git add Signed-off-by line using format.signoff not working

My client git version is 1.7.0.4.

I want to automatically add a "Signed-off-by" line for the committer at the end of the commit log message when commit a message.

1. When I set git config --global format.signoff true, and run git commit -m "modify something", I see no "Signed-off-by" in git log.
2. If I use git commit -m -s "modify something", then "Signed-off-by" does show in git log.

Can anyone help?

Update for Git 2.14.x/2.15: as I mentioned in "Git - Detect if commit is signed off programmatically", you will be able to parse a commit message trailer for Signed-off-By line.

"git interpret-trailers" has been taught a "--parse" and a few other options to make it easier for scripts to grab existing trailer lines from a commit log message.

See stefanct's answer for commit-msg client-side hook which would use git interpret-trailers.

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Original answer (2013)

format.signoff is about patch (as illustrated, for instance, in this patch):

format.signoff

A boolean value which lets you enable the -s/--signoff option of format-patch by default

It has no bearing on git commit -s.

In other words, you don't have to signoff every commit, but when you are publishing them as patch for others to use (as in "git: submitting patches"), then you should sign them.

For the exact signification of Signed-of-by, see "What is the Sign Off feature in Git for?".