Remove odd or even lines from a text file

Question

I need to remove odd lines in a text file to make a down-sampling. I've found this command,

awk 'NR%2==0' file 

but it only prints the odd lines in the terminal. How to really remove them?

I don't really care for even or odd, I want them removed from the file or printed in another file. This only prints them in the terminal.

Answer 1

awk

The % is a modulus operator and NR is the current line number, so NR%2==0 is true only for even lines and will invoke the default rule for them ({ print $0 }). Thus to save only the even lines, redirect the output from awk to a new file:

awk 'NR%2==0' infile > outfile 

sed

You can accomplish the same thing with sed. devnulls answer shows how to do it with GNU sed. Below are alternatives for versions of sed that do not have the ~ operator:

keep odd lines

sed 'n; d' infile > outfile 

keep even lines

sed '1d; n; d' infile > outfile 

Answer 2

Using GNU sed:

sed -i '0~2d' filename 

to remove the even numbered lines from the file.

For removing odd numbered lines:

sed -i '1~2d' filename 

The -i option would cause the changes to be saved to the file in-place.

Quoting from the manual:

`FIRST~STEP'      This GNU extension matches every STEPth line starting with line      FIRST.  In particular, lines will be selected when there exists a      non-negative N such that the current line-number equals FIRST + (N      * STEP).  Thus, to select the odd-numbered lines, one would use      `1~2'; to pick every third line starting with the second, `2~3'      would be used; to pick every fifth line starting with the tenth,      use `10~5'; and `50~0' is just an obscure way of saying `50'.