如何在不影响标题的情况下使用此awk命令

Question

Good nigt. I have this two files:

File 1 - with phenotype informations, the first column are the Ids, the orinal file has 400 rows:

ID  a b  c          d 
215 2 25 13.8354303 15.2841303
222 2 25.2 15.8507278 17.2994278
216 2 28.2 13.0482192 14.4969192
223 11 15.4 9.2714745 11.6494745

File 2 - with SNPs information, the original file has 400 lines and 42,000 characters per line.

ID  t u j l
215 2 0 2 1 
222 2 0 1 1 
216 2 0 2 1 
223 2 0 2 2 
217 2 0 2 1 
218 0 2 0 2 

And I need to remove from file 2 individuals that do not appear in the file 1, for example:

ID  t u j l
215 2 0 2 1 
222 2 0 1 1 
216 2 0 2 1 
223 2 0 2 2

I used this code:

awk 'NR==FNR{a[$1]; next}$1 in a{print $0}' file2 file1 > file3

and I can get this output(file 3):

215 2 0 2 1 
222 2 0 1 1 
216 2 0 2 1 
223 2 0 2 2

but I lose the header, how do I not lose the header?

Answer 1

To keep the header of the second file, add a condition{action} like this:

awk 'NR==FNR {a[$1]; next}
     FNR==1  {print $0; next}  # <= this will print the header of file2.
     $1 in a {print $0}' file1 file2

NR holds the total record number while FNR is the file record number, it counts the records of the file currently being processed. Also the next statements are important, so that to continue with the next record and don't try the rest of the actions.