Jan 25 Review

/etc/passwd UID < 1000 - system users

​ UID >=1000 - regular users
/etc/shadow
/etc/group
Permissions
Ls -l
_ rwx rwx rwx
rwx rwx rwx

(c/l/-/d) Owner user (u) Group (g) Others (o)

File Directory

r cat ls

w vi Rm, mv, touch…

Mkdir, rmdir

x /file Cd, lss

Jan 25 Class
How to change permission
Chmod [taget permission] file/dir
●​ 3 different ways to do this:
○​ Using the Octal Value (most ideal)
■​ Rw- r- - - -x ((4+2), 4, 1 —> 641)
●​ Readable for user group others, writable for users only,
executable for none (octal value: 644, how to do: chmod
644 file)
○​ Using the assign function by assigning directly (ideal when there are a
lot of changes)
■​ chmod u=rw,g=r,o=r file
●​ Another example = u=rx,g=x,o=w [no spaces - execute as:
chmod u=rx,g=x,o=w file]
○​ Add and remove (=/-) (ideal when the changes are minimal)
■​ If we want to change from r-x -w- -wx to -w- r-x rwx
●​ chmod u-rx,u+w,g-w,g+rx,o+r file

Default permission: System is configured for new file/directories when created

●​ When you make a new file, the default permissions are: - rw- rw- r- -
●​ When you make a new directory, the default permissions are: d rwx rwx r-x
●​ Base permissions for files are rw- rw- rw- (666)
●​ Base permissions for directory are rwx rwx rwx (777)

umask
●​ The higher the number, the more secure it is
○​ However, this is less convenient for the users
●​ The smaller the umask, the less secure
○​ More convenient for the users

Sticky bit u g o

●​ Example: 0 002
○​ Step 1: put in binary value
■​ 0 0 2 = 000 000 010
○​ Step 2: flip the binary value
■​ 000 000 010 → 111 111 101
○​ Step 3: logical AND
Flipped binary 1 1 1 1 1 1 1 0 1
value

Default file 1 1 0 1 1 0 1 1 0
value

Logical AND 1 1 0 1 1 0 1 0 0

Permission r w - r w - r w -

In class work through example 243

●​ Step 1
○​ 2 4 3 = 010 100 011
●​ Step 2
○​ 010 100 011 → 101 011 100
●​ Step 3
Flipped binary 1 0 1 0 1 1 1 0 0
value

Default file value 1 1 0 1 1 0 1 1 0

Logical AND 1 0 0 0 1 0 1 0 0

Permission r - - - w - r - -
○​ Umask 243
Subtraction
●​ For directory
○​ Example
■​ 777 - 243 = 534
○​ Example
■​ 777 - 537 = 240
○​ Example
■​ 777 - 431 = 346
●​ For file
○​ Example 666 - 243 = 423 DOES NOT WORK
■​ 243 is an odd number (executable can’t work in a file) - if it is even, we
can complete it correctly
●​ We would need to minus each digit by one if it is an odd number
○​ 666 - 242 = 424
○​ Example 537
■​ 537 → 426
■​ 666 - 426 = 240
○​ Example 431
■​ 431 → 420
■​ 666 - 420 = 246

Quiz 2
●​ Cat, more, less, head (top 10 lines), tail (bottom 10 lines)

Cat Displays the text of the file Cat [file]

line by line

more Enables user to view the More [file]

contents of a file one screen
at a time

less Enables user to view the Less [file]

contents of a file one screen
at a time

head Enables user to view n Head [file]

number of lines from the top. Head -5 [file]
Default 10

tail Enables user to view n Tail [file]

number of lines from the Tail -5 [file]
bottom. Default 10

●​ Filtering
○​ Wc, sort (-n, -k), cut (-f, -d, -b), uniq, | (STUDY)
wc Gives the number of lines, Wc [file]
words, and characters in the output:
data L# W# C# file name

sort Sorts the content of a file. Sort [file]

Default alphabetically

Sort -n Sorts the content of a file Sort -n [file]

according to string numerical
value

cut

Cut -f

Cut -d

Cut -b

uniq Removes duplicate lines that Uniq [file]

are duplicated in succession.
If you want to sort and then
delete duplicates, you would
need to do: sort [file] | uniq

| (pipeline) Connect the output of one

command directly into the
input of another

grep Used to search particular Grep [word to search] [file]

information from a text file Output:
[word] line content
[word] line content

●​ Ln, ln-s (hard vs soft)

●​ User permissions
●​ Chmod (no umask)

Jan 30 - Class
Sudoers - sudo command arguments
1.​ Customize certain admin privilege by adding user to a group such as adm, sudo
2.​ Maintain all activity log for using sudo
3.​ No admin password needed, use user’s own password

Quiz 2
Sort -n -k2
cut -d” “ -f1,2
Cur -d” “ -f2 sort | -n uniq | wc -l
Feb 12: Lab and A2 Notes

Shell Special Character Meaning Example

Double Quotes “ ” Characters inside the double

quotes are treated as
ordinary characters; special
characters lose their
meaning. Exceptions: $, \, `

Single Quotes ‘ ‘ Suppresses ALL expansions,

no exceptions. If the single
quotes are inside double
quotes, however, they lose
their special character status
and are just regular.

