Week 5: Files, Directories, and Pointers

Question 1

What is a file?

Answer 1

A container for data which is persistent and accessible by name

Question 2

What are UNIX files?

Answer 2

Unix file types: regular, directory, device, link and
etc.

Unix files are identified by a name in a directory

Question 3

What is a “Regular” file?

Answer 3

essentially a sequence of bytes

can access these bytes in order

Question 4

What is a “Directory” file?

Answer 4

a file containing name and pointer (inode number)

pairs for files and subdirectories in the directory

Question 5

What is a “Device” file?

Answer 5

access a device (like a soundcard, mouse,

terminal, or …) like it is a file

Question 6

What are “Links”

Answer 6

hard link: create another name for a file

soft link: a pointer to another file

Question 7

What option can be used to get file information?

Answer 7

-F

Question 8

What does the UNIX file system represent? What represents the root?

Answer 8

An upside-down tree

/ represents root

Question 9

What is the difference between an absolute pathname and a relative pathname?

Answer 9

Absolute: to identify where a file is, concatenate the directories
together, can be anywhere in the UNIX file system

Relative: pathnames relative to the current working
directory

Question 10

What pathways signify home?

Answer 10

$HOME

/user/home

~

or simply cd

Question 11

What do the following represent in file locations?

/
~
.
..

Answer 11

/ means root
~ means home
. means current directory
.. means parent directory

Question 12

What does mkdir dirname do?

Answer 12

Makes a new directory in the current directory with the name “dirname”

Question 13

What does rmdir do? When does it work? Not work?

Answer 13

Removes a directory, only works if the directory exists and is empty

Question 14

What does cp text1 text2 do if:

text1 is a file

text2 is a directory?

Answer 14

Copies text1 to directory text2

Question 15

What does cp text1 text2 do if:

text1 is a directory

text2 is a directory?

Answer 15

Nothing, you need to add the
-r option to make it recursive:

cp -r text1 text2

Question 16

What does cp -r text1 text2 do if:

text1 is a directory

text2 is a directory?

Answer 16

Copies directory text1 and all of its contents to directory text2

Question 17

What is the command to create a link? Format?

Answer 17

ln readme.txt unix_is_easy

Creates a link to readme.txt called unix_is_easy

Question 18

What is a hard link?

Answer 18

An indirect pointer to another file or directory

Question 19

What command searches through directories for a file? Format?

Answer 19

find

find PATH EXPRESSION

e.g. Find all files and directories named README
starting from the current directory:
find . –name “README”

Note: the dot before -name

Question 20

What does ls -a do?

Answer 20

list all files including those

beginning with a . (dot)

Question 21

What does head do?

Answer 21

head displays the

top lines of a file

Question 22

What command can you use within a “man” veiwer to go to the next page?

Answer 22

Spacebar

Question 23

What command can you use within a “man” veiwer to quit?

Answer 23

q

Question 24

What characters are valid as file names?

Answer 24

Almost any character is valid in a file name, the exception is the / (slash) character and null character

Question 25

Are UNIX file names case-sensitive?

Answer 25

Yes

Question 26

gcc looks for what extensions by default?

Answer 26

.c for C program files .h for header files .o for an object file

Question 27

What does the file command do? file filename

Answer 27

Print the type of the file

Question 28

How do you delete a file with the name -i?

Answer 28

rm -- -i -- means no options

Question 29

For wildcarding, what does * mean (asterisk)

Answer 29

matches a sequence of zero or more characters

Question 30

For wildcarding, what does ? mean

Answer 30

Matches any single character

Question 31

For wildcarding, what does [...] mean

Answer 31

Matches any of the one characters between the braces

Question 32

What do double quotes do in shell?

Answer 32

Stops interpretation of some | shell special characters (whitespace mostly, *, ~)

Question 33

What do single quotes do in shell?

Answer 33

Stops interpretation of even more specials like $ ex: $HOME

Question 34

What does Ctrl-V do in shell?

Answer 34

Quotes the next character, even if it is a control | character

Question 35

What are the three levels of file permission?

Answer 35

user = the owner of the file group users = a group of people set by the user others = everyone else

Question 36

What command shows file permission?

Answer 36

ls -l

Question 37

What is this? drw-r-xr--

Answer 37

First character (d here) indicates file type The rest is permission in the order of user, group, others In the order of rwx (read write execute)

Question 38

What is the syntax (using numbers) of chmod and what does it do?

Answer 38

Sets file permissions chmod DDD file [file ...] chmod 644 file Where the three numbers (up to 7) indicate binary numbers

Question 39

What is the symbolic syntax of chmod?

Answer 39

chmod [ugoa][+-=][rwx] file [...] [ugoa] indicates who [+-=] indicates add, remove, or exactly equal to [rwx] indicates what permission(s)

Question 40

What does the -R flag do for chmod?

Answer 40

It recursively changes the mode for each chmod operand that is a directory.

Question 41

What is umask? How does it work?

Answer 41

Sets the default permissions for any file you will create Format is backwards to the chmod command – tells you which permissions will NOT be given umask 077 means don't let anyone but the User do anything with my files by default

Question 42

Pointers must have a

Answer 42

Type

Question 43

What does an * do for pointers?

Answer 43

Declare a pointer AND what the address points to

Question 44

What does an & do for pointers?

Answer 44

Return the address of the variable

Question 45

All pointer variables in a 16-bit architecture are ... bytes

Answer 45

2 bytes

Question 46

All pointer variables in a 32-bit architecture are ... bytes

Answer 46

4 bytes

Question 47

Pointer variables contain

Answer 47

The address in memory of another variable

Question 48

All pointer variables in a 64-bit architecture are ... bytes

Answer 48

8 bytes

Question 49

What does int j = *&i; do?

Answer 49

/* same as j = i; */

Question 50

What does the following do? char ca[3],*cap; cap = &(ca[1]); *cap = 7;

Answer 50

Creates a character array with 3 elements and a character pointer called cap The pointer cap is assigned the address of the SECOND element in the array Go to the address location and make it 7

Question 51

In pointer arithmetic, what is important

Answer 51

The type of the variable

Question 52

How many bytes will the following pointer arithmetic move if the array contains integers? ``` cp = &(ca[0]); *(cp+2) = 8; ```

Answer 52

8 bytes (2 x 4 bytes)

Question 53

How many bytes will the following pointer arithmetic move if the array contains characters? ``` cp = &(ca[0]); *(cp+2) = 8; ```

Answer 53

2 bytes (2 x 1 bytes)

Question 54

How many bytes will the following pointer arithmetic move if the array contains double floats? ``` cp = &(ca[0]); *(cp+2) = 8; ```

Answer 54

16 bytes (2 x 8 bytes)

Question 55

What is a void pointer? How do you reference it?

Answer 55

Declared by: void *pV; A void pointer in c is called a generic pointer, it has no associated data type. printf("c = %c\n\n",*((char*)pV)); printf(“db = %lf\n\n",*((double *)pV));

Question 56

What is an inode number?

Answer 56

A pointer to a directory in UNIX