Six

Question 1

locate the binary file of a command

Answer 1

whereis command

Question 2

short description of a command

Answer 2

whatis command

Question 3

to repeat most recent command

Answer 3

!!

Question 4

to turn on a environmental variable

Answer 4

set -o variable_name
set +o variable_name will turn it off
shopt -s optionName : enable it
shopt -u optionName: disable it

Question 5

what is the if expression

Answer 5

if condition
then
elif condition
then
else
fi

Question 6

grep format

Answer 6

grep pattern file

Question 7

date

Answer 7

date +Format

Question 8

positional parameters

Answer 8

$0 is the script itself
$1 is the first argument
$2 the second
.
after $9
it must be written as ${}
$# is the total number of inputs

Question 9

[ -z “string” ]

Answer 9

the length of the string is nonzero

Question 10

case expression

Answer 10

case $var in
var1)
;;
var2)
;;
*)
;;
esac

Question 11

translate string

Answer 11

tr [option] set1 set2

Question 12

another way to avoid using tr

Answer 12

case $input in

[Ss][Oo])…….

Question 13

Third way to turn on case-insensitiveness

Answer 13

shopt -s nocasematch
case $opt in
shopt -u nocasematch

Question 14

for loop

Answer 14

for var in item1 item2
do
….
done

Question 15

for loop (C heritage)

Answer 15

for (( EXP1; EXP2; EXP3))
do
done

Question 16

command substitution

Answer 16

$(command)

this store the command output to a string or a variable

Question 17

while loop

Answer 17

while condition
do
done

Question 18

how to define a function in Bash

Answer 18

function function_name () {
statement
}

Question 19

function in a single line

Answer 19

function function_name () {; ; ;}
the last statement must have a semicolon.

Question 20

The limitation of the function

Answer 20

it must be defined before the first call

Question 21

A function cannot be empty

Answer 21

even if it has comments in it, it is empty. It shouldn’t be empty.

Question 22

command: basename

Answer 22

basename strips directory information and suffixes from filenames
-a –multiple : support multiple arguments and treat

Question 23

what does backtick mean

Answer 23

backtick ` runs the contents of the enclosed command and take the output as a string

Question 24

Leading dollar sign before single quotes

Answer 24

it causes escape sequences to be interpreted.

Question 25

what does (( t = a

Answer 25

if a

Question 26

How to use let command

Answer 26

``` let command let the expression to do arithmetic operation ex. if you do var3=$var1*$var2, if $var1 is 1 and $var2 is 2, you will get $var3 as 1*2 which is plain text. But if we run: let var3=$var1*$var2, we will get $var3 as an integer 2 instead. It does what (()) does. ```

Question 27

how to use expr

Answer 27

concatenates and evaluates the arguments according to the operation give. Arguments must be separated by spcaes

Question 28

extract substring of $length characters, starting at $position

Answer 28

expr substr $string $position $length

Question 29

eval command

Answer 29

ex. pro="ls -l" if you type $pro, it will do nothing , becuase "ls -l " is plain text. We can use eval to force the expression to execute

Question 30

let command

Answer 30

let command is used to erform arithmetic operations on shell variables

Question 31

what is ${parameter+alt_value}

Answer 31

no matter parameter is null or not, if it is set, it will be altered. This function is to determine if it is set or not while ${parameter:+alter_value} will substitute it with null string if it is a null string

Question 32

Rename command syntax

Answer 32

rename is a perl script. syntax: rename oldname newname *.files ex. rename .bak .txt *.bak rename all *.bak file as *.txt

Question 33

cut command

Answer 33

cut -cN1-N2 file : this will give specified columns cut -d "delimiter" -fN1,N2-N3,N4 cut -d "delimiter" -s -fN1,N2-N3,N4 will not give the line that doesnt contain this delimiter