Let’s have a file readme.txt and such a branch structure

master
  |
branch_a 
  |
branch_b

How this structure is organized:

1. When we are in master:

   git checkout -b branch_a
   echo “a” >> readme.txt
   git add .
   git commit -am “a”

2. git checkout -b branch_b
   echo “b” >> readme.txt
   git add .
   git commit -am “b”

Right now what we are in branch_b and we have in readme.txt:

a
b

Now, I’d like to make adjustment to readme.txt in branch_a. First, switch to branch_a:

git checkout branch_a

Then edit readme.txt like this:

a_1

Then I’d like to switch to branch_b, merge with branch_a and preserve the adjustments made in branch_a.

In other words, I’d like to run some commands so that in branch_b in readme.txt I would have:

a_1
b

Could you help me?