Transcript TECH 154 Midterm

TECH 154 Midterm
Solutions
Question 1: Short Definitions
a) superblock
superblock
An area on the disk indexed by inode values in which
are stored file properties such as permissions, file type
and link count and times related to the file, and
pointers
Does NOT include the file name, which is stored along
with the inode number.
A number of people memorized and reproduced a
googled answer “contains characteristics of a file
system”. This is very incomplete and IMV did not
illustrate understanding of what it was.
b) hidden file
hidden files
Any file that begins with a period. Not shown normally
when using shell patterns in commands like ls or stat.
Hidden files include . (current directory), .. (parent
directory), vim swap files and other resources such as
.vimrc.
They have NOTHING to do with security. “Security by
Obscurity” is generally a very bad idea as it is not an
adequate defence against a knowledgeable user. The
purpose is to prevent users from being distracted or
inadvertantly editing, modifying or deleting these files.
c) tty
tty
Either a terminal (or terminal simulation) or
the command to display which terminal # you
are using.
d) metacharacter
metacharacter
I never met a character I didn’t like. 
(never use this on a test!)
A character that is interpretted for its function
rather than than taken literally.
Found in shell patterns, such as ?*[^]{}, globbing:
$ ~, regular expressions, redirection such as | >
<, or command grouping: & ; || &&
e) flags vs arguments
flags vs arguments
Both are found on the command line.
A flag is usually preceded by a single dash or a double
dash and then a word. They modify the behaviour of
the command.
Arguments are non-flag strings on the command line.
(They do not include the command itself.) They often
represent files or are associated with the flag that
comes directly before it.
½ mark for each definition.
Question 2: Commands
Like any tool such as a hammer or a voltmeter, in order to
understand how to use a command you have to use it. You have to
pour yourself over what that command can do.
If you mereley copy someone else on an assignment or memorize
a phrase you read you won’t be able to demonstrate that you
understand what it does.
a) last
last
Displays all the login and logout times and
point of access of all or specified users since
the system log was created.
The last command uses the log file /var/2tmp.
Our SysAdmin rolls over this log file at the
beginning of January each year.
b) finger
finger
Given a userid, supplies their real name, shell, home
directory, when they last logged in and current .plan
and .project information.
Beyond the above it does not give access to a user’s
files.
It also indicates if they have any mail messages,
however mail has been mostly disabled on our system.
finger was designed to help people discover
information about each other in order to encourage
cooperation in a shared environment.
c) uptime
uptime
Not only shows how long the system has been
running since the system was last started, it
also shows the average load on the system
and the number of people logged on.
It has nothing to do with individual logins or
users.
d) cal
cal
Displays a calendar for the current or specifed
month or year.
If you answered “displayed the (calendar)
date” – that answer could be confused for the
date command.
e) stat
stat
Displays properties of files such as inode, link
count, permissions, times, ownership, device it is
on.
Does not display content.
The use of the word “statistics” is contained in
the question and is annotated as “circular”.
It may have worked in your high school. It has no
value at a college level.
f) touch
touch
Create empty files and updates their modified
and accessed time values.
g) vim commands
• :20,30d
or
:20
11dd
• :w ~/lab6.html (you can’t save in /usr)
• :1,$s/Unix/UNIX/g (OK if used as sed pattern)
Question 3: Misc
a) date format codes
date +’%A, Month: %B, %Y %T’
Format codes consist of a % followed by a letter.
The letter specifies a value such at the year %Y or
the time %T. Plain text is quoted literally.
Formatting has to do with the look of the output.
It has nothing to do with export TZ (setting
different time zones.)
This is a question where an example is helpful.
b) hard link vs symbolic link
Hard links are files that share the same inode value.
A symbolic link is a file that contains a path to the file it represents.
Hard links do not work across physical devices. Symbolic links do.
The ln command (used for both) cannot create a hard link to a
directory. Saying that a symbolic link can, just emphasizes the
same point of difference. However . and .. are created as hard
links to the current directory and the parent directory respectively.
Anyone who stated that either of these is a
copy of a file deserves to be flogged with a
wet noodle.
c)
/usr/share/zoneinfo parent directory of timezone information
files.
/dev/null used with redirection to discard unwanted
output. It is not a recycle bin and is not used
to delete files. It is not a directory.
/usr
Unix systems resource directory. Subdirectories
include bin, lib, include, src. Nothing to do with
users!
. . (dot dot) - parent, not previous directory. cd - takes you
to the previous directory.
d) 2>
|
>>
<
2> - redirect output of stderr to a a file
|
- pipe the output (stdout) of one command to
the input of the next.
>> - append the output (stdout) of a command to
a file
<
- replace stdin with a file
e) Matching: ??[4-9].[^Dd]oc
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??
any 2 characters
[4-9] - a single digit in the range of 4-9
. the character . itself. This isn’t a wildcard
[^Dd] any character BUT D or d
oc literally the characters ‘oc’.
xy7.Moc works
xyz.doc does not
Question 4: HTML
….next week
Question 5: Security
a) What are the minimal permissions on all files
and directories do you set to make a web page
visible to the outside world and nobody else,
not even yourself?
For directories: o=x
For html files: o=r
(navigable only)
(readable only)
That’s It
b) What are the octal values you would use to
set the following permissions: rw- r-x r – x
read is 4, write is 2, execute is 1.
rw = 4+2
655
r-x is 4+1
is your answer
c) What is the symbolic form of this chmod
command: chmod
0636 ?*.doc
chmod uo=rw,g=wx ?*.doc
Just writing out rw- -wxr-x is only worth ½.
Who is the root user?
root
She’s the systems admin. She is all powerful.
Worship at her feet. Defy her at your peril.
She can completely ignore any and all
permissions.
It is NOT you nor the current user. (Unless you
are root, in which case you still need to
provide an explanation similar to the above.)
Die Hard – The SysAdmin version
http://xkcd.com/705
Question 6: Tasks
Create This
– TECH154
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|-- Course
| |-- Assign2.doc
| |-- Assign3.doc
| `-- Assign4.doc
|-- Fees
| |-- Ins.txt
| |-- Misc.txt
| |-- SL.txt
| `-- Tuit.txt
`-- Summary.xls
mkdir -p TECH154/{Course,Fees}
touch TECH154/Course/Assign{2..4}.doc
touch TECH154/Fees/{Ins,Misc,SL,Tuit}.txt
touch Summary.xls
The longer (and harder) way requires use of cd to
navigate up and down subdirectories and takes
longer to write out. Save time using command
flags and shell patterns.
extract lines 500-700
head –n 700 dataFile | tail –n 201
or
sed -n –e ‘500,700p’ dataFile
extract lines relating to Dr. Sarav on
March 3rd
... | egrep “Dr Sarav” | egrep “March 3”
Requires 2 egreps, not one
Extract time, patient name and reason for the appointment
... | cut –d’;’ -f2 > theTime
... | cut –d’;’ -f3 > patient
... | cut -d’;’ -f5 > reason
display information in this format:
Eye Exam: Jim Smith, 2:15 PM
paste -d’:,’ reason patient theTime