Five:SPECIAL CHARACTERS

Question 1

semicolon ;

Answer 1

A semicolon allows to write multiple lines of command in one line.
Note: there is a space after ever semicolon

Question 2

double semicolon ;;

Answer 2

The double semicolon is used in the case switch expression

Question 3

The case expression

Answer 3

case $variable in
var1) statement ;;
var2) statement ;;
*) statement ;;
esac

Question 4

dot .

Answer 4

as a component of a filename

also file is hidden if it is prefixed by a dot

Question 5

dot . in character match

Answer 5

match only one character

Question 6

double quote

Answer 6

preserve most of the special chracter

Question 7

comma operator

Answer 7

all evaluated, but only the last one returned

Question 8

comma in concatenation

Answer 8

/usr/{,share/}bin
represents
/usr/bin
/usr/share/bin

Question 9

command colon

Answer 9

”:” is itself a bash built-in and may be equivalent to true
:&raquo_space; filename This combination do nothing to an existing file but create it if it does not exist
: > filename This empty a file

Question 10

colon as a field separator

Answer 10

PATH=PATH1:PATH2

Question 11

a double parenthesis

Answer 11

((condition ? state1 : state2))

Question 12

${parameter?err_msg}, ${parameter:?err_msg}

Answer 12

if parameter is set, use it, else print err_msg

Question 13

? serves as a single wild card

Answer 13

?name

Question 14

listing arguments

Answer 14

”$@” and “$*” note that these two should be quoted

Question 15

$?

Answer 15

exit status

Question 16

(command group)

Answer 16

The variables in the parenthesis is local inside the parenthesis

Question 17

cat {file1,file2,file3} > filename

no space is allowed in the bracket

Answer 17

this combine these three files to just one file

Question 18

cp file{extension1,extension2}

Answer 18

this copies two files with the different extension

Question 19

block of code

Answer 19

{command block}
the content in the block can be seen by the remain of the script  unlike the condition in the parenthesis
{read line1
read line2
}

Question 20

equivalence of test

Answer 20

[]
[
test
[[]]
difference between [] and [[]]. The [] doesn’t evaluate
[[]] evaluates
There must be a space between the [ and the chactaters
if you want to use && and || operator, best use [[ ]]

Question 21

if command expression

Answer 21

if takes the exit value from the command expression

Question 22

(()) and $[]

Answer 22

evaluate integer expansion

Question 23

cd -

Answer 23

get into the previous working directory

Question 24

previous working directory

Answer 24

~-

Question 25

current working directory

Answer 25

~+

Question 26

in the test expression

Answer 26

test 5 > 2 && statement || statement

The both sides of the > must contain a space

Question 27

where should we put ! in the test expression

Answer 27

right after the keyword test

Question 28

what is $()

Answer 28

as far as I can guess, $(command) this will give the output of command as the input

Question 29

what is the format for find

Answer 29

find path -name pattern