Shell Programming and Scripting

Thanks a lot, it worked perfectly :-)
But would you mind explaining those complex expressions used in the snippet

Code:

join -t"|" -11 -21 -o1.1,2.2,1.3,1.4 file1 file2

Sure,

While reading the first non empty input file (NR == FNR, by the way there is a patch for the latest GNU awk that enables support
for the new blocks BEGINFILE and ENDFILE that help avoid the problems with empty files),
build the associative array _ (you can use another name, if you wish ):
for every key ($1) concatenate the associated records.
The ?: is the ternary operator, so the code above implements the following logic:

Code:

_[$1] ?                  # if the value of _[$1] is not null or 0
_[$1] RS $0              # concatenate RS (the current record separator, 
                         # newline in this case) and the current input record $0 
                         # to the existing value
: $0                     # if _[$0] is null or 0 (not yet defined, in this case)
                         # set it to the current input record $0.

So, given your example data, after the first input file is processed
the associative array _ contains the following keys/values:

Code:

key: 
D456 

value:  
D456|1.1|msg d 2|remarks 1
D456|1.2|msg d 3|remarks 2
D456|1.3|msg d 4|remarks 3
D456|1.4|msg d 5|remarks 4

key: 
B234 

value: 
B234|1.1|msg b 2|remarks 1
B234|1.2|msg b 3|remarks 2

The next statement forces awk to immediately stop processing the current record
and go on to the next record. This means that no further rules are executed
for the current record, and the rest of the current rule’s action isn’t executed .
[from the GAWK manual: Effective AWK Programming by Arnold Robbins]

Code:

($1 in _ && $0 = $0 RS _[$1]) || 1

Now processing the second input file.
If the first field matches a key in the _ array
evaluate the second part of the logical short-circuit operator AND (&&):
concatenate RS and the value of the _[$1] to the current record,
i.e. print

Code:

B234|msg b 1|date b 1|other info b 1
B234|1.1|msg b 2|remarks 1
B234|1.2|msg b 3|remarks 2
[...]

Otherwise (|| is logical OR) trigger the default action
(which in awk is print the current record: 1 in boolean context is a synonym for true
(everything different than the null string "" and 0 is considered to be true).

Hope this helps.

Yes indeed, that was very well explained. Thanks a ton.