git son: add command to create independent child repositories

.gitignore

@@ -164,6 +164,7 @@
 /git-show-branch
 /git-show-index
 /git-show-ref
+/git-son
 /git-sparse-checkout
 /git-stage
 /git-stash

Documentation/git-son.adoc

@@ -0,0 +1,64 @@
+git-son(1)
+==========
+
+NAME
+----
+git-son - Create an independent child repository that knows its parent
+
+SYNOPSIS
+--------
+[verse]
+'git son' [--inherit] [--branch <branch>] <name>
+
+DESCRIPTION
+-----------
+
+Create a new independent Git repository inside the current working
+tree as a subdirectory named `<name>`. Unlike a submodule, the child
+repository is not tracked by the parent; instead, `<name>/` is added
+to the parent's `.gitignore`.
+
+The child repository is configured with a remote called `parent`
+pointing back to the parent repository's origin URL (or local path
+if no origin is set), allowing the child to fetch from the parent
+at any time.
+
+OPTIONS
+-------
+--inherit::
+	Fetch the parent's history into the child repository at
+	creation time. Without this flag, the child starts with a
+	single initial commit.
+
+--branch <branch>::
+	When used with `--inherit`, check out the given branch from
+	the parent instead of the default branch. This option
+	requires `--inherit`.
+
+<name>::
+	The name of the subdirectory (and child repository) to create.
+	Must not already exist.
+
+EXAMPLES
+--------
+
+Create a simple child repository:
+
+	git son my-tool
+
+Create a child that inherits the parent's history:
+
+	git son --inherit my-fork
+
+Create a child starting from a specific parent branch:
+
+	git son --inherit --branch feature my-experiment
+
+Later, from within the child, fetch updates from the parent:
+
+	cd my-tool
+	git fetch parent
+
+GIT
+---
+Part of the linkgit:git[1] suite

Documentation/meson.build

@@ -139,6 +139,7 @@ manpages = {
   'git-show-ref.adoc' : 1,
   'git-show.adoc' : 1,
   'git-sh-setup.adoc' : 1,
+  'git-son.adoc' : 1,
   'git-sparse-checkout.adoc' : 1,
   'git-stage.adoc' : 1,
   'git-stash.adoc' : 1,

Makefile

@@ -728,6 +728,7 @@ SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
 SCRIPT_SH += git-quiltimport.sh
 SCRIPT_SH += git-request-pull.sh
+SCRIPT_SH += git-son.sh
 SCRIPT_SH += git-submodule.sh
 SCRIPT_SH += git-web--browse.sh
 

command-list.txt

@@ -186,6 +186,7 @@ git-show                                mainporcelain           info
 git-show-branch                         ancillaryinterrogators          complete
 git-show-index                          plumbinginterrogators
 git-show-ref                            plumbinginterrogators
+git-son                                 mainporcelain
 git-sparse-checkout                     mainporcelain
 git-stage                                                               complete
 git-stash                               mainporcelain

git-son.sh

@@ -0,0 +1,97 @@
+#!/bin/sh
+#
+# git-son: create an independent child repository that knows its parent
+#
+
+SUBDIRECTORY_OK='Yes'
+OPTIONS_SPEC='git son [options] <name>
+--
+inherit    fetch parent history into the son
+branch=    start the son from a specific parent branch
+'
+
+. git-sh-setup
+require_work_tree
+cd_to_toplevel
+
+inherit=
+branch=
+while test $# -gt 0
+do
+	case "$1" in
+	--inherit)
+		inherit=1 ;;
+	--branch)
+		shift
+		branch="$1" ;;
+	--)
+		shift; break ;;
+	-*)
+		usage ;;
+	*)
+		break ;;
+	esac
+	shift
+done
+
+name="$1"
+test -n "$name" || usage
+
+if test -n "$branch" && test -z "$inherit"
+then
+	die "fatal: --branch requires --inherit"
+fi
+
+parent_dir="$(pwd)"
+parent_remote="$(git remote get-url origin 2>/dev/null)" || parent_remote=
+
+if test -e "$name"
+then
+	die "fatal: '$name' already exists"
+fi
+
+mkdir "$name" || die "fatal: could not create directory '$name'"
+
+if ! echo "$name/" >> "$parent_dir/.gitignore" 2>/dev/null
+then
+	rm -rf "$name"
+	die "fatal: could not update .gitignore"
+fi
+
+cd "$name" || die "fatal: could not enter directory '$name'"
+
+if ! git init
+then
+	rm -rf "$parent_dir/$name"
+	die "fatal: could not initialize repository in '$name'"
+fi
+
+if test -n "$parent_remote"
+then
+	git remote add parent "$parent_remote"
+else
+	git remote add parent "$parent_dir"
+fi
+
+if test -n "$inherit"
+then
+	git fetch parent || die "fatal: could not fetch from parent"
+	if test -n "$branch"
+	then
+		git checkout -b "$branch" "parent/$branch" ||
+			die "fatal: could not checkout branch '$branch'"
+	else
+		git checkout -b main parent/HEAD 2>/dev/null ||
+			git checkout -b main "parent/$(git remote show parent | sed -n 's/.*HEAD branch: //p')" 2>/dev/null ||
+			echo "warning: could not determine parent HEAD, starting empty"
+	fi
+else
+	echo "# $name" > README.md
+	git add README.md
+	git commit -q -m "Initial commit"
+fi
+
+echo ""
+echo "Created son repository '$name'"
+echo "  parent: ${parent_remote:-$parent_dir}"
+echo "  inherit: ${inherit:-no}"

meson.build

@@ -1973,6 +1973,7 @@ scripts_sh = [
   'git-mergetool.sh',
   'git-quiltimport.sh',
   'git-request-pull.sh',
+  'git-son.sh',
   'git-sh-i18n.sh',
   'git-sh-setup.sh',
   'git-submodule.sