If the records are sorted, and the in the same order there is not even a need to do the compare. You could just use:
$ awk '{new = $NF; getline < "b.txt"; regexp = $NF "$"; sub(regexp,new); print}' a.txt
B L1983A B1N 20090701 HUECDP +0000000000000000.00 F 20090701 01 A QBLH
B L1983A B1N 20090701 HUHFDP +0000000000000000.00 F 20090701 01 A QBL1
That remembers the last field a the line from the first file, reads the other file, replaces its last field with the one from the first and prints.
If the lines are sorted but potentially NOT equal then you will need to add some code to read along into whichever file that has fallen behind until caught up.
if file a is
10
12
and file b is
10
11
12
then the program has to skip that line 11 from b.
if file a is
10
11
13
and file b is
10
12
13
then the program needs to skip a line from file from each before processing.
Below an example of how to solve such program in awk.
Note, I used 28 instead of 38 in the example, because that's how the data showed up in the forum, and while you indicated 4 fields, you actually showed 5, so that's not to be trusted either.
Also please note how you wasted James's time by being imprecise initially.
You did NOT just need to find matching lines... for which GREP is perfect, but you also needed data from EACH provide file for which GREP is useless.
hope this helps,
Hein.
-------------- update.awk ----------------
BEGIN { a_skip = b_skip = c_lines = 0 }
{ a_match = substr($0,1,28)
a_last = $NF
while (a_match != b_match) {
if (a_match > b_match) {
b_skip++
if ((getline < "b.txt") != 1 ) { exit }
b_match = substr($0,1,28)
b_last = $NF
c = $0
}
if (a_match < b_match) {
a_skip++
if (getline != 1) { exit }
a_match = substr($0,1,28)
a_last = $NF
}
}
regexp = b_last "$"
sub (regexp, a_last, c)
print c
c_lines++
b_skip--
}
END { print c_lines " printed to C. " a_skip, " skipped from a, ", b_skip " from b." > "/dev/stderr"
}
-------------- sample execution ----------
/cygdrive/c/temp
$ awk -f update.awk < a.txt > c.txt
2 printed to C. 1 skipped from a, 0 from b.
/cygdrive/c/temp
$ cat c.txt
B L1983A B1N 20090701 HUECDP +0000000000000000.00 F 20090701 01 A QBLH
B L1983A B1N 20090701 HUHFDP +0000000000000000.00 F 20090701 01 A QBL1
