Looking for way to move even lines to the beginning of odd lines

Question

I am trying to figure out how to move all even lines to the beginning of all odd lines as follows:

before:

AAA
BBB
CCC
DDD

after:

BBBAAA
DDDCCC

Something I can run on multiple files at once would be great since combining all the files using cat results in a file that is too big to deal with. Something similar to the following command for doing text swapping so that I can add it to a bash file:

  find /directory/. -type f -exec sed -i 's/oldtext/newtext/g' {} \;

I have searched but I haven't found something suitable.

To make things easier I can add 'string1' to the beginning of all odd lines and 'string2' to the beginning of all even lines as follows:

  before:
  string1AAA
  string2BBB
  string1CCC
  string2DDD

  after:
  string2BBBstring1AAA
  string2DDDstring1CCC 

Big thanks.

Answer 1

Forget about the idea with the prefix. The script you are looking for is

sed 'N;s/\(.*\)\n\(.*\)/\2\1/'

That is, read the next line with N, then change order and remove the newline.

And you can combine this with the find as you wrote. Try without option -i first to test whether this is what you want.

Answer 2

gawk solution:

for f in /directory/*; do gawk -i inplace '{getline nl; print nl$0}' "$f"; done

---

for f in /directory/* - for each file in specified path

gawk -i inplace - enable in place file editing

getline nl - read the next record from awk's input into variable nl (next line)

print nl$0 - print the current and the next record swapped

Answer 3

With GNU sed:

sed '1~2 { h; d; }; G; s/\n//'

With awk:

awk '{ if(NR%2) save = $0; else print $0 save }'

With perl:

perl -pe '$_ = <> . $_; s/\n//'

Answer 4

To complete RomanPrekhrest's answer, if you have and old version of gawk like me that does not support inplace edition, or have the getline instruction:

find /path/to/directory/ -exec awk 'NR%2!=0{line=$0; next}{print line $0}END{if(NR%2!=0) print line}' {} > /tmp/mytmpfile.txt \; -exec mv /tmp/mytmpfile.txt {} \;

Also added something to handle files with an odd number of lines.

Answer 5

eval "`echo 'NL=qsq' | tr 'qs' '\047\012'`"; # newline
set -- 
while IFS= read -r l; do
   case $# in
      0 ) set -- "$l" ;;
      * ) printf '%s%s\n' "$l" "$1"; set -- ;;
   esac
done < input.data
printf '%s' ${1+"$1$NL"} # to take care of dangling line

awk '{ if ( NR % 2 ) ORS = $0 RS; else print }' input.data

perl -lpe '$_ = (<>.$_) =~ s/\n//r' input.data

sed -ne '
   ${p;d;}
   h;n;G;s/\n//p
' input.data

Result

BBBAAA
DDDCCC