How to use 'sed or gawk' to delete a text block until the third line previous the last one

Question

Good day, I was wondering how to delete a text block like this:

1    
2    
3    
4    
5    
6    
7    
8

and delete from the second line until the third line previous the last one, to obtain:

1    
2    
6    
7    
8

Thanks in advance!!!

BTW This text block is just an example, the real text blocks I working on are huge and each one differs among them in the line numbers.

Answer 1

Getting the number of lines with wc and using awk to print the requested range:

$ awk 'NR<M || NR>N-M' M=3 N="$(wc -l file)" file
1
2
6
7
8

This allows you to easily change the range by just changing the value of M.

Answer 2

This might work for you (GNU sed):

sed '3,${:a;$!{N;s/\n/&/3;Ta;D}}' file

or i f you prefer:

sed '1,2b;:a;$!{N;s/\n/&/3;Ta;D}' file

These always print the first two lines, then build a running window of three lines. Unless the end of file is reached the first line is popped off the window and deleted. At the end of file the remaining 3 lines are printed.

Answer 3

since you mentioned huge and also line numbers could be differ. I would suggest this awk one-liner:

awk 'NR<3{print;next}{delete a[NR-3];a[NR]=$0}END{for(x=NR-2;x<=NR;x++)print a[x]}' file

• it processes the input file only once, without (pre) calculating total line numbers
• it stores minimal data in memory, in all processing time, only 3 lines data were stored.
• If you want to change the filtering criteria, for example, removing from line x to $-y, you just simply change the offset in the oneliner.

add a test:

kent$  seq 8|awk 'NR<3{print;next}{delete a[NR-3];a[NR]=$0}END{for(x=NR-2;x<=NR;x++)print a[x]}'
1
2
6
7
8

Answer 4

Using sed:

sed -n '
    ## Append second line, print first two lines and delete them.
    N; 
    p; 
    s/^.*$//;
    ## Read next three lines removing leading newline character inserted
    ## by the "N" command.
    N; 
    s/^\n//; 
    N; 
    :a; 
    N;
    ## I will keep three lines in buffer until last line when I will print
    ## them and exit.
    $ { p; q };
    ## Not last line yet, so remove one line of buffer based in FIFO algorithm.
    s/^[^\n]*\n//; 
    ## Goto label "a".
    ba
' infile

It yields:

1
2
6
7
8