Topic 4 - Unix Files and Directories

Question 1

What is a computer file?

Answer 1

A container for a sequence of bytes

Question 2

What are the 4 types of files?

Answer 2

1) Regular normal file
2) Directory
3) Link
4) Device (Unix treats a device like a file

Question 3

Does the shell remember the current directory for you or do you have to remember the current directory on your own.

Answer 3

The shell remembers the current directory for you

Question 4

What is the cmd for determining the current directory?

Answer 4

pwd (print working directory)

Question 5

cmd for changing directories?

Answer 5

cd

Question 6

The cmd “ls” does what?

Answer 6

listing the contents of a directory

Question 7

How is the Unix filesystem organized?

Answer 7

As un upside-down tree; top of the filesystem is the root

Question 8

Whats the symbol for the root directory?

Answer 8

/

Question 9

How do you change from the current directory to the root directory?

Answer 9

cd /

Question 10

Examples of file names in unix:
a.out(
data@
readme.txt
tmp/
Are the letters *,@, and / part of the file name?

Answer 10

No, just indicators to give more information about the file

Question 11

A typical Unix file system spans _____ disks?

Answer 11

Many

Question 12

As a user, do you need to know which physical disk is used to store your things?

Answer 12

No

Question 13

What is the cmd for reporting on all mounted file system?

Answer 13

df.

Question 14

Definition of Absolute Pathnames?

Answer 14

• start from the root directory ie. /
• string together all directory names that are in your path to the file
• separate each directory name by a single slash

Question 15

What does ~ mean?

Answer 15

Refers to the $HOME directory

Question 16

What does “cd” with no arguments do?

Answer 16

Changes the working directory to your home directory

Question 17

Define Relative Pathnames

Answer 17

Specifying pathnames relative to the current working directory using . and ..

Question 18

what does “ .. “ mean?

Answer 18

Start from parent directory

Question 19

What does “ . “ mean?

Answer 19

Start from the current directory

Question 20

How do you create a new directory?

Answer 20

mkdir

Question 21

How do you remove a directory? (as long as it exists and is EMPTY)

Answer 21

rmdir

Question 22

How do you move a file or directory?

Answer 22

mv

Question 23

How do you copy a file from one directory to another?

Answer 23

cp [filename]

Question 24

How do you copy a directory?

Answer 24

Use a recursive copy: “cp -r” to copy the directory

Question 25

Changes the current working directory, and also remembers the previous one by pushing it on to ta stack

Answer 25

pushd

Question 26

Popd does what?

Answer 26

change the current working directory back to the last directory placed on the stack by pushd

Question 27

Keeps track of what you have so far in the stack

Answer 27

dirs

Question 28

How do you reveal hidden files?

Answer 28

ls -a

Question 29

How you display the first few lines of a file? (Default 10)

Answer 29

head

Question 30

How do you display the last few lines of a file (default 10)

Answer 30

tail

Question 31

What is the only invalid character in a Unix file name?

Answer 31

the Slash ( / ) - used for pathnames

Question 32

Which ones are not legal Unix file names? a a. .a a.b.c

Answer 32

None, they're all legal - Unix does not enforce file name extensions. Just remember: any name beginning with a dot is hidden where "ls" command cannot see them but the "ls -a" command can. (Note: the . and .. are reserved for current and parent directories)

Question 33

Give an example of an extension for executable files.

Answer 33

Executable files usually have no extensions. This does make distinguishing executable files from data files a bit difficult.

Question 34

How can you determine if a file is executable?

Answer 34

using the "file" command --> file filename

Question 35

Give 3 examples of Wildcard Characters:

Answer 35

1) * 2) ? 3) [...]

Question 36

What does the wildcard * do?

Answer 36

matches a sequence of zero or more characters: | a*.c --> matches abbb.c, aaaa.c, aldkfjaklf.c

Question 37

What does the wildcard ? do?

Answer 37

matches any single characters

Question 38

What does the wildcard [...] do?

Answer 38

Matches any single character between the braces: b[aei]t --> matches bat, bet, or bit, not baet, or bt You can use a hyphen: [A-Z] to match a range. in this example, it'll match the alphabet of capital letters

Question 39

What acts as a boundary for wildcards to ensure matching does not go beyond filenames into the file path?

