Remove odd or even lines from a text file
This might work for you (both GNU and non-GNU sed):
sed -n 'p;n' file # keep odd
sed -n 'n;p' file # keep even
-n
: suppress printing
p
: print current line
n
: next line
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'.
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