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Top Forums Shell Programming and Scripting Print rows, having pattern in specific column... Post 302359162 by Scrutinizer on Tuesday 6th of October 2009 02:16:00 AM
Old 10-06-2009
A[$1] = $1 is used to set up the array of patterns. It is set when NR is equal to FNR, in other words when the first file is read i.e. the pattern file. It means fill the associative array "$1" to the value of "$1", $1 being the first field of your pattern file So with the OP's provided inputs it gets filled like so:
Code:
A[cd003]=cd003
A[cd005]=cd005
A[cd007]=cd007
A[cd008]=cd008

So once the pattern file is done it starts reading the input file FNR is no longer equal to NR, so it will just execute the part "A[$3]" for each line, which means: print the current line if field 3 exist as a key in the array.

The script is only testing the existence of the array elements, not using its contents. So IMO the use of $1 is a tiny bit superfluous. I think we could also just set it to 1 instead:

Code:
awk 'NR == FNR { A[$1]=1; next } A[$3]' pattern_file input_file

or, since pattern file does not have a third column.
Code:
awk 'NR == FNR { A[$1]=1 } A[$3]' pattern_file input_file


Last edited by Scrutinizer; 10-06-2009 at 03:28 AM..
 

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GREP(1) 						      General Commands Manual							   GREP(1)

NAME
grep - search a file for a pattern SYNOPSIS
grep [ option ... ] pattern [ file ... ] DESCRIPTION
Grep searches the input files (standard input default) for lines (with newlines excluded) that match the pattern, a regular expression as defined in regexp(6). Normally, each line matching the pattern is `selected', and each selected line is copied to the standard output. The options are -c Print only a count of matching lines. -h Do not print file name tags (headers) with output lines. -i Ignore alphabetic case distinctions. The implementation folds into lower case all letters in the pattern and input before interpre- tation. Matched lines are printed in their original form. -l (ell) Print the names of files with selected lines; don't print the lines. -L Print the names of files with no selected lines; the converse of -l. -n Mark each printed line with its line number counted in its file. -s Produce no output, but return status. -v Reverse: print lines that do not match the pattern. Output lines are tagged by file name when there is more than one input file. (To force this tagging, include /dev/null as a file name argument.) Care should be taken when using the shell metacharacters $*[^|()= and newline in pattern; it is safest to enclose the entire expression in single quotes '...'. SOURCE
/sys/src/cmd/grep.c SEE ALSO
ed(1), awk(1), sed(1), sam(1), regexp(6) DIAGNOSTICS
Exit status is null if any lines are selected, or non-null when no lines are selected or an error occurs. GREP(1)
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