W1: Linux - Bash Language

Question 1

1. Keywords: bash

Answer 1

GNU Bourne-Again SHell
sh-compatible language interpreter, executes commands read from an input .sh filew

Question 2

2. Keywords: Global Variables

Answer 2

Global variables can be used by all Bash scripts on your system

Question 3

3. Keywords: Local Variables

Answer 3

Local Variables can only be used within the script (or shell) in whech they’re defined

Question 4

4. Keywords: printenv

Answer 4

prints environment variables, which are the values kept in the machine that are in use by programs running in terminals/subshells.

ALSO prints previously specified environment variables in the shell.

aka attributes with a title and a value.

Question 5

5. Keywords: Index/associative arrays

Answer 5

array: stores multiple data.
with index and the value of each array element accessed by its corresponding index value.

Question 6

6. Keywords: for, while, until loops

Answer 6

N=10
for i in $(seq 1 $N); do
echo “$i-th run”
done

i=0
while [ $i -lt 100 ]; do
i=expr $i + 1
echo $i
done

i=0
until [ $i -ge 100 ]; do
i=expr $i + 1
echo $i
done

Question 7

7. Keywords: break,

Answer 7

break 1

terminates current loop, passes program control to the command that follows the terminated loop.

Question 8

8. Keywords: continue

Answer 8

continue 1

skips the remaining commands in the current loop iteration, passes program control to the next loop iteration.

Question 9

9. Keywords: select

Answer 9

https://linuxhint.com/bash_select_command/

used in scripts for menu creation
#!/bin/bash
echo “Which Operating System do you like?”

Operating system names are used here as a data source
select os in Ubuntu LinuxMint Windows8 Windows7 WindowsXP
do

case $os in
# Two case values are declared here for matching
“Ubuntu”|”LinuxMint”)
echo “I also use $os.”
;;

Three case values are declared here for matching
“Windows8” | “Windows7” | “WindowsXP”)
echo “Why don’t you try Linux?”
;;

Matching with invalid data
*)
echo “Invalid entry.”
break
;;
esac

done

Question 10

10. Keywords: PS3

Answer 10

PS1: primary prompt string
PS2: secondary prompt string
PS3: this parameter’s value is used as prompt for the select command
PS4:

Question 11

11. Keywords: unset

Answer 11

unsetting or deleting a variable directs the shell to remove the variable from the list of variables that it tracks

https://unix.stackexchange.com/questions/382391/what-does-unset-do

Question 12

12. Keywords: if statement

Answer 12

!/bin/bash

# Basic if statement
if [ 1000 > 100 ]
then
echo Hey that\'s a large number.
pwd
fi
date

Question 13

13. Keywords: case statement

Answer 13

!/bin/bash

simplifies complex conditions with multiple different choices. easier to read and maintain than nested if.

echo “Which color do you like best?”
echo “1 - Blue”
echo “2 - Red”
echo “3 - Yellow”
echo “4 - Green”
echo “5 - Orange”
read color;
case $color in
1) echo “Blue is a primary color.”;;
2) echo “Red is a primary color.”;;
3) echo “Yellow is a primary color.”;;
4) echo “Green is a secondary color.”;;
5) echo “Orange is a secondary color.”;;
*) echo “This color is not available. Please choose a different one.”;;
esac

Question 14

14. Keywords: function

Answer 14

!/bin/bash

BashFunction () {
echo Hello, my linux friends
}

BashFunction

Question 15

15. Keywords: /dev/stdin

Answer 15

standard input stream file. what’s input there is what’s input to the console,

what’s input to stdout is console output

Question 16

1. What is the meaning of #!/bin/bash

Answer 16

“shabang”.

inscructs the program loader to use the /bin/sh program instead of any other, passing the oath of the script as the first argument.

Question 17

2. How do you declare local/global vars?

Answer 17

global variables are declared outside of the function block. local variables are declared inside the function block

other source claims: all variables in bash are global by default,
set a variable as local x=$1 or sush to be a local variable.

Question 18

3. How do you make multiline comments?

Answer 18

: ‘
this will not get executed
‘

Question 19

4. How do you access associative array fields?

Answer 19

!/bin/bash

BashFunction () {
declare -A assArray1
assArray1[fruit]=Mango
assArray1[bird]=Cockatail
assArray1[flower]=Rose
assArray1[animal]=Tiger

declare -A assArray2=( [HDD]=Samsung [Monitor]=Dell [Keyboard]=A4Tech )

echo ${assArray1[bird]}
echo ${assArray2[HDD]} }

BashFunction

Question 20

5. What does the following command do?

echo $(elements[-1]}

Answer 20

supposedly prints the last element in the array

Question 21

6. How do you iterate through an array’s elements in a for loop?

Answer 21

arr=( “apple” “banana” “cherry” )

for item in “${arr[@]}”
do
echo $item
done

Question 22

7. Given the following arrays:
   my_array=(3 4 5 6)
   or
   my_array=(3,4,5,6)
   What will the following commands return?

echo ${#my_array[@]}
echo ${#my_array[*]}

Answer 22

!/bin/bash

provides the length of the element,

elements need to be seperated by spaces. the second ‘array’ simply consists of a single element ‘3,4,5,6’

testSeven () {
my_array=(3 4 5 6)
echo ${#my_array[@]}
echo ${#my_array[]}
echo stops here
my_array=(3,4,5,6)
echo ${#my_array[@]}
echo ${#my_array[]}
}
testSeven

4
4

1
1

Question 23

8. What is the meaning of the following switches within the square bracket of an ‘if’ statement? -d -w -x -a

Answer 23

https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html
-d : true if file exists and is a directory
-w : true if file exists and is writable
-x : true if file exists and is executable

Question 24

9. How do you check if a variable is set?

Answer 24

-v variable
-z${variable}

Question 25

10. How would you return a result of a function?

Answer 25

bash functions don’t return a value when called.bash function return value is the status of the last statement executed in the function.

easily just set a global variable to the ‘return’ value. technically all variables in bash are global, play with that as you will.

Question 26

11. Is it possible to write a function with the name cp?

Answer 26

apparently yes

Question 27

12. How would you read a password variable from STDIN, keeping it hidden?

Answer 27

!/bin/bash

secretInput () {
echo “Please enter your password”;
read -s password;
#pass = $(</dev/stdin);

echo break;
echo $password; }

secretInput

Question 28

13. How many times will the following loop run?

counter=1
while [ $counter -le 10 ]
do
. . .
done

Answer 28

ad infinitum

Question 29

14. How many times will the following loop run?

counter=1
until [ $counter -le 10 ]
do
. . .
done

Answer 29

it will not.

as long as the test case is true, it won’t start

Question 30

15. When would you choose “while loop” and “until loop”?

Answer 30

since until won’t run until it is false, you use until

Question 31

Exercise 1.
Write a function in a bash script file, that returns the number of lines in that file.

Answer 31

!/bin/bash

function countMe () {
wc -l < countMe.sh
}
lineCount=$(countMe)
echo $lineCount
#beep boop bop
#line 9

Question 32

2. Please assign the value of the command ls /usr | wc -l to the variable ls_output

Answer 32

!/bin/bash

function assignMe () {
ls /usr | wc -l
}
ls_output=$(assignMe)
echo $ls_output

Question 33

!/bin/bash

3. Fix the following script: The script should prompt the user to choose a mentor name,
   the script then prints the mentor’s email.
   If the user chooses ‘Exit’ then the program exits.

declare -A mentors

mentors=([“Naftali”] ????? [“Exit”])
mentors_emails=(“naftali@infinitylabs.co.il” “tal@infinitylabs.co.il” “ilya@infinitylabs.co.il”)

PS3=”Please enter your choice: “

select name in ?????
do
echo ${??????}
done

Answer 33

!/bin/bash

declare -A mentors=( [Naftali]=”naftali@infinitylabs.co.il” [Tal]=”tal@infinitylabs.co.il” [Ilya]=”ilya@infinitylabs.co.il” [Exit]=”Exit” )
PS3=”Please enter your choice: “

select name in ${!mentors[@]}
do
if [ $name == Exit ]; then
exit 0
fi

echo ${mentors[$name]} done