Compare strings using awk command - bash

Question

I am at initial stage of learning shell scripting. So please explain me the steps for better understanding.
Consider I have two files
Content of the two files are as below:
File1.txt
ABC=10
DEF=20
XYZ=30
File2.txt
DEF=15
XYZ=20
I want to write a simple shell script to check both the files and add the values and print the final output as below. like
ABC=10
DEF=35
XYZ=50

Answer 1

You can use awk:
awk 'BEGIN{FS=OFS="="} FNR==NR{a[$1]=$2;next} {a[$1]+=$2}
END{for (i in a) print i, a[i]}' file1 file2
ABC=10
XYZ=50
DEF=35
Breakup:
NR == FNR { # While processing the first file
a[$1] = $2 # store the second field by the first in an array
next # move to next record
}
{ # while processing the second file
a[$1]+=$2 # add already stored value by 2nd field in 2nd file
}
END{..} # iterate the array and print the values
If you want to keep original ordering intact then use:
awk 'BEGIN{FS=OFS="="} FNR==NR{if (!($1 in a)) b[++n]=$1; a[$1]=$2;next} {a[$1]+=$2}
END{for (i=1; i<=n; i++) print b[i], a[b[i]]}' file1 file2
ABC=10
DEF=35
XYZ=50