Answer 39

the slash ( / ) in the pathnames

Question 40

Using * or ?, can you match a dot (.) at the beginning of a filename? Anywhere else in the filename? Comment on your answer.

Answer 40

No. Yes. A dot ( . ) at the beginning of a filename must be matched explicitly i.e. with a literal "."

Question 41

What do double quotation marks do?

Answer 41

- Stops the interpretation of some shell special characters (whitecaps and wild cards) - Forces the shell to deal with whatever between the two quotes as one token

Question 42

What do single quotation marks do?

Answer 42

Does everything double quotation marks do plus stops variable expansion ($HOME, etc.)

Question 43

What do Back Quotes do?

Answer 43

Text between back quotes leads to executing the text as a command and returning the results instead

Question 44

What is the name of the single control structure Unix creates for each file?

Answer 44

Index Node (ie. inode) for each file

Question 45

Wha happens when a file is created in terms of its inode?

Answer 45

When the file is created, there is one link to the file's inode.

Question 46

Ls -i displays ____?

Answer 46

The inode number of each file.

Question 47

Ls -l displays _____?

Answer 47

Displays permissions AND the number of existing links per file.

Question 48

Each hard link acts like a _______?

Answer 48

a pointer to the actual file.

Question 49

Whats a symbolic link?

Answer 49

"ln -s" is an indirect pointer to another file or directory

Question 50

Removing any one of a file's hard links except one ______ [will or will not] remove the contents of the file?

Answer 50

will not

Question 51

Removing all of a file's hard links ______ [will or will not] remove the contents of the file?

Answer 51

Will

Question 52

Removing a file's symbolic link ______ [will or will not] affects the file

Answer 52

will not

Question 53

Removing the symbolic link ________ [will or will not] remove the contents of the file.

Answer 53

will not

Question 54

What happens when you remove the original file that has a symbolic pointer?

Answer 54

The symbolic pointer will point to an unknown file

Question 55

Who can do what when it comes to these 3 actions: 1) Creating a symbolic link to directories or files 2) Creating hard links to files 3) Creating hard links to directories

Answer 55

1) Any user 2) Any user 3) Only the super user / root user

Question 56

File Permissions: How does Unix divide up users in terms of who has what permissions with files?

Answer 56

3 Divison --> Owner User, Group User, Other users 1) Owner user: owner of the file 2) Group user: each file is assigned to one and only one group (superuser sets up groups) 3) Other users: everyone else

Question 57

How do you view Permissions?

Answer 57

the command ls -l

Question 58

what does "r" mean?

Answer 58

read permission

Question 59

what does "w' mean?

Answer 59

write permission

Question 60

what does "x" mean?

Answer 60

execute permission

Question 61

How do you change / assign permissions?

Answer 61

Command: chmod

Question 62

What are the 2 syntaxes / methods you can use with the chmod command?

Answer 62

2 syntaxes / methods you can use: 1) numeric method 2) symbolic method.

Question 63

Describe the numeric method for chmod command.

Answer 63

Numeric method: chmood DDD file [file..] | 1 2 4 = --x -w- r--

Question 64

Describe the symbolic method for chmod command.

Answer 64

Symbolic method; chmod [ugoa][+-=}[wrx] file[ [...] Relative: chmod u+rwx file -- gives owner user R,W,X Absolute: chmod g=r file --> gives group users R, makes sure it has nothing else Relative and Absolute permission assignment can be appended with commas: chmod u=rwx, g-w,o-rwx file

Question 65

How do you set default permissions for any file you will create?

Answer 65

command: umask

Question 66

Describe the umask command:

Answer 66

value is opposite to that of the chmod command: tells which permissions will not be given - umask 077 means do not let anyone but the owner user to do anything with my files by default Generally, set umask once in your .cshrc file and never set it again

Question 67

Directory permissions are different from file permissions: Listing file names requires:_____ Creating file names requires: _____ Accessing files information (if you know the name) requires:_____

Answer 67

read permission write permission execute permission

Question 68

How can you change the permissions for a whole tree of files and sub directories?

Answer 68

chmod -R Example: chmod -R a-x+X publicDocs Recursively (i.e. on all files and directories in publicDocs) removes execute permission for all classes and adds special execution permission for all classes it recursively changes the mode for each chmod operand that is a directory and all sub-directories and files receive those permissions.