How to delete every second line from a file?

Question

File:

Data inserted into table. Total count 13
No error occurred
Data inserted into table. Total count 45
No error occurred
Data inserted into table. Total count 14
No error occurred
Data inserted into table. Total count 90
No error occurred

Expected output file:

Data inserted into table. Total count 13
Data inserted into table. Total count 45
Data inserted into table. Total count 14
Data inserted into table. Total count 90

I want the output to look this way: every second line will be deleted but there will be no gap between lines.

Answer 1

Solving this by deleting every second line can be error prone (for example, when process sometimes generates two meaningful lines instead one). May be it is better to filter out the garbage:

grep -v "No error occurred" file

It can run as filter, you can add more garbage patterns here and improve the result.

Answer 2

With sed:

sed -e n\;d <file

With POSIX awk:

awk 'FNR%2' <file

If you have older awk (like oawk), you need:

oawk 'NR%2 == 1' <file

With ex:

$ ex file <<\EX
:g/$/+d
:wq!
EX

will edit the file in-place.

• g mark a global command
• /$/ match every lines
• +d delete the next line
• wq! save all changes

This approach share the same ideal with sed approach, delete every next line of current line start from line 1.

With perl:

perl -ne 'print if $. % 2' <file

and raku:

raku -ne '.say if $*IN.ins % 2' <file
raku -ne '.say if ++$ % 2' <file

Edit

Raku IO::Handle.ins was removed in this commit.

Answer 3

Ассоrding to the question, with GNU sed:

sed '0~2d' file

will delete every second line but I'd like to offer filter lines by it content:

sed '/Data/! d' file

or with same result

sed '/No error/d' file

Answer 4

Here is a way using sed:

sed -n 'p;n' filename

Another way with GNU sed:

sed -n '1~2p' filename

Output from above commands:

Data inserted into table. Total count 13
Data inserted into table. Total count 45
Data inserted into table. Total count 14
Data inserted into table. Total count 90

Answer 5

Another answer, you could use vi/vim!

qdjddq

And then if your file was 500 lines (for example) type

250@d

And then to write and exit type

:x

Or if something goes wrong and you don't want to save:

:q!

Explanation:

q      #Start Recording
 d     #Put the recording into register 'd'
  j    #Move the cursor down
   dd  #Delete the line
     q #Stop recording

250    #Number of repeats
   @d  #Playback the recording in register 'd'.

Answer 6

You can try with awk:

awk 'NR % 2 != 0' file

or you can print only lines containing Data inserted:

awk '$0 ~ /Data inserted/' file

Answer 7

Here's quite a different way of doing it:

< file paste - - | cut -f1

This assumes that the odd-numbered lines do not contain tabs. If they do, then you'll need to pick another separator character, e.g. : here:

< file paste -d: - - | cut -d: -f1

Answer 8

Another option (shorter)

sed 'n; d' file

Answer 9

It also solves the problem, though it is a bit slower:

vim -c "%normal jdd" -c "wq" file

Answer 10

Using Raku (formerly known as Perl_6)

raku -pe 'get;' file

Sample Input:

Data inserted into table. Total count 13
No error occurred
Data inserted into table. Total count 45
No error occurred
Data inserted into table. Total count 14
No error occurred
Data inserted into table. Total count 90
No error occurred

Sample Output:

Data inserted into table. Total count 13
Data inserted into table. Total count 45
Data inserted into table. Total count 14
Data inserted into table. Total count 90

There's already an answer using Raku from @cuonglm, which uses the one-liner raku -ne '.say if ++$ % 2'. Above is a different approach using Raku's get routine instead. In combination with the -pe (autoprinting) command-line flags, Raku's get routine advances the readline-cursor by a line, which (because -pe prints return values not the $_ topic variable) results in that line get-ting discarded. Only the first, third, fifth, etc. lines are printed.

Need to print the second, fourth, sixth lines, etc. instead of above? Just use Raku's -ne command-line flags. Here Raku's get routine advances the readline-cursor by a line, and then .put is called, giving the following output:

~$ raku -ne 'get.put;'  file
No error occurred
No error occurred
No error occurred
No error occurred

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