git alias with optional parameter

Question

I want to ensure that my latest commit has the current date before I push. Since I always rebase to master before a merge + push, I made this alias:

[alias]
    sync = !git commit --amend --date=today && git rebase master

Problem is, it keeps launching my text editor asking for a new commit message. Is there a way to have an optional parameter, so that I can choose to use either:

git sync 'my commit message'

or

git sync

where the latter will simply use the existing commit message, whatever it happens to be?

Answer 1

To apply the extra parameters to anything except the end of your alias’ “command line”, you will need to put your shell commands in a script. You can do it with an external script (like jdelStrother’s answer), or you can do it with an “inline” shell script.

You can use -m to feed git commit your new message or use the -C HEAD/--reuse-message=HEAD option to have it use the existing message and author (it would also reuse the author timestamp, but you are resetting that with --date=…). Using any of these options will prevent Git from opening an editor for your commit message.

Here it is as an “inline” shell script:

git config --global alias.sync '!sh -c '\''git commit --amend --date=today ${1+-m} "${1---reuse-message=HEAD}" && git rebase master'\'' -'

The core of this small script is the pair of conditional parameter expansions:

${1+-m} "${1---reuse-message=HEAD}"

When you call it with an extra parameter (i.e. your replacement log message), these expand to two shell words: -m "<your new log message>". When you do not supply the extra parameter, they expand to just a single word: "--reuse-message=HEAD".

The trailing dash is also important; it could be any shell word, the point is that something must be there because the shell will use it to initialize its $0 parameter (which usually has a default value, so it is useless for the conditional expansion itself).

---

If I misunderstood and you actually want to see the editor when you do not supply the extra parameter, then use the single expansion ${1+-m "$1"} instead of the pair of expansions.

Answer 2

When your aliases start getting more complex, it's probably easiest just to create a separate script for them. If you add a file 'git-sync' to your path, it will be automatically called when you do 'git sync'.

So, if you created that file with something along the lines of -

#!/bin/sh

if [ -z "$1" ]; then
  git commit --amend --date=today
else
  git commit --amend --date=today -m "$1"
fi
git rebase master

- that would probably work. It's typed off the top of my head though, so caveat lector.