git log show one commit id only

You can use git show referencing the third parent from your current commit (i.e. the second ancestor from HEAD). Also, git show accepts the same format string as git log:

git show HEAD~2 --pretty=format:"%h" --no-patch

Update (2016-12-01)

An even better way would be to use the rev-parse plumbing command with the --short option to output the abbreviated (7 characters) commit SHA-1:

git rev-parse --short HEAD~2

Or you could also specify the exact length of the commit SHA-1:

git rev-parse --short=4 HEAD~2

Since at least git version 2.3.8, you can use the --skip option:

   git log -1 --skip 2 --pretty=format:"%h"

Not sure which earlier versions of git support --skip.

There is a tool for that:

git log -3 --pretty=format:"%h" | tail -n 1

You can include n characters of the hash (instead of the default) with the following flag:

--abbrev=n 

Relevant pieces of the Unix Philosophy

1) Make each program do one thing well. To do a new job, build afresh rather than complicate old programs by adding new "features".

2) Expect the output of every program to become the input to another, as yet unknown, program. Don't clutter output with extraneous information. Avoid stringently columnar or binary input formats. Don't insist on interactive input.

... [i.e.]

• Write programs that do one thing and do it well.
• Write programs to work together.

https://en.wikipedia.org/wiki/Unix_philosophy