Git - find parent branch of child branch

Problem:

A_branch:
  - commit a1
  - commit a2
  - ......

A_branch - checkout -> B_branch

B_branch:
  - commit a1
  - commit a2
  - ......
  - commit b1
  - commit b2
  - ......

A few time later, when we on branch B, but we forget what branch we checkout from. How do we know that A is parent of B ?

Solution:

Using reflog and grep, we can do it. When we on branch A_branch and run git checkout -b B_branch, we will see some logs with reflog:

.....
......
85b4b25 HEAD@{150}: checkout: moving from A_branch to B_branch
85b4b25 HEAD@{150}: checkout: moving from B_branch to XYZ
85b4b25 HEAD@{152}: checkout: moving from A_branch to B_branch

So we need keyword: to B_branch, in last line (last activity log)

git reflog | grep "to $(git rev-parse --abbrev-ref HEAD)" | tail -1

// return
// 85b4b25 HEAD@{152}: checkout: moving from A_branch to B_branch

So, A_branch is parent of B_branch