Dear All,

I have text file like this:

Header
Record 1
Record 2
.......
Record n
Tail

This line of code :
awk '{ if ( NR == 1 ) { head=substr($0,1,300);} else { last = substr($0,1,300);}END{printf "Header is : %-300s Trailer is : %-300s\n", head, last}' filename

converted Header and Trailer in one line and within 300 character each.The output is just one line.

Now I want to overwrite this one line above (Header+ Tail) over the first line of original file (orginal header) and delete the Tail........i can copy this header and tail to new file and then append Record 1 to Record n to this new file, but that will cause too low performance........so I think better to update first and the last line of the this file.................so how can i do that?

i.e. 1-update the header with new line i.e. (header+trailer)
2-delete the tail in original file (may be using cut tail -1 filename?)