git prepare-commit-msg hook

notify [dev]‽ $ cat .git/hooks/prepare-commit-msg
printf "1i\n$SOMEENVVAR\n.\nwq\n" | ed -s "$1"


notify [dev]‽ $ export SOMEENVVAR="some environment variable"


notify [dev]‽ $ git commit -m "hiya"
[dev d02fd1a] some environment variable hiya
 1 file changed, 0 insertions(+), 0 deletions(-)
 delete mode 100644 foo


notify [dev] $ git show
commit
Author:
Date:

    some environment variable
    hiya

diff --git a/foo b/foo
deleted file mode 100644
index e69de29..0000000