Shell Programming and Scripting

Hello all,
I have a pattern file some what like this,

cd003
cd005
cd007
cd008


and input file like this,

abc cd001 cd002 zca
bca cd002 cd003 cza
cba cd003 cd004 zca
bac cd004 cd005 zac
cba cd005 cd006 acz
acb cd006 cd007 caz
cab cd007 cd008 azc



I want to print all the rows which have the pattern in column 3 ?
Is it possible to give patterns from a file, in AWk ?
How to do it in shell scripting (without using AWk) ?

THANKS IN ADVANCE

Hi.

Something like:

Code:

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

Hi scottn,

Very nice code. The only thing I can't figure out is why you need the next statement in this case.

Code:

awk 'NR == FNR { A[$1] = $1 } A[$3]' pattern_file input_file

seems to produce the same result..

S.

The idea is to read the pattern file completely before doing anything else.

But you're right, in this case A[$3] would never be set as there is no field three in the pattern file. But if that was to change, using next means you wouldn't have to worry about it later!

Hi,

Can I know what is the relationship between NR == FNR { A[$1] = $1; next } A[$3] ?
I quite confusing about the reason why you using the A[$X] to print rows, having pattern in specific column.
Thanks for your reply

The quick answer:

First realize that awk arrays are indexed by a value not a number. While array indexes may look like a number, that is not how awk sees them.

The first clause sets up an array (A) indexed by values from the pattern file.
Indexes to A are cd003, cd005, etc... so A["cd003"] is a valid entry in the A array.
NR is the number records awk has ever read. NR is set to 1 when awk starts.
FNR is the number of records read from the current file. FNR is reset to 1 when a new file is opened.
Both are incremented when a record is read.

So, if NR is equal to FNR, then we are reading from the first file (pattern file) since the record counts are the same.
If NR is not equal to FNR, then we are reading from a subsequent file (i.e. data file).

The A[$3] (where $3 is the third field from the data file) says if the entry exists (i.e A["cd003"] then do the default action (print the line), else ignore that entry.

This clause is not executed on the pattern file because the next statement says "skip all following code. read next line, and start processing clauses from the top". The 'next' statement adds to the robustness of the code.

Hi,

Really thanks for your explanation.
It is excellent.
Besides that, can you told me what is the meaning of
A[$1] = $1Thanks for your help again.