How can I use text-processing tools to insert a new line after every N lines?
Example for N=2:
INPUT:
sadf asdf yxcv cxv eqrt asdf OUTPUT:
sadf asdf yxcv cxv eqrt asdf 
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6 Answers
With awk:
awk ' {print;} NR % 2 == 0 { print ""; }' inputfile With sed (GNU extension):
sed '0~2 a\\' inputfile With bash:
#!/bin/bash lines=0 while IFS= read -r line do printf '%s\n' "${line}" ((lines++ % 2)) && echo done < "$1" 
- 2Arithmetic evaluation can be used directly as condition, no need for the
[[ ]]to test:while read line; do echo "$line"; ((lines++ % 2)) && echo; done.manatwork– manatwork2011-10-29 14:57:40 +00:00Commented Oct 29, 2011 at 14:57 - 2Be aware that the above documented command
sed '0~2 a\ 'adds a space to each inserted newline. If you wanted to add a newline after each line, any of these similarly work:sed '0~1 a\ ',sed 'a\ ', or justsed G.Asclepius– Asclepius2013-11-07 19:20:54 +00:00Commented Nov 7, 2013 at 19:20
sed (GNU)
With (GNU) sed:
sed '0~2G' Short (ugly for N=100):
sed 'n;G' man sed explains ~ as:
first ~ step
Match every step'th line starting with line first. For example, ``sed -n 1~2p'' will print all the odd-numbered lines in the input stream, and the address 2~5 will match every fifth line, starting with the second. first can be zero; in this case, sed operates as if it were equal to step. (This is an extension.)
sed (other)
With other sed (Count new lines):
sed -e 'p;s/.*//;H;x;/\n\{2\}/{g;p};x;d' Or, to be more portable, written as (remove comments for some versions of sed) :
sed -e ' # Start a sed script. p # Whatever happens later, print the line. s/.*// # Clean the pattern space. H # Add **one** newline to hold space. x # Get the hold space to examine it, now is empty. /\n\{2\}/{ # Test if there are 2 new lines counted. g # Erase the newline count. p # Print an additional new line. } # End the test. x # match the `x` done above. d # don't print anything else. Re-start. ' # End sed script. awk
With awk, probably:
awk '1 ; NR%2==0 {printf"\n"} ' Using paste
paste -d'\n' - - /dev/null <file 
sed n\;G <infile ... is all you need ...
For example:
seq 6 | sed n\;G OUTPUT:
1 2 3 4 5 6 ...(and a blank follows the 6 as well)...or...
seq 5 | sed n\;G OUTPUT:
1 2 3 4 5 (and no blank follows the 5)
If a blank should always be omitted in a last line case:
sed 'n;$!G' <infile 
Another awk flavour:
awk '{ l=$0; getline; printf("%s\n%s\n\n", l, $0) }' 
- 1"This answer was flagged as low-quality because of its length and content." I'll accept it... ;-)Hauke Laging– Hauke Laging2015-01-27 10:31:29 +00:00Commented Jan 27, 2015 at 10:31
Using Perl
~$ perl -ne 'print $_; if ($. % 2 == 0) { print "\n"};' file Using Raku (formerly known as Perl_6)
~$ raku -ne '.put; if (++$ %% 2) { print "\n"};' file The Perl answer uses $_ which is Perl's topic variable, and $. which is Perl's line-number variable. If the line number % modulo 2 equals zero, insert a \n newline.
The Raku answer uses $_ but in an abbreviated form: .put is short for $_.put. The Raku answer uses ++$ an anonymous pre-incrementing counter variable. Raku's %% 'Divisibility operator' is set to return True if ++$ % 2 == 0, so again (like the Perl answer), a \n newline is inserted.
Sample Input:
sadf asdf yxcv cxv eqrt asdf Sample Output:
sadf asdf yxcv cxv eqrt asdf Perl References:
https://perldoc.perl.org
https://www.perl.org
Raku References:
https://docs.raku.org
https://raku.org
