Hello. I want to append a string with value "NULL" only to the end of the registers which end with semicolon:

The command I´m using is:

It doesn´t work, I don´t know what am I doing wrong.
Any ideas of why it´s not working? Thanks for the help.

If you mean "lines" when you say "registers", then it works fine here. Note that the "g" has no function because you are matching only once--ie at the end of the line.

Another way of doing the same thing:
sed '/;$/s/$/NULL/' filename

If you have data in multiple columns, then you may need something like AWK.

Please post an example of the data, and what is not working correctly.

Thanks. I am making a script for parsing a raw file text with database registers into a file with INSERT INTO sql instructions.

The plain text document may have something like this:
id;name;last name; address; telephone; cellphone; email
Some of the atributes may have null values, like this:
id;name;;address;;cellphone;

In this case I need to fill with the word NULL the null fields so I can later create an INSERT INTO sql instruction later on (using awk), such like this:

id;name;NULL;address;NULL;cellphone;NULL

I have been able to insert every NULL string except for the one at the end.

The problem is that when I use both commands I am still getting
1;... ; instead of 1; .... ;NULL

When you say "both commands", I'm not sure what you are referring to.

Here's one crude way to do it:
sed -e 's/;;/;NULL;/g' -e 's/;$/;NULL/' filename > newfilename

Really good SED tutorial (and much more) here:
http://www.grymoire.com/Unix/Sed.html

Bash documents here:
http://tldp.org

If the file was created under Windows/DOS it could have line endings CRLF, i.e. \r\n.
Unix files use only LF, i.e. \n
If this is the cause of the problem and you have GNU sed try:-

I just realized that the last character of the line was not the semicolon, even though it seemed to be, that is why the suggestions given above did not work. As you said, this file was created in windows and there were several spaces between the semicolon and the end of line.
I removed those spaces with this:
And then I put the NULL string at the end of every line ended with semicolon with this:

You can also use [[:space:]] to remove all whitespace---it works on both spaces and tabs. I have not tried it for your particular example.