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?

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

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