Bash Script

Question 1

What is set command?

Answer 1

set returns all environment variables + extras like functions etc which env would not return

Question 2

What is $#?

Answer 2

Number of all parameters

Question 3

What is $0, $1

Answer 3

Program name, first argument

Question 4

What is $*

Answer 4

All arguments

Question 5

How to print parameter 12?

Answer 5

${12}

Question 6

What does this do?
var=”one two three four five”
set – $var

Answer 6

sets the input parameters as one two three four five

Question 7

for i
do
echo $i
done

Answer 7

when range is neglected the for loop loops through the input parameters so $i is $1, $2 etc

Question 8

Difference between echo $varname and echo “$varname”?

Answer 8

Same if there are no multiple white spaces. In case there are, $varname will print only 1 space while “$varname” will print all the spaces.

Question 9

hello=”K L M”
1. Output: echo “$hello”
2. Output: echo $hello
3. Output: echo ‘$hello’

Answer 9

1. K L M
2. K L M
3. $hello

Question 10

IFS=’!’ read a b
echo a is $a
echo b is $b

1. input = kostas mylonas
2. input = kostas!mylonas

Answer 10

1. a is kostas mylonas
   b is

2.
a is kostas
b is mylonas

Default IFS is whitespace

Question 11

str=”kostas”
echo ${str^^}

Answer 11

KOSTAS

Question 12

str=”KOSTAS”
echo ${str,,}

Answer 12

kostas

Question 13

str=”KOStas”
echo ${str,}

Answer 13

kOStas

Question 14

str=”kOstAs”
echo ${str^}

Answer 14

KOstAs

Question 15

anArray=(‘el1’ ‘el2’ ‘el3’)
1. echo ${anArray[@]}
2. echo ${!anArray[@]}
3. echo ${#anArray[@]}

Answer 15

1. whole array: el1 el2 el3
2. array indices: 0 1 2
3. size of array: 3

Question 16

a=2
let b = a+2
Will it work? If not what should be changed to make it work

Answer 16

No, space is not allowed. If you want to use space then include evreything in “” like
let “b = a+2”

Question 17

How to properly use expr command?

Answer 17

Always put it inside ` ` and spaces are mandatory!
e.g.
a =expr 3 + 5
a =expr $k + 3

Question 18

string=ABigString
pos=4
length=6
z=expr substr $string $pos $length
echo $z

Output?

Answer 18

gStrin

Question 19

What is
“echo $?”

Answer 19

Prints the exit status of the last executed command

Question 20

set – echo "p1 p2 p3 p4 p5 p6"

for j in ${*:2:3}
do
echo -n $j
done
echo

Answer 20

p2p3p4

“Start from 2nd param and use up untill 3 params”

Question 20

set – echo "p1 p2 p3 p4 p5 p6"

for j in ${*:2}
do
echo -n $j
done
echo

Answer 20

p2p3p4p5p6

“Start from 2nd param and go till the end”

Question 21

!export my_new_env_var=”Harry”

Answer 21

create env var only for current session. If you want it globally then you have to go through bashrc

Question 22

How to multiply using expr?

Answer 22

a=expr 5 \* 3
# 15 – with expr in multiplication use *

Question 23

z=expr substr $string $pos $length

Answer 23

from string start from pos and take up to length chars
ABigString pos = 4 len = 6 will give
gStrin

Question 24

A=0.04
B=0.03
let “comp=echo $A-$B\>0 | bc”

Answer 24

True

Question 25

a=1000
b=$(echo “scale=3; $a/100” | bc)

Answer 25

10.000

Question 26

multiline comments in bash?

Answer 26

! /bin/bash

: ‘
This is the segment of multi-line comments
Through this script, you will learn
How to do multi-line commenting
‘

Question 27

How to call a bash script from python

Answer 27

import subprocess
subprocess.call([‘sh’, ‘./exitStatus.sh’])

Question 28

if [ “$age” -gt 18 ] && [ “$age” -lt 40 ]

if [[ “$age” -gt 18 && “$age” -lt 40 ]]

if [ “$age” -gt 18 -a “$age” -lt 40 ]

Are these three the same?

Answer 28

Yes

Question 29

How to right if (count > 9.8) in bash?

Answer 29

if (( $(bc «<”$count > 9.8” )))
first write as normal and then append $(bc «< and fix parenthesis

Question 30

How to check if string $a is alphabetically smaller than $b?

Answer 30

if [ $a /< $b ]

Question 31

How to check if string a is not null
and how if its null?

Answer 31

if [ -n “a” ]
if [ -z “a” ]

Question 32

if [ -e file ]

Answer 32

check if file exists

Question 33

if [ -r file ]

Answer 33

check if user has read permissions on file

Question 34

for j in ${*:3}

Answer 34

from second parameter to the end

Question 35

for j in ${*:3:3}

Answer 35

from 3d parameter and use 3 parameters 3d 4th 5th