NR and FNR in awk

NR indicates the number of data rows read after the awk starts executing the program.

FNR is similar to Nr. The difference is that FNR accumulates from 0 every time awk opens a new file.

 

The following are two examples:

1. The output results for a single file NR and FNR are the same:

# Awk '{print NR, $0}' file1 1 a B c d 2 A B D C 3 A C B D # Awk '{print FNR, $0}' file1 1 a B c d 2 A B D C 3 A C B D

2, but for multiple files:

# Awk '{print NR, $0}' file1 file2 1 a B c d 2 A B D C 3 A C B D 4 aa bb cc dd 5 aa bb dd CC 6 aa cc bb dd # Awk '{print FNR, $0}' file1 file2 1 a B c d 2 A B D C 3 A C B D 1 aa bb cc dd 2 aa bb dd CC 3 aa cc bb dd

 

Let's look at another example of typical application of NR and FNR:

There are two file formats:

# Cat account James | 000001 Li Si | 000002 # Cat CDR 000001 | 10 000001 | 20 000002 | 30 000002 | 15

The result is to print the username, account, and amount in the same line, as shown below:

Michael Jacob | 000001 | 10 Michael Jacob | 000001 | 20 Lee IV | 000002 | 30 Li Si | 000002 | 15

Run the following code:

# Awk-F/| 'nr = FNR {A [$2] = $0; next} {print a [$1] "|" $2} 'account CDR

Note:

When Nr = FNR is true, the first file account is read, and then {A [$2] = $0; next} cyclically stores each row of the account file into array A, and uses $2 2nd fields as subscript reference.

When Nr = FNR is false, judge that the second file CDR is read, and skip {A [$2] = $0; next }, execute {print a [$1] "|" $2} unconditionally for each row of the Second file Cdr. At this time, the variable $1 is the first field of the Second file, when reading the first file, the second field $2 of the first file is used as the array subscript. therefore, you can use a [$1] to reference arrays here.