Print several lines after nth occurence in bash

awkgrepperltext processing

I'm looking for a way to search for the nth occurrence of a pattern and print k lines after the pattern. I guess awk would work better than grep in this case but I cannot figure out the way to do it properly …

Say I have the following text file:

Draft  
blablablabla  
tralalalalala  
Draft  
blablablabla  
tralalalalala  
Draft  
important line 1  
important line 2  
Draft   
blablablabla   
tralalalalala

In this case n=3 and k=2, I want to print the 2 lines following the 3th occurrence of the pattern "Draft". In my specific case, n and k may vary.

Is it a simple way to do this ?

Best Answer

You can use grep and tail to achieve this:

$ n=3
$ k=2
$ grep -m "$n" -A "$k" 'Draft' input.txt | tail -n "$k"
important line 1  
important line 2  
$

The -m "$n" option to grep specifies to stop after the nth match, and -A "$k" tells grep to output k lines from after each match. We then pipe this to tail -b "$k" to output only those k lines.

Related Solutions

Text Processing – Print Line After nth Occurrence of a Match

awk -v n=3 '/<Car>/ && !--n {getline; print; exit}'

Or:

awk '/<Car>/ && ++n == 3 {getline; print; exit}'

To pass the search pattern as a variable:

var='<car>'
PATTERN="$var" awk -v n=3 '
  $0 ~ ENVIRON["PATTERN"] && ++n == 3 {getline; print; exit}'

Here using ENVIRON instead of -v as -v expands backslash-escape sequences and backslashes are often found in regular expressions (so would need to be doubled with -v).

GNU awk 4.2 or above lets you assign variables as strong typed regexps. As long as its POSIX mode is not enabled (for instance via the $POSIXLY_CORRECT environment variable, you can do:

# GNU awk 4.2 or above only, when not in POSIX mode
gawk -v n=3 -v pattern="@/$var/" '
  $0 ~ pattern && ++n == 3 {getline; print; exit}'

Search a pattern and print preceding lines starting with another pattern

Here's a solution in Perl:

perl -nlE '
    if    (/a/)   { @buffer = ($_) }
    elsif (/xyz/) { push @buffer,$_; say for @buffer }
    else          { push @buffer,$_}
' your_file

How this works

It reads through the file line-by-line and does one of three things:

If the current line matches the pattern a, it assigns the current line to the @buffer array.
If the current line matches the pattern xyz, it pushes the current line onto the buffer and prints the contents of the buffer
If none of the two cases above is true, it simply appends the current line to the @buffer array.

Thus, whenever a new line matches the pattern a, the contents of the @buffer are erased and replaced by the current line only. This guarantees you will find the closest a preceding xyz.

You should of course replace the regexes I used with the actual regexes relevant to your case.

Best Answer

Related Solutions

Text Processing – Print Line After nth Occurrence of a Match

Search a pattern and print preceding lines starting with another pattern

Related Question