Do you know a command in linux to retrieve specified line from a file?

Thanks.

You can use sed for this. e.g. print line number 42:

For example I have file like

bla1 ddd1
bla2 eee2
bla3 fff3

how i can retrive specified word on a specified line?

for exapmle I want to retrive second word on third line? (fff3)

Do you want to retrieve based on name or location? For the former, just use grep, eg:
cat <filename>|grep <keyword>
returns every line in <filename> which contains <keyword>. For more sophisticated stuff, read up on sed and awk. I would start with the Bash Guide for Beginners by Machtelt Garrels--its on tldp.org.

That will retrieve the second word from the forty second line. Change 42 to the line you want and change $2 to the word you want.

If you are looking to extract part of a line, using awk as Andrew mentioned is a good method when the you have fields de-limited by whitespace. If you have a delimited records on some othere character, you can use awk with the -F option. For example to print the third field as delimited by a colon character:
.
Delimited fields can alse be extracted using the cut command, which is lighter weight than awk. You specify the delimiting character with the -d option, and the field number(s) with the -f option. You can do the same as the awk command above with cut like this:
Cut can also extract by character position:
...will print output characters 13-19 of the input line(s).

There are several other options. See the cut manual page for more information.

Third question, how can i change the data at specified location? For example my data file like this :

bla1 ddd1
bla2 eee2
bla3 fff3

How can i change the words in second word in second line? (eee2)
Do i have to use bash programming?

You could do it with sed - the stream editor. It's unclear how you with to identify the line... by the line number, by some pattern...? I chose to do it by the pattern eee2 here, but there are other ways depending on how you want it to work.

Maybe not, but it is the easiest.
Did you read the bash tutorial I suggested?

I recommend asking the whole question up front--not in little pieces.

I want to write a bash code but i cannot know how to use bash variable. For example :

#fileoperations.sh

getline()
{
sed -n '$2 p' $1 #???
}

And also in awk..