Diff 2 settings files and replace the difference - linux

Question

I have 2 files with settings:
file1.txt and file2.txt
A=1 A=2
B=3 B=3
C=5 C=4
D=6 .
. E=7
I am looking for the best approach to replace the values of the file1.txt with the diff values of file2.txt, so the file1.txt would look like:
file1.txt:
A=2
B=3
C=4
D=6
E=7
Currently i didn't write any code, but the only approach i think about is to write a bash script that diffs both files (provided as positional arguments), and use sed to replace non-matching strings. Something in this vein:
./diffreplace.bash file1.txt file2.txt > NEWfile1.txt
I wonder whether there is something more elegant that alerady exists?

Answer 1

All of the following solutions may change the order of assignments. I assumed that would be ok.
Lazy Solution
If you use these assignments in some way that allows overwriting, then you can simple append file2 to the end of file1. All old values will be overwritten be the new ones when you execute result.
cat old new > result
Slightly Better Solution
Extending the previous approach, you can iterate over the lines of result and for every variable, keep only the last assignment:
cat new old |
awk -F= '{if (a[$1]!="x") {print $0; a[$1]=x}}'
Alternative Solution
Use join to combine both files, then filter out the values from the first file by using cut. When your files are sorted, use
join -t= -a1 -a2 new old | cut -d= -f1,2
if not, use
join -t= -a1 -a2 <(sort new) <(sort old) |
cut -d= -f1,2

Answer 2

I'm a little puzzed over your comment the structure of the file must remain untouched. Sort mixes the order so I'm assuming that the As are always on line 1 or line 1 is . etc:
$ awk '
BEGIN { RS="\r?\n" } # in case of Windows line-endings
$0!="." { # we dont store . (change it to null if you need to)
a[FNR]=$0 # hash using line number as key
}
END { # after all that hashing
for(i=1;i<=FNR;i++) # iterate in line number order
print a[i] # output the last met version
}' file1 file2 # mind the file order
Output:
A=2
B=3
C=4
D=6
E=7
Edit: A version with a whitelist:
$ cat whitelist
A
B
E
Script:
$ awk -F= '
NR==FNR { # process the whitelist
a[FNR]=$1 # for a key is linenumber, record as value
b[$1]=FNR # bor b record is key, linenumber is value
n=FNR # remember the count for END
next
} # process file1 and file2 ... filen
($1 in b) { # if record is found in b
a[b[$1]]=$0 # we set the record to a[linenumber]=record
}
END {
for(i=1;i<=n;i++) # here we loop on linenumbers, 1 to n
print a[i]
}' whitelist file1 file2
Output:
A=2
B=3
E=7