Escape Character \ “Quotes” only a single Echo “The balance for

character (precede the $USER is: \$5.00”
character you want the
expansion suppressed on. The balance for user me is:
Can escape and escape $5.00
character by putting two \\. In
single quotes the \ loses
meaning and is treated as a
regular character.

The following strings are stored in a file called regexfile.txt (one string per line).
cat, cot, cut, ct, caat, caut, caout, caouaout, cAt, cAot,cabt
Write the output of the following grep commands. (If there is not a match, just
write down “no match”).
grep -E c[aou]t regexfile.txt

grep -E c[aou]?t regexfile.txt

grep -E c[aou]{2\,4}t regexfile.txt

grep -E c[^aou]t regexfile.txt

grep -E c[aou]*t regexfile.txt

Grep -e c[^aou]*t

Grep -E c.?t regexfile.txt

If, then, else
-f = file
-d = directory
-e = exist

if…Elif - ls; who; date

read -p “Please provide your input.” number (where -p indicates the user prompt)
if [ $number -gt 0 ] ; then​ ​ ​ (note the space before and after square brackets)
​ Echo “$number is a positive”
elif [ $number -lt 0 ]
then
​ Echo “$number is a negative”
elife [ $number -eq 0 ]
then
​ echo “$number is zero”
else
​ echo “not a number”

Nested If
FILE=$1 ​ ​ ​ ​ ​ ​ ($1 command line argument)
if [ -e “$1” ]; then​ ​ ​ ​ ​
​ if [ -f “$FILE” ] ; then​ ​ ​ ​
​ ​ Echo “This is a regular file”​ ​
​ fi
​ if [ -d “$FILE” ] ; then
​ ​ Echo “this is a dir:
​ fi
​ if [ -r “$FILE” ] ; then​ ​ ​ (-r = readable -w = writable -x = executable)
​ ​ Echo “This is readable by owner”
​ fi
else
​ echo “file does not exist”
fi

For Loop
Example 1
for index in 1 2 3 4
do
​ Echo $index
done

Example 2
for loop_variable in 1 2 3
do
​ Echo $loop_variable
done

Example 3
for i in *​ ​ ​ ​ (*= file name expansion)
do
​
​ if [ -d “$i” ]
​ then
​ ​ echo dir found “$i”​ ($ indicates variable (keeps the special character
properties within double quotation) “” = boundary that is protected)
​ fi
done

Example 4
for i in $(ls)​ ​ ​ ​ (command substitution)
do
​ if [ -f $i ]
​ then
​ ​ count=$((count+1))​ ((())=arithmetic evaluation)
​ fi
done
echo total number of files are
$count​​ ​ ​ ​ (when referencing the variable, need $)

Midterm Review

[CHAPTER 1 - NOT ON MIDTERM]History of Linux/Unix

●​ GNU + linux kernel; GPL
●​ Linux distribution
○​ Fedora
○​ Ubuntu
○​ Mint
○​ Etc… (Over 800 in existence)

Linux utilities (20 COMMANDS)

○​ Ls
■​ -l
■​ -a
■​ -r
○​ Mkdir
■​ -p (parent directories)
○​ Rm
■​ -r directory name
■​ -f (don’t ask, just delete)
○​ Rmdir (ONLY WORKS ON EMPTY DIRECTORIES)
○​ Cd
■​ .
■​ ..
■​ / (root)
■​ ~ (home)
■​ - (previous working directory)
○​ Cut
■​ -f
■​ -d
■​ -b
○​ Sort (like in assignment and quizzes)
■​ | (pipeline)
○​ Uniq
○​ Wc
■​ -l
■​ -w
■​ -c

File system (10 NEED TO KNOW)

●​ / (root)
●​ /etc
○​ /etc/passwd
○​ /etc/group
●​ /bin
●​ /var
○​ /var/log

Absolute vs relative pathname

Permissions
What does r w x mean for file and directory
How to give permissions to file and directory with octal value (chmod)
●​ Chmod
○​ Octal value
○​ =
○​ +/-
Octal value ← → rwx rwx rwx
Umask
●​ To change and evaluate your systems securite
●​ What are base permissions and default permissions
○​ Base permissions for file: 666
○​ Base permissions for directory: 777
○​ Umask with base permissions gives the default permission
○​ Example
■​ Umask 022
●​ File 666-022=644
●​ Directory 777-022=755
■​ Umask 035
●​ File 666-035 (035 → 024)
○​ 666-024=642
●​ Directory 777-035=742

Special character
●​ I/O redirection (quiz 3/lab 4)
●​ $
●​ ;
●​ (space)
●​ \
●​ “”
●​ ‘’
●​ #
●​ ` (back tick)
●​ *
●​ ?
●​ &

Single and double quotation examples: (assignment 2)

name=mark
Echo “\$name”​ → $name
Echo “ ‘$name’ ”​ → ‘mark’
Echo ‘ “\$name” ’​ → “\$name”

User management
●​ Adduser ​ useradd
●​ Groupadd​ addgroup
●​ Usermod
○​ -g (primary group)
○​ -G (supplement group)
●​ Userdel
Regex (lab 4)

●​ Phone number (midterm)

○​ 403/587/895
●​ Credit card number (midterm)
●​ IP address (hard… possibly for the final)

Scripting (up to lab 5)

●​ No conditions, loops, ifs…

ALL

●​ Labs
●​ Quizzes
●​ Assignments