Removing unwanted lines in text file

Simply:

sed 'n;d' file 

for even lines, and

sed '1!n;d' file 

for odd lines.

With the GNU implementation of sed, you can also use this syntax:

Odd:

sed '1~2d' file 

Even:

sed '2~2d' file 

You can remove said lines of unwanted text by like so

sed -i '/[REGEX]/d' <FILE> 

Where [REGEX] is a regular expression that matches the unwanted line of text, and <FILE> is the name of the file you want to remove the text from. You can nest delete (//d) commands like so:

sed '/[REGEX]/d ; /[REGEX]/d ; /[REGEX]/d' 

Here's an example:

echo "a" > file ; echo "abcd" >> file sed -i '/^a$/d' file cat file # => abcd 

If your version of sed doesn't support the -i option, you can use to following to the same effect.

cat file | sed '/[REGEX]/d' > file_2 && cat file_2 > file && rm file_2 

A word to the wise

Redirecting the output of cat file right back into file will not have the desired effect. It will in fact, truncate file completely, that is it will completely erase file.

So don't do this:

cat file | sed '/[REGEX]/d' > file 

Here is awk solution:

For deleting odd line:

awk 'NR%2==0{ print $0 > "infile" }' infile 

For deleting even lines:

awk 'NR%2{ print $0 > "infile" }' infile 

Note that above commands deletes the related lines in-place from infile input file, so be careful you write the output in to the same input file.

You can write them into another separate file like as following:

This will create the EvenLines file contain even-number of lines:

awk 'NR%2==0{ print $0 > "EvenLines" }' infile 

And this one will create the OddLines file contain odd-number of lines:

awk 'NR%2{ print $0 > "OddLines" }' infile 

sed '0~2d' file 

This command is used to delete the even lines in the files.