How do I add numbers from two txt files and write it to the same file?

Question

I want to add numbers from two txt files. The number will change in file1, and file2 should update itself like this file2 = file1 + file2. Decimals not needed.

Example:

file1

3

file2

7

Output:

file1

3

file2

10

I tried $ paste file1 file2 | awk '{$0 = $1 + $2}' > file2 but all it does is copy the number from file1 to file2.

Answer 1

In-place replacements are not natively supported in awk. You can use sponge from moreutils to rewrite to a file you read in.

awk-way to add two files line-by-line

awk ' FNR==NR { a[NR]=$1 } FNR!=NR { print $1+a[FNR] }' file1 file2 | sponge file2

Simpler non-awk way, that also supports floating points:

paste -d'+' file1 file2 | bc -l | sponge file2

Both methods support multi-line files.

Answer 2

You could do this:

let tot=$(cat file1)+$(cat file2)
echo $tot

Answer 3

I have tried with command and it worked fine too

paste f1 f2| awk '{print $1+$2}' >f2_tmp && mv f2_tmp f2


praveen@praveen:~$ cat f1
3
praveen@praveen:~$ cat f2
10
praveen@praveen:~$