Extract Nth line after matching pattern

Question

I want to extract the Nth line after a matching pattern using grep, awk or sed.

For example I have this piece of text:

Answer 1

Printing the lnb^th line after first blank/empty line:

Index of line to print (in bash shell):

lnb=2

• Using sed:

  sed -ne '/^\s*$/{:a;n;0~'"$lnb"'!ba;p;q}' my_file`
  

• Using perl:

  perl -ne '/^\s+$/ && $k++;$k!=0 && $k++ && $k=='"$lnb"'+2 && (print,last)' my_file`
  

Printing the lnb^th line after regular-expression match:

• Using sed:

  sed -ne '/regex/{:a;n;0~'"$lnb"'!ba;p;q}' my_file
  

• Using perl:

  perl -ne '/regex/ && $k++;$k!=0 && $k++ && $k=='"$lnb"'+2 && (print,last)' my_file
  

  Bonus 1, Windows PowerShell (install Perl first) :

  $lnb=2
  
  perl -ne "/regex/ && `$k++;`$k!=0 && `$k++ && `$k==$lnb+2 && (print,last)" my_file
  

  Bonus 2, Windows DOS command line :

  set lnb=2
  
  perl -ne "/regex/ && $k++;$k!=0 && $k++ && $k==%lnb%+2 && (print,last)" my_file
  

Printing ALL lnb^th lines after regular-expression match:

• Using perl(bash example):

  perl -ne '/regex/ && $k++;$k!=0 && $k++ && $k=='"$lnb"'+2 && (print,$k=0)' my_file
  

Answer 2

To extract the Nth line after a matching pattern you want:

awk 'c&&!--c;/pattern/{c=N}' file

e.g.

awk 'c&&!--c;/Revision:/{c=5}' file

would print the 5th line after the text "Revision:"/.

FYI the following idioms describe how to select a range of records given a specific pattern to match:

a) Print all records from some pattern:

awk '/pattern/{f=1}f' file

b) Print all records after some pattern:

awk 'f;/pattern/{f=1}' file

c) Print the Nth record after some pattern:

awk 'c&&!--c;/pattern/{c=N}' file

d) Print every record except the Nth record after some pattern:

awk 'c&&!--c{next}/pattern/{c=N}1' file

e) Print the N records after some pattern:

awk 'c&&c--;/pattern/{c=N}' file

f) Print every record except the N records after some pattern:

awk 'c&&c--{next}/pattern/{c=N}1' file

g) Print the N records from some pattern:

awk '/pattern/{c=N}c&&c--' file

I changed the variable name from "f" for "found" to "c" for "count" where appropriate as that's more expressive of what the variable actually IS.

Answer 3

I like solutions I can learn to redo, without having to google for it every time. This solution is not perfect, but uses simple grep commands that I can write from memory.

grep -A7 "searchpattern" file | grep -B1 "^--$" | grep -v "^--$"

You can change the 7 to the nth line you want after the search pattern. It then searches for the "group separator" -- and shows the last line before that. Then remove the group separators.

The only case this does not work correctly is if your data contains lines with only "--".