70 shell script interview questions and answers

We have chosen the expected 70 individual shell Script questions and answers . For all system administrators , It's really important to know the script or at least the basics , This in turn helps automate many tasks in your work environment . In the past few years , We have seen all linux Job specifications require scripting skills .

1） How to pass parameters to scripts ？

./ Script parameters

Example ： The script will display the file name

./show.sh file1.txt

cat show.sh

#!/bin/bash

cat $1

2) How to use parameters in scripts ？

The first parameter ：$1,
The second parameter ：$2

Example ： Script will file (arg1) Copy to target (arg2)

./copy.sh file1.txt /tmp/

cat copy.sh

#!/bin/bash

cp $1 $2

3） How to calculate the number of passed parameters ？

$#

4） How to get the script name in the script ？

$0

5） How to check whether the previous command runs successfully ？

$？

6） How to get the last line from the file ？

tail -1

7) How to get the first line from the file ？

head -1

8) How to get the third element from each line of the file ？

awk '{ print $3}'

9) If the first equals FIND, How to get the second element from each line of the file

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

10)  How to debug bash Script

take -xv Add to #!/bin/bash

Example

#!/bin/bash –xv

11） For example, how to write functions ？

function example {
echo "Hello world!"
}

12) How to add a string to a 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 whether there are files on the file system ？

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

15) write down shell Syntax of all loops in the script ？

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 the cycle ：

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

16) At the beginning of each script #!/bin/sh or #!/bin/bash What does that mean? ？

This line tells which shell.#!/bin/bash Use /bin/bash Executed script . If it is python Script , There will be #!/usr/bin/python

17) How to get the number... From a text file 10 That's ok ？

head -10 file|tail -1

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

#

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

0

20) What order “export”？

In the child shell Open variables in

21) How to run scripts in the background ？

Add... At the end of the script “&”

22) “chmod 500 Script ” What does it do ？

Enable script owners to execute scripts

23） “>” What do you do ？

Redirect the output stream to a file or another stream .

24) & and && What's the difference?

& - We use it when we want to put the script in the background
&& - When we want to execute orders / Script time , If the first script completes successfully

25) When we're in [ Conditions ] Previous needs "if" when ？

If the conditions are met , We need to run multiple commands .

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

My name is $name

27) bash shell What are the symbols used for comments in the script ？

#

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

variable

29) ' and " What's the difference between quotation marks ？

' - We use it when we don't want to evaluate variables as values
" - All variables will be evaluated and assigned their values .

30) How to integrate stdout and stderr The stream is redirected from within the script to log.txt file ？

add to “exec >log.txt 2>&1” As the first command in the script

31) How to use only echo Command to get some string variables ？

echo ${variable:x:y}
x - The starting position
y - length
Example ：
variable=" My name is Petras, I'm a developer ."
echo ${variable:11:6} # Will be displayed Petras

32) Only if string variables =“User:123:321:/home/dir” When given , How to use echo Command acquisition home_dir？

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

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

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

34) How to list UID Less than 100 (awk) Users of ？

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

35) Programming , Calculate a unique primary group for the user and display only the count and group name

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 be in bash shell Change the standard field separator to “:”？

IFS=“：”

37) How to get variable length ？

${#variable}

38) How to print the end of a variable 5 Characters ？

echo ${variable: -5}

39) ${variable:-10} and ${variable:-10} What's the difference? ？

${variable:-10} - If in ${variable: -10} No variables have been allocated before , Then give 10 - give
At the end of the variable 10 Symbols

40) How to use only echo Command to replace part of the string ？

echo ${ Variable // Pattern / Replace }

41) Which command replaces the string with uppercase ？

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

42) How to calculate local accounts ？

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

43) How in the absence of wc Command to calculate the words in the string ？

set ${string}
echo $#

44) Which is right “export $variable” or “export variable”？

export variable

45) How to list the second letter as a or b The file of ？

ls -d ?[ab]*

46) How to put integers a Add to b And assigned to c ？

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

47) How to delete all spaces from a string ？

echo $string|tr -d " "

48) Rewrite the command to print sentences and convert variables to plural numbers ：item="car"; Echoes “ I like $item”？

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

49) Writing will print from 0 To 100 And display every third (0 3 6 9 ...) The order of ？

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 the parameters provided to the script ？

echo $*
or
echo [email protected]

51) [ $a == $b ] and [ $a -eq $b ] What's the difference?

[ $a == $b ] - Should be used for string comparison
[ $a -eq $b ] - Should be used for digital testing

52) = and == What's the difference between

= - We use it to assign values to variables
== - We use it to compare strings

53) Write commands to test $a Is it greater than 12 ？

[$a-gt 12]

54) Write commands to test $b Is it equal to 12 ？

[ $b -le 12 ]

55) How to check whether a string is in “abc” Beginning of letter ？

[[ $string == abc* ]]

56) [[ $string == abc* ]] and [[ $string == "abc*" ]] What's the difference?

[[ $string == abc* ]] - The string will be checked to see if it starts with abc Beginning of letter
[[ $string == "abc*" ]] - Will check whether the string is exactly equal to abc*

57) How to list with ab or xy The user name at the beginning ？

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

58) what $！ It means in bash in ？

Recent background commands PID

59) what $？？

The nearest foreground exits .

60) How to print the current shell Of PID？

echo $$

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

echo $#

62) $* and [email protected] What's the difference?

$* - Treat all parameters passed to the script as a single string
Provide [email protected] - Provide all parameters passed to the script as a separated list . Separator $IFS

63) How to be in bash In the definition of array ？

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 delete id by 2 Array elements of ？

unset array[2]

68) How to add id by 333 New array elements of ？

array[333]="New_element"

69) shell How the script gets the input value ？

a) Through parameters

./script param1 param2

b) By reading commands

read -p "Destination backup Server : " desthost

70) How do we use in scripts “expect” command ？

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