Define a process.

Answer 2

Instance of running program that includes all system resources used for that program. Has a unique PID. Processes only interact with each other through kernel (ecall instructions).

Question 3

How are processes created using fork?

Answer 3

fork() system call creates a copy of the calling process and assigns it a new PID. Both the parent and the child processes continue from the same fork() call, but in child fork() returns 0, in parent it does not.

Question 4

What does the wait system call do?

Answer 4

wait(int *status)
Puts calling process to sleep until one of its children processes terminates with exit(status)

Question 5

What does the exec command do?

Answer 5

exec(prog, arguments[]) - replaces the running program in the calling process with the program in the file prog. first argument is name of file, subsequent arguments are command line arguments.

Question 6

Syntax for opening, reading, writing into a file.

Answer 6

fd = open(“filename”, flags, permissions)
write(fd, message, number of bytes);
read(fd, data, num);
close(fd)

Question 7

What are the flags for the open command?

Answer 7

O_RDONLY (read only)
O_WRONLY (write only)
O_RDWR (read and write)
O_CREATE (whether to create if not there)

Question 8

Pipe syntax.

Answer 8

int p[2];
pipe(p);
read from p[0]
write to p[1]

Question 9

Syntax of simple commands in bash.

Answer 9

command arg1 arg2 … argN

Question 10

Distinguish between operands and options.

Answer 10

Operands are object arguments
Options modify default behavior of the command.

Question 11

What do \, ‘ and “ do?

Answer 11

\ removes special meaning of following symbol.
‘ removes special meaning of all symbols surrounded by them.
“ removes special meaning of all symbols surrounded by them except $, \ and ``.

Question 12

How are non-built in commands run?

Answer 12

If the command specified by the user is not a built-in command, the shell tries to find an executable file in the file system with the name matching the command and runs it as a child process using exec().

Question 13

What does pwd do?

Answer 13

Print the full name of the current working directory.

Question 14

How does one run an executable file from the current working directory?

Answer 14

./exeName

Question 15

What do the nano and cat fileName commands do?

Answer 15

nano -> starts a text editor
cat fileName -> displays the contents of the file with the name file.

Question 16

Inside the nano text editor how do you save changes.

Answer 16

Ctrl - O to write out data
Ctrl - X to exit

Question 17

How do you copy, move and delete files in UNIX?

Answer 17

cp foo bar -> copy the contents of file foo into file bar (create if doesn’t exist)
mv foo bar -> move the file foo into bar
rm foo delete file foo

ln foo bar (create a hard link bar to the file foo)

Question 18

How do u make and remove directories?

Answer 18

mkdir dirname = create directory with the name dirname
rmdir dirname = delete directory with the name dirname

Question 19

To instruct shell that process should take input from file, write output to file and output error messages to file, syntax.

Answer 19

command arg1 <inputFile>outputFile 2>errorFile (arg2 ... argN)</inputFile>

Question 20

When we want to disregard output data what can we do?

Answer 20

Redirect output stream to /dev/null

Question 21

What is syntax to send output and errors to same file? What does&raquo_space; do?

Answer 21

command >outputFile 2>&1

Using&raquo_space; means it appends instead of overwriting.

Question 22

How can a string be passed as a command line argument?

Answer 22

«<string

Question 23

Use bc to calculate 2^128.

Answer 23

bc «< “2^128”

Question 24

I want to pass as input everything in a file until a particular word.

Answer 24

«word
/
/
contents we want to append
/
/
word

Question 25

What does find and wc commands do? Give an example using both.

Answer 25

find generates list of all C files in the current directory on separate lines. wc (prints number of lines, words or bytes depending on options) find . -name "*.c" | wc -l . (signifies current directory and all sub-directories) -name (is what find is looking for and "*.c" the ending name. -l tells wc to count the number of newline characters. | pipes the output on the left into the right.

Question 26

What does tr set1 [set2] do? How can tr access all whitespace, upper or lowercase letters?

Answer 26

Replaces character listed in set1 with corresponding character in set2. "[:blank:]" "[:upper:]" "[:lower:]"

Question 27

What does uniq file 1 file2 do? What does sort -u file do? What does cut -b a,b,c do?

Answer 27

uniq discards all but one of successive identical lines from standard input and writes to standard output. sort concatenation of all files. -u removes duplicate lines. cut -b takes bytes a,b,c where a,b and/or c could be ranges and drops all other bytes.

Question 28

What do commands head and tail do?

Answer 28

Print first / last 10 lines from file respectively.

Question 29

What do more, less, tee do?

Answer 29

More and less ask user if they want to keep reading file after certain length. tee outputs to SO and file.

Question 30

How are shell variables declared in bash? What does set do?

Answer 30

VariableName=VariableValue Displays all shell variables

Question 31

How can the result of a compound arithmetic expansion be computed (e.g. display 1 + 3)?

Answer 31

echo $((1+3))

Question 32

How does brace expansion work?

Answer 32

Arguments that contain optional parts -> my_file.{c, h} -> replaced with all possible combinations resulting from substituting optional parts into the argument.

Question 33

How can a shell script be run?

Answer 33

chmod +x my_script //marks it as executable ./my_script to run

Question 34

What does every bash script begin with?

Answer 34

#!/bin/bash

Question 35

How is a for loop implemented in bash?

Answer 35

for var [in words]; do commands; done

Question 36

Syntax for reading into a variable in bash?

Answer 36

read varName

Question 37

Give basic case syntax for bash.

Answer 37

case var in option1 | option2) commands;; option3) commands;; *) commands;; esac

Question 38

How can the exit status be accessed?

Answer 38

$? command

Question 39

Give if/while syntax in bash.

Answer 39

if [ "$A" = "1" ]; then echo "A is 1" else echo "A is not 1" fi while ["$A" = "1"]; do commands; done

Question 40

What is copy-on-write?

Answer 40

Memory optimization technique that minimizes the time spent copying memory during a fork() and exec() sequence. Child and parent process share same memory pages until one of them attempts to modify that memory then copy is made.

Question 41

What is transaction logging?

Answer 41

Logs all changes in a system where data consistency and recovery are critical such as databases. 1. A command either runs fully or not at all 2. If it runs it is permanent 3. Changes made recorded before change actually implemented.

Question 42

What is lazy memory allocation?

Answer 42

Memory allocated only when used (i.e. when variable accessed).