What are NR and FNR and what does NR FNR imply

Question

I am learning file comparison using awk   I found syntax like below   awk  NR  FNR a  1  next  1 in a print  1   file1 file2   I couldn t understand what is the significance of NR  FNR in this  If I try with FNR  NR then also I get the same output    What exactly does it do

User · Answer

Look up NR and FNR in the awk manual and then ask yourself what is the condition under which NR  FNR in the following example     cat file1 a b c    cat file2 d e    awk   print FILENAME  NR  FNR   0   file1 file2 file1 1 1 a file1 2 2 b file1 3 3 c file2 4 1 d file2 5 2 e

User · Answer

Look for keys  first word of line  in file2 that are also in file1  Step 1  fill array a with the first words of file 1   awk   a  1     file1   Step 2  Fill array a and ignore file 2 in the same command  For this check the total number of records until now with the number of the current input file   awk  NR  FNR a  1    file1 file2   Step 3  Ignore actions that might come after   when parsing file 1  awk  NR  FNR a  1  next   file1 file2    Step 4  print key of file2 when found in the array a  awk  NR  FNR a  1  next   1 in a print  1   file1 file2

User · Answer

Assuming you have Files a txt and b txt with  cat a txt a b c d 1 3 5 cat b txt a 1 2 6 7   Keep in mind NR and FNR are awk built-in variables  NR  - Gives the total number of records processed   in this case both in a txt and b txt  FNR - Gives the total number of records for each input file  records in either a txt or b txt   awk  NR  FNR a  0    if  0 in a print FILENAME     NR     FNR      0   a txt b txt a txt 1 1 a a txt 2 2 b a txt 3 3 c a txt 4 4 d a txt 5 5 1 a txt 6 6 3 a txt 7 7 5 b txt 8 1 a b txt 9 2 1   lets Add  next  to skip the first matched with NR  FNR  in b txt and in a txt  awk  NR  FNR a  0  next  if  0 in a print FILENAME     NR     FNR      0   a txt b txt b txt 8 1 a b txt 9 2 1   in b txt but not in a txt   awk  NR  FNR a  0  next  if    0 in a  print FILENAME     NR     FNR      0   a txt b txt b txt 10 3 2 b txt 11 4 6 b txt 12 5 7  awk  NR  FNR a  0  next    0 in a   a txt b txt 2 6 7

User · Answer

In awk  FNR refers to the record number  typically the line number  in the current file and NR refers to the total record number  The operator    is a comparison operator  which returns true when the two surrounding operands are equal   This means that the condition NR  FNR is only true for the first file  as FNR resets back to 1 for the first line of each file but NR keeps on increasing   This pattern is typically used to perform actions on only the first file  The next inside the block means any further commands are skipped  so they are only run on files other than the first   The condition FNR  NR compares the same two operands as NR  FNR  so it behaves in the same way

User · Answer

There are awk built-in variables   NR - It gives the total number of records processed   FNR - It gives the total number of records for each input file

[awk] What are NR and FNR and what does "NR==FNR" imply?

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