Remove previous line to a match string?

Question

How I can remove the previous line of a match pattern?

Or

the opposite of:

sed -n '/pattern/{g;1!p;};h'

Answer 1

Use tac | sed | tac (Linux/Solaris) then it's next line after a match pattern :)

Answer 2

sed is an excellent tool for simple substitutions on a single line, for anything else just use awk:

$ cat file
here is a
a bad line before
a good line
in a file

$ awk 'NR==FNR{if (/good/) del[NR-1]; next} !(FNR in del)' file file
here is a
a good line
in a file

You can use the above idiom to delete any number of lines before and/or after a given pattern, e.g. to delete the 3 lines before and 2 lines after a given target:

$ cat file
-5
-4
-3
-2
-1
target
+1
+2
+3
+4
+5
$
$ awk 'NR==FNR{if (/target/) for (i=-3;i<=2;i++) del[NR+i]; next} !(FNR in del)' file file
-5
-4
+3
+4
+5

or to leave the target in place and just delete the lines around it:

$ awk 'NR==FNR{if (/target/) for (i=-3;i<=2;i++) if (i!=0) del[NR+i]; next} !(FNR in del)' file file
-5
-4
target
+3
+4
+5

All very clear, trivial, and scalable...