Delete all lines after n lines below the line containing the first match

Question

If a file contains the following:

Line 1
FooLine 5
Line 2
Line 55
Line 3
Line 4
Line 5
Line 6
Line 7
Line 5
Line 8
Line 9

How to delete the lines below two lines after "Line 5", so that the file will now contain only the following?

Line 1
FooLine 5
Line 2
Line 55
Line 3
Line 4
Line 5
Line 6
Line 7

Note: This is a slightly improved and reversed version of the question Delete all lines before n lines above the line containing the first match

Answer 1

With sed:

$ sed '/^Line 5$/{n;n;q;}' file
Line 1
FooLine 5
Line 2
Line 55
Line 3
Line 4
Line 5
Line 6
Line 7

To edit the file in place, add the -i option for GNU sed or compatible or -i '' for FreeBSD sed or compatible.

Answer 2

Using any awk:

$ awk -v n=2 'c&&!--c{exit} $0 == "Line 5"{c=n+1} 1' file
Line 1
FooLine 5
Line 2
Line 55
Line 3
Line 4
Line 5
Line 6
Line 7

See printing-with-sed-or-awk-a-line-following-a-matching-pattern/17914105#17914105 for an explanation and related idioms.

Answer 3

Here are a couple of ways, both of which assume you want to start deleting two lines after a line that contains nothing but the exact string Line 5:

• perl

  $ perl -lne '$k=$. if /^Line 5$/; print; exit if $k and $.==$k+2;' file
  Line 1
  FooLine 5
  Line 2
  Line 55
  Line 3
  Line 4
  Line 5
  Line 6
  Line 7
  

  You can use perl -i -lne '$k=$. if /^Line 5$/; print; exit if $k and $.==$k+2;' file to edit the file in place.

• awk

  awk '(/^Line 5$/){ k=NR } 1; (k && NR==k+2){exit};' file
  Line 1
  FooLine 5
  Line 2
  Line 55
  Line 3
  Line 4
  Line 5
  Line 6
  Line 7
  

Answer 4

Using awk:

$ awk -v var=2 '1;/^Line 5$/{c=NR+var}NR==c{exit}'

Using 2 pass way

$ awk -v var=2 
    '
    pass==1 && ($0 == "Line 5"){c=FNR+var; nextfile} 
    pass==2 && FNR<=c
    '
 pass=1 file pass=2 file

---

Or using ed slightly changed:

$ printf '%s\n' '1,/^Line 5$/2p'  | ed -s file

Answer 5

Using Raku (formerly known as Perl_6)

~$ raku -pe 'if m/^Line \h 5$/ {.put; get.put xx 2; last};'  file

OR:

~$ raku -pe 'm/^Line \h 5$/ && .put && get.put xx 2 && last;'  file 

OR:

~$ raku -ne 'm/^Line \h 5$/ ?? (.put; get.put xx 2; last) !! .put;'  file

Here are answers coded in Raku, a member of the Perl-family of programming languages. Raku features high-level support for Unicode, built-in.

In the first pair of answers, the -pe autoprinting linewise flags are used. Lines will autoprint until the desired line is read, as determined via Regex. At that point the desired line is output, the code instructs to get two more lines and output them, then that's the last (i.e. the code exits). Note, for the second of these two examples andthen can be used instead of &&.

In the third answer, the -ne non-autoprinting linewise flags are used. Each line is read into Raku's Test ?? True !! False ternary operator to find the desired line via Regex. If the desired line is True, it is output, the code instructs to get two more lines and output them, then that's the last (i.e. the code exits). If the desired line via Regex is False, it is simply output and the linewise reading continues, potentially to the end of the file.

Sample Input:

Line 1
FooLine 5
Line 2
Line 55
Line 3
Line 4
Line 5
Line 6
Line 7
Line 5
Line 8
Line 9

Sample Output:

Line 1
FooLine 5
Line 2
Line 55
Line 3
Line 4
Line 5
Line 6
Line 7

---

NOTE: With all three answers above, blank lines are handled fine, but the code will throw an error if you try to get.put return lines that don't exist (e.g. if you're too close to the end of the file).

To handle this more gracefully (return whatever lines are available after a matching line, up to the requested number), use the following code which is similar to @terdon's Perl answer. Below set $k to equal "n+1" lines to return the current line plus "n"-lines that follow ($k=3 gives same output as above):

~$ raku -ne 'my $k=3 if m/^Line \h 5$/; $k.put && --$k; last unless $k;'  file

https://docs.raku.org
https://raku.org