Linux - 70 Useful Linux Scripting Techniques

70 Shell Scripting Interview Questions & Answers

May 18, 2015 | By Petras Liumparas in SHELL SCRIPTS

We have selected expected 70 shell scripting question and answers for your interview preparation. Its really vital for all system admin to know scripting or atleast the basics which in turn helps to automate many tasks in your work environment. In the past few years we have seen that all linux job specification requires scripting skills.

1) How to pass argument to a script ?

./script argument

Example : Script will show filename

./show.sh file1.txt

cat show.sh
#!/bin/bash
cat $1

2) How to use argument in a script ?

First argument: $1,
Second argument : $2

Example : Script will copy file (arg1) to destination (arg2)

./copy.sh file1.txt /tmp/

cat copy.sh
#!/bin/bash
cp $1 $2

3) How to calculate number of passed arguments ?

$#

4) How to get script name inside a script ?

$0

5) How to check if previous command run successful ?

$?

6) How to get last line from a file ?

tail -1

7) How to get first line from a file ?

head -1

8) How to get 3rd element from each line from a file ?

awk '{print $3}'

9) How to get 2nd element from each line from a file, if first equal FIND

awk '{ if ($1 == "FIND") print $2}'

10) How to debug bash script

Add -xv to #!/bin/bash

Example

#!/bin/bash –xv

11) Give an example how to write function ?

function example {
echo "Hello world!"
}

12) How to add string to string ?

V1="Hello"
V2="World"
let V3=$V1+$V2
echo $V3

Output

Hello+World

13) How to add two integers ?

V1=1
V2=2
V3=$V1+$V2
echo $V3

Output
3

Remember you need to add "let" to line V3=$V1+$V2
then echo $V3 will give 3

if without let , then it will be

echo $V3 will give 1+2

14) How to check if file exist on filesystem ?

if [ -f /var/log/messages ]
then
echo "File exists"
fi

15) Write down syntax for all loops in shell scripting ?

for loop :

for i in $( ls ); do
echo item: $i
done

while loop :

#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done

until loop :

#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done

16) What it means by #!/bin/sh or #!/bin/bash at beginning of every script ?

That line tells which shell to use. #!/bin/bash script to execute using /bin/bash. In case of python script there there will be #!/usr/bin/python

17) How to get 10th line from the text file ?

head -10 file|tail -1

18) What is the first symbol in the bash script file

#

19) What would be the output of command: [ -z "" ] && echo 0 || echo 1

0

20) What command "export" do ?

Makes variable public in subshells

21) How to run script in background ?

add "&" to the end of script

22) What "chmod 500 script" do ?

Makes script executable for script owner

23) What ">" do ?

Redirects output stream to file or another stream.

24) What difference between & and &&

& - we using it when want to put script to background
&& - when we wand to execute command/script if first script was finished successfully

25) When we need "if" before [ condition ] ?

When we need to run several commands if condition meets.

26) What would be the output of the command: name=John && echo 'My name is $name'

My name is $name

27) Which is the symbol used for comments in bash shell scripting ?

#

28) What would be the output of command: echo ${new:-variable}

variable

29) What difference between ' and " quotes ?

' - we use it when do not want to evaluate variables to the values
" - all variables will be evaluated and its values will be assigned instead.

30) How to redirect stdout and stderr streams to log.txt file from script inside ?

Add "exec >log.txt 2>&1" as the first command in the script

31) How to get part of string variable with echo command only ?

echo ${variable:x:y}
x - start position
y - length
example:
variable="My name is Petras, and I am developer."
echo ${variable:11:6} # will display Petras

32) How to get home_dir with echo command only if string variable="User:123:321:/home/dir" is given ?

echo ${variable#*:*:*:}
or
echo ${variable##*:}

33) How to get “User” from the string above ?

echo ${variable%:*:*:*}
or
echo ${variable%%:*}

34) How to list users which UID less that 100 (awk) ?

awk -F: '$3<100' /etc/passwd

35) Write the program which counts unique primary groups for users and displays count and group name only

cat /etc/passwd|cut -d: -f4|sort|uniq -c|while read c g
do
{ echo $c; grep :$g: /etc/group|cut -d: -f1;}|xargs -n 2
done

36) How to change standard field separator to ":" in bash shell ?

IFS=":"

37) How to get variable length ?

${#variable}

38) How to print last 5 characters of variable ?

echo ${variable: -5}

39) What difference between ${variable:-10} and ${variable: -10} ?

${variable:-10} - gives 10 if variable was not assigned before
${variable: -10} - gives last 10 symbols of variable

40) How to substitute part of string with echo command only ?

echo ${variable//pattern/replacement}

41) Which command replaces string to uppercase ?

tr '[:lower:]' '[:upper:]'

42) How to count local accounts ?

wc -l /etc/passwd|cut -d" " -f1
or
cat /etc/passwd|wc -l

43) How to count words in a string without wc command ?

set ${string}
echo $#

44) Which one is correct "export $variable" or "export variable" ?

export variable

45) How to list files where second letter is a or b ?

ls -d ?[ab]*

46) How to add integers a to b and assign to c ?

c=$((a+b))
or
c=`expr $a + $b`
or
c=`echo "$a+$b"|bc`

47) How to remove all spaces from the string ?

echo $string|tr -d " "

48) Rewrite the command to print the sentence and converting variable to plural: item="car"; echo "I like $item" ?

item="car"; echo "I like ${item}s"

49) Write the command which will print numbers from 0 to 100 and display every third (0 3 6 9 …) ?

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

50) How to print all arguments provided to the script ?

echo $*
or
echo $@

51) What difference between [ $a == $b ] and [ $a -eq $b ]

[ $a == $b ] - should be used for string comparison
[ $a -eq $b ] - should be used for number tests

52) What difference between = and ==

= - we using to assign value to variable
== - we using for string comparison

53) Write the command to test if $a greater than 12 ?

[ $a -gt 12 ]

54) Write the command to test if $b les or equal 12 ?

[ $b -le 12 ]

55) How to check if string begins with "abc" letters ?

[[ $string == abc* ]]

56) What difference between [[ $string == abc* ]] and [[ $string == "abc*" ]]

[[ $string == abc* ]] - will check if string begins with abc letters
[[ $string == "abc*" ]] - will check if string is equal exactly to abc*

57) How to list usernames which starts with ab or xy ?

egrep "^ab|^xy" /etc/passwd|cut -d: -f1

58) What $! means in bash ?

Most recent background command PID

59) What $? means ?

Most recent foreground exit status.

60) How to print PID of the current shell ?

echo $$

61) How to get number of passed arguments to the script ?

echo $#

62) What difference between $* and $@

$* - gives all passed arguments to the script as a single string
$@ - gives all passed arguments to the script as delimited list. Delimiter $IFS

63) How to define array in bash ?

array=("Hi" "my" "name" "is")

64) How to print the first array element ?

echo ${array[0]}

65) How to print all array elements ?

echo ${array[@]}

66) How to print all array indexes ?

echo ${!array[@]}

67) How to remove array element with id 2 ?

unset array[2]

68) How to add new array element with id 333 ?

array[333]="New_element"

69) How shell script get input values ?

a) via parameters

./script param1 param2

b) via read command

read -p "Destination backup Server : " desthost

70) How can we use "expect" command in a script ?

/usr/bin/expect << EOD
spawn rsync -ar ${line} ${desthost}:${destpath}
expect "*?assword:*"
send "${password}\r"
expect eof
EOD

Good luck !! Please comment below if you have any new query or need answers for your questions. Let us know how well this helped for your interview :-)

This page is a reference to the site:  http://linoxide.com/linux-shell-script/shell-scripting-interview-questions-answers/

MSSQL - Double Hopping

Getting the error below when running a SSRS report through SharePoint or a SQL using Linked Server to another server when NTLM security is used instead of SQL account.

SOLUTION:


That error message is almost always related to the "Double Hop" problem. The easiest way to fix it is to make sure all trusted connections are connecting using kerberos instead of NTLM. The alternative would be to use a SQL user instead of trusted auth.

Setting up kerberos is not a simple undertaking, but will resolve this problem and other problems you'll likely experience in the future. I have a many page document specific to our environment (so I can't share all of it) that describes how to set it up. My opening description is as follows:

Configuring SPN's Allows for machines to connect to SQL Server with Kerberos authentication instead of NTLM. Microsoft does not allow NTLM authentication for connections using what is known as the "Double Hop". Microsoft does allow connections using Kerberos to connect using the "Double Hop".

A typical Scenario where you would connect through the "Double Hop" would be by using a linked server. If you connect to SQLSRV1 using Trusted Windows Authentication, and then Select from a linked server that is configured to pass the login's current security context, you have just made a "Double Hop".

If you attempt to do this utilizing NTLM you will get an error saying "Login Failed for user 'NT AUTHORITY\ANONYMOUS LOGON'" This error will occur whether you're trying to connect through IIS first, through SSIS, SSRS, or even while trying to do bulk file operations to a file server as the second hop.


Run the following code on your backend SQL server to determine whether the connections to the server are using kerberos or not.

SELECT    sys.dm_exec_connections.session_id AS SPID, 
  sys.dm_exec_connections.connect_time AS Connect_Time, 
  DB_NAME(dbid) AS DatabaseName, 
  loginame AS LoginName, 
  sys.dm_exec_connections.auth_scheme as Auth_Scheme,
  sys.dm_exec_connections.net_transport AS Net_Transport,
  sys.dm_exec_connections.protocol_type as Protocol_Type,
  sys.dm_exec_connections.client_net_address as Client_Net_Address,
  sys.dm_exec_connections.local_net_address as Local_Net_Address,
  sys.dm_exec_connections.local_tcp_port as Local_TCP_Port
FROM sys.sysprocesses 
Right Outer JOIN sys.dm_exec_connections
ON sys.sysprocesses.spid=sys.dm_exec_connections.session_id
Order By Auth_Scheme, Net_Transport

 

Answer from SQLServerCentral:  http://www.sqlservercentral.com/Forums/Topic1391332-2799-1.aspx