Shell Programming and Scripting

Ok here's my pickle. I have a file in which every line must be the same length. Each field within the line is a certain length. None of these can be changed. What I need to do is look at a specific field within this file, let's say it starts with character 30 and ends with 50. If this field is empty, I need it to be replaced with a bit of text.

An example. Note that field 3 below in the first line is blank. I need to make it look like line2. Also, it should ignore the line if there is anything at all in the field as in line 2 and 3.

Code:

field1field2                       field4
field1field2field3                field4
field1field2sometext           field4

Any help would be appreciated as I'm horrible with sed.

Could you please also show us the output so that it would be helpful for us to provide the code. how many characters would be there in field 3? do you want to remove all the white spaces?

Unfortunately I can't provide an example, it's confidential patient data and can't exactly be sanitized. I can however provide what character/column the field begins and ends at. The field starts at colum 308 and ends at column 327. Instead of being blank, I want the field to contain "See Free Text" without the quotes.

No I do not want to change the size of the line in any way. The field, nor the line containing the field can not change in size, otherwise the importer has a fit.

FYI, I figured it out through use of another resource. The following is what did the trick, just in case anyone else needs to know.

perl -pe 's/^(.{307})\s{20}/$1SEE FREE TEXT /' input.file > output.file