Sed – How to Delete the Last Line from a File

Here's one way using GNU sed:

sed -n '/--/,/Content-Length: 0/ { H; /Content-Length: 0/ { g; s/\n\(.*\)\n.*--.*/\1/p } }'

Result:

line 1 "--"  
line 2   
line 3 "Content-Length: 20"  

Explanation:

Match between the pattern range. Append this range to the hold space. On the last
line of the range, copy the hold space to pattern space to work with it. Then use
find/replace regex to remove everything after the last occurrence of '--'. HTH.

You could use awk for this I believe like so:

awk -vN=3 '/PATTERN/ {skips[FNR+N]=1;} {if(!(FNR in skips)) print;}' <file>

so each time we hit PATTERN we'll record the line that is N away from here, and only print those lines we have not marked for skipping.

with gawk you could use -i inplace as well to do it in place

As you noted, that wouldn't handle multiple files. Of course, you could wrap with a for loop to iterate over all the files, but if there aren't enough to make the commandline too long you could also do it like so:

 awk -vN=3 '{if(FNR==1) split("", skips, ":");} /PATTERN/ {skips[FNR+N]=1;} {if(!(FNR in skips)) print;}' *

where we reset skips to an empty array each time FNR hits 1, so the start of each file.
With gnu awk you could write it as:

gawk -i inplace 'FNR==1{delete nr};/PATTERN/{nr[FNR+3]++};!(FNR in nr)' file*