Interview Q&A for Shell Script Part-III

Q41:-Which one is correct "export $variable" or "export variable" ? 
Ans:- export variable=value

Q42:-How to list files where second letter is a or b ? 
Ans:- ls -d ?[ab]*

Q43:-How to add integers a to b and assign to c ?
Ans:-
c=$((a+b))
or
c=`expr $a + $b`
or
c=`echo "$a+$b"|bc`

Q44:-How to remove all spaces from the string ? 
Ans:-echo $string|tr -d " "

Q45:-Rewrite the command to print the sentence and converting variable to plural: item="car"; echo "I like $item" ? 
Ans:- item="car"; echo "I like ${item}s"

Q46:-Write the command which will print numbers from 0 to 100 and display every third (0 3 6 9 …) ? 
Ans:-
for i in {0..100..3}; do echo $i; done
or
for (( i=0; i<=100; i=i+3 )); do echo "Welcome $i times"; done

Q47:-What difference between [ $a == $b ] and [ $a -eq $b ] 
Ans:-
[ $a == $b ] - should be used for string comparison
[ $a -eq $b ] - should be used for number tests

Q48:-What difference between = and == ?
Ans:-
"=" - we using to assign value to variable
"==" - we using for string comparison

Q49:-Write the command to test if $a greater than 12 ? 
Ans:- [ $a -gt 12 ]

gt - Greater Than
ge - Greater Than or Equal to
lt - Less Than
le - Less Than or equal to
eq - Equal to

Q50:-How to check if string begins with "abc" letters ? 
Ans:-[[ $string == abc* ]]

Q51:-What difference between [[ $string == abc* ]] and [[ $string == "abc*" ]] 
Ans:-
[[ $string == abc* ]] - will check if string begins with abc letters
[[ $string == "abc*" ]] - will check if string is equal exactly to abc*

Q52:-How to list usernames which starts with ab or xy ? 
Ans:- egrep "^ab|^xy" /etc/passwd|cut -d: -f1

Q53:- What difference between $* and $@
Ans:-
$* - Print all passed arguments to the script as a single string
$@ - Print all passed arguments to the script as delimited list. Delimiter $IFS

$0 - Print the Script Name
$! - Most recent background command PID
$? - Most recent foreground process exit status 0 if success else any arbitiry number.
$$ - Give the PID of current shell
$# - Give the number of passed arguments

Q54:-How to define array in bash ? 
Ans:- array=("Hi" "my" "name" "is")

Q55:-How to print the first array element ? 
Ans:- echo ${array[0]}

echo ${array[0]}     - Print first element of array
echo ${array[@]}     - Print all elements of array
echo ${!array[@]}   - Print all indexes of array

Q56:- How to remove array element with id 2 ? 
Ans:- unset array[2]

Q57:-How to add new array element with id 8 ? 
Ans:- array[8]="New_element"

Q58:-How shell script get input values ? 
Ans:- There are 2 ways to pass value to shell script
a) via parameters
./script param1 param2

b) via read command
read -p "Destination backup Server : " desthost

Q59:-How can we use "expect" command in a script ?
Ans:-
/usr/bin/expect << EOD
spawn rsync -ar ${line} ${desthost}:${destpath}
expect "*?assword:*"
send "${password}\r"
expect eof
EOD