Re: Git alias syntax help

From: Jeff King <peff@peff.net>
Date: Jan 14, 2020, 22:28 UTC
Message-ID: <20200114222802.GC3957260@coredump.intra.peff.net>
In-Reply-To: <CAHMHMxWpLAnj3w8DGLMFbfy-A-pBjDxNdMeiM-fyuu-gnZyg+Q@mail.gmail.com>

On Tue, Jan 14, 2020 at 05:21:20PM +0200, Σταύρος Ντέντος wrote:

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> I am having an issue with git-aliases - specifically, the intricacies
> involved in their syntax.
> 
> In general, the syntax is confusing to me, especially when it is
> _wise_ to use quotes inside a `!sh` alias.
> e.g. which one would be the correct one
> new = "!f() { : git log ; git log \"${1}@{1}..${1}@{0}\" \"$@\" ; } ; f"
> new = !f() { : git log ; git log "${1}@{1}..${1}@{0}" "$@" ; } ; f

Only the first one is correct. In addition to the quotes in the second one being eaten by the config parser, the unquoted semicolon starts a comment.

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> The alias confusing me is more specifically this:
> https://git.wiki.kernel.org/index.php/Aliases#simple_diff_ignoring_line_number_changes
> 
> diffsort = !sh -c 'git diff "$@" | grep "^[+-]" | sort --key=1.2 | uniq -u -s1'
> 
> The output of:
> $  colordiff -su <(git diffsort HEAD^..HEAD) <(git diffsort HEAD^^..HEAD^)
> Files /dev/fd/63 and /dev/fd/62 are identical
> is a little unexpected, since I know for a fact that one of the
> referced commits is not a code block moved.

The issue here isn't with Git's alias mechanism, but a quirk of how "sh -c" works. You can run with GIT_TRACE to see what we're passing to the shell (though note that your double-quotes don't make it through):

  $ GIT_TRACE=1 git diffsort HEAD^..HEAD
  17:22:47.644542 [pid=3959333] git.c:708           trace: exec: git-diffsort HEAD^..HEAD
  17:22:47.644648 [pid=3959333] run-command.c:663   trace: run_command: git-diffsort HEAD^..HEAD
  17:22:47.645038 [pid=3959333] run-command.c:663   trace: run_command: 'sh -c '\''git diff $@ | grep ^[+-] | sort --key=1.2 | uniq -u -s1'\''' HEAD^..HEAD
  17:22:47.650319 [pid=3959336] git.c:439           trace: built-in: git diff

The problem is that "sh -c" takes the first non-option argument as $0, not $1. For example:

  $ sh -c 'echo 0=$0, @=$@' foo bar baz
  0=foo, @=bar baz

You can add any extra string there to become $0, like:

  diffsort = "!sh -c 'git diff \"$@\" | grep \"^[+-]\" | sort --key=1.2 | uniq -u -s1' --"

which will do what you want. You can use whatever string you like, since you know that your "-c" snippet does not ever look at $0.

-Peff