Csdrhrt

I know how to merge two tables to print alternative lines in a new file, but I want to merge every two lines from file1.txt with one line from file2.txt. As an example:

file1.txt is

A a aa

B b bb 

C c cc

D d dd

and file2.txt is

E e ee 

F f ff

I want to have

A a aa

B b bb 

E e ee 

C c cc

D d dd

F f ff

Using GNU sed, you can read and insert one line from the second file after every other line ("two skip two") of the first:

$ sed '2~2R file2.txt' file1.txt

A a aa

B b bb 

E e ee 

C c cc

D d dd

F f ff

Slightly more long-winded implementation of the same in awk:

awk '{print} !(NR%2) {if ((getline < "file2.txt") > -1) print}' file1.txt

You can use the paste command for this:

% paste -d 'n' - - file2.txt < file1.txt

A a aa

B b bb 

E e ee 

C c cc

D d dd

F f ff

paste, by default, takes lines from each input file, then merges them into a single line separated by tabs. Using -d 'n', we tell it to use newlines as the separator instead. - is standard input, which is redirected in from file1.txt (< file1.txt). So paste takes one line from standard input, then another line from standard input (effectively two lines from file1.txt, then a line from file2.txt and prints them out separated by newlines.

Original shellscript

I thought the following script would do it. The trick is to open the two files on separate file descriptors (here #4 and #5).

#!/bin/bash



while read -r -u 4 line1 && read -r -u 4 line2 && read -r -u 5 line3

do

  echo "$line1"

  echo "$line2"

  echo "$line3"

done 4<file1.txt 5<file2.txt

Test

Make the script executable and run it,

$ ./script 

A a aa

B b bb

E e ee

C c cc

D d dd

F f ff

The original script stops, when reading to the end of file in one of the input files (so that only matching 2+1 lines are written to the output).

Modified shellscript

The following script will continue reading and writing until all the files have reached end of file (so that all available lines are written to the output). This should match what the OP wants.

#!/bin/bash



end1=false

end2=false

cont=true

while $cont

do

 read -r -u 4 line1

 if [ $? -eq 0 ]

 then

  echo "$line1"

 else

  end1=true

 fi



 read -r -u 4 line2

 if [ $? -eq 0 ]

 then

  echo "$line2"

 else

  end1=true

 fi



 read -r -u 5 line3

 if [ $? -eq 0 ]

 then

  echo "$line3"

 else

  end2=true

 fi



 if $end1 && $end2  # modify this line to change when to stop

 then

  cont=false

 fi

done 4<file1.txt 5<file2.txt