Seven

Question 1

Elements in a regular expression
RE is used in matching patterns
link:
http://tldp.org/LDP/abs/html/x17129.html

Answer 1

A character set
An anchor: these designate the position in the line of text that the RE is to match.
Modifiers: These expands or narrow the range of text of txt

Question 2

sed command

Answer 2

sed in a nutshell is used to modify text content.

Question 3

echo ${string/what/A}

Answer 3

This will substitute what in string by A

Question 4

echo ${string:0:number}

Answer 4

Output 0:number length which doesn’t contain number

Question 5

: ‘multiple lines here’

Answer 5

This is another way to write the comment here

Question 6

cat > filename

Answer 6

output the comments to the filename

Question 7

eval echo \${$var}

Answer 7

Usually, echo ${$var} is a syntax error, because we can only have one variable name in the curly parenthesis. eval takes the multiple strings as its input like they are typed from the command line when the $ will be translated. And then it puts them together and run them.

Question 8

array initialization

Answer 8

array=(var1 var2 var3 var4)

Question 9

what is the function of a code block

Answer 9

{code block} : I/O redirection

Note: redirect everything in the block

Question 10

expr

Answer 10

expr arithmetic expression
ex. expr 3 * 2
sometimes, it has to be escaped

Question 11

=~

Answer 11

=~ is used to match regex
ex.  
variable="This is a fine mess"
if [[ "$variable" =~ T.......fin*es* ]]
then
....
fi
double quote for T.....fin*es* is not necessary, and it may cause error