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I'm trying to change the order of lines in a specific pattern. Working with a file with many lines (ex. 99 lines). For every three lines, I would like the second line to be the third line, and the third to be the second line.
EXAMPLE.
1- Input:
gi_1234
My cat is blue.
I have a cat.
gi_5678
My dog is orange.
I also have a dog.
...
2- Output:
gi_1234
I have a cat.
My cat is blue.
gi_5678
I also have a dog.
My dog is orange.
...
$ seq 9 | sed -n 'p;n;h;n;G;p'
1
3
2
4
6
5
7
9
8
That is, print the current line, get the next one, hold it, get the next one, Get the held line (append it to the pattern space) and print that 2-line pattern space with the third and second lines swapped.
Using awk and integer maths:
awk 'NR%3 == 1 { print } NR%3 == 2 { delay=$0 } NR%3 == 0 { print; print delay; delay=""} END { if(length(delay) != 0 ) { print delay } }' /path/to/input
The modulus operator performs integer division and returns the remainder, so for each line, it will return the sequence 1, 2, 0, 1, 2, 0 [...]. Knowing that, we just save the input on lines where the modulus is 2 for later -- to wit, just after printing the input when it's zero.
NR%3 == 0 { print; print delay; delay=""} END { if(length(delay) != 0 ) { print delay }.
Jun 1, 2017 at 15:14
Another awk approach:
awk '{print $0; if ((getline L2)>0 && (getline L3)>0){ print L3 ORS L2 }}' file
The output:
gi_1234
I have a cat.
My cat is blue.
gi_5678
I also have a dog.
My dog is orange.
(getline L2)>0 && (getline L3)>0 - extracts next 2 records if they exist
each 2nd and 3rd records are assigned to L2 and L3 variables respectively
line2, etc.
Jun 1, 2017 at 16:23
Using perl and a short script:
user@pc:~$ cat input.txt
gi_1234
My cat is blue.
I have a cat.
gi_5678
My dog is orange.
I also have a dog.
user@pc:~$ perl -ne '$l2=<>; $l3=<>; print $_,$l3,$l2;' input.txt
gi_1234
I have a cat.
My cat is blue.
gi_5678
I also have a dog.
My dog is orange.
The script process the whole file, for each line (stored in $_) it will get the next two lines ($l2 and $l3) and print them in the requested order: line1, line3, line2.
One way could be as follows:
sed -e '
/\n/s/\(.*\)\(\n\)\(.*\)/\3\2\1/;//b
$!N;$q;N; # load up the pattern space with 3 lines provided eof not reached
P;D; # first just print the first line then interchange the two and print them
' yourfile
Alternatively,
perl -ne 'print $_, reverse scalar <>, scalar <>' yourfile
gi_1234
I have a cat.
My cat is blue.
gi_5678
I also have a dog.
My dog is orange.
Why not just make a while loop? In expanded form:
( while read a
do
read b
read c
echo "$a"
echo "$c"
echo "$b"
done
) < input.txt
In "single line format":
( while read a ; do read b ; read c ; echo "$a" ; echo "$c" ; echo "$b" ; done) < input.txt
Outputs:
gi_1234
I have a cat.
My cat is blue.
gi_5678
I also have a dog.
My dog is orange.
perl -ne 'print if $.%3==1;$var=$_ if $.%3==2;print $_ . $var if $.%3==0' input.txt
The idea here is that we use modulo operator % with line number $. variable, to figure out which one is every first, which one is every second, and which one is every 3rd line. For every 3rd line remainder is 0, while for every 1st and 2nd line it will have corresponding numbers.
Test:
$ cat input.txt
gi_1234
My cat is blue.
I have a cat.
gi_5678
My dog is orange.
I also have a dog.
$ perl -ne 'print if $.%3==1;$var=$_ if $.%3==2;print $_ . $var if $.%3==0' input.txt
gi_1234
I have a cat.
My cat is blue.
gi_5678
I also have a dog.
My dog is orange.
The approach with storing second line into a variable has a flaw. What if the last line is the "second" one , i.e. for that line number remainder is 2 ? The original code in my and DopeGhoti's answer won't print My dog is orange if we leave out the last line. The fix for that in both cases is to use END{} code block, with unsetting the temporary variable after printing. In other words:
$ awk 'NR%3 == 1 { print } NR%3 == 2 { delay=$0 } NR%3 == 0 { print; print delay;delay=""}END{print delay}' input.txt
and
$ perl -ne '$s=$_ if $.%3==2;print $_ . $s and $s="" if $.%3==0 or $.%3==1;END{print $s}' input.txt
This way, the code will work for arbitrary number of lines in a file, not just those divisible by 3.
In awk's case, if the last line in the file produces output of 1 for $. % 3, the previous code has issue of outputting blank newline because of unconditional printing of END{print delay}, since print function mentioned in the comments always appends newline to whatever variable it is operating on. In case of perl version this issue doesn't occur, since with -ne flags print function doesn't append the newline.
Nonetheless, the fix in awk's case is to make conditional, as mentioned by Dope Ghoti in the comments is to verify the length of the temporary variable. The perl version of the same fix would be:
$ perl -ne '$s=$_ if $.%3==2;print $_ . $s and $s="" if $.%3==0 or $.%3==1;END{print $s if length $s}' input.txt
awk) NR%3 == 0 { print; print delay; delay=""} END { if(length(delay) != 0 ) { print delay }.
Jun 1, 2017 at 15:22
-ne flags doesn't output a newline. It does indeed print, but it's a null string, no trailing newline. Nontheless, I've added the mention of the issue and the same fix into my answer. Thanks !
Jun 1, 2017 at 22:28
Not suitable for long files, but still handy if you were just editing a file and wanted, for example, to reorder some yaml stanzas.
First record a macro:
gg qq j ddp j q
And then repeat desired number of times:
@q @q @q ...
Or just e.g.
3@q
Explanation:
@q @q @q, it is possible doing by this way 3@q - repeat three times. 100@q - repeat the macro 100 times.
Usage: ./shuffle_lines.awk input.txt
Check shebang #!/usr/bin/awk -f, because the awk location may differ on your system.
#!/usr/bin/awk -f
{
if ((NR + 1) % 3 == 0) {
buffer = $0;
} else if (NR % 3 == 0) {
print $0 ORS buffer;
buffer = "";
} else {
print;
}
}
