05-20-2015
In awk, NR is the number of records read so far from all of the input files and FNR is the number of records read so far from the current input file.
So, when you read from the 1st input file a[FNR] will be the default uninitialized empty string and length(a[FNR]) will be zero which will then cause a[FNR] to be set to the contents of field 2 from that line, field 3 from that line, and the field separator ($2$3FS). And, when the awk reads the corresponding line from subsequent input files, length(a[FNR]) will not be zero and a[FNR] will be set to the current a[FNR] with field 2 from the current line in the new file, field 3 from the current line in the new file, and FS appended to the end of it (a[FNR]$2$3FS).
Is this any clearer for you?
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As I know:
FNR: The ordinal number of the current record in the current file.
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merge(1) merge(1)
NAME
merge - three-way file merge
SYNOPSIS
merge [-Llabel1 [-Llabel3]] [-p] [-q] file1 file2 file3
DESCRIPTION
merge incorporates all changes that lead from file2 to file3 into file1. The result goes to standard output if -p is present, into file1
otherwise. merge is useful for combining separate changes to an original. Suppose file2 is the original, and both file1 and file3 are
modifications of file2. Then merge combines both changes.
An overlap occurs if both file1 and file3 have changes in a common segment of lines. On a few older hosts where diff3 does not support the
-E option, merge does not detect overlaps, and merely supplies the changed lines from file3. On most hosts, if overlaps occur, merge out-
puts a message (unless the -q option is given), and includes both alternatives in the result. The alternatives are delimited as follows:
<<<<<<< file1 lines in file1 ======= lines in file3 >>>>>>> file3
If there are overlaps, the user should edit the result and delete one of the alternatives. If the -L label1 and -L label3 options are
given, the labels are output in place of the names file1 and file3 in overlap reports.
DIAGNOSTICS
Exit status is 0 for no overlaps, 1 for some overlaps, 2 for trouble.
IDENTIFICATION
Author: Walter F. Tichy.
Revision Number: 1.1.6.2; Release Date: 1993/10/07.
Copyright (C) 1982, 1988, 1989 by Walter F. Tichy.
Copyright (C) 1990, 1991 by Paul Eggert.
SEE ALSO
diff3(1), diff(1), rcsmerge(1), co(1)
merge(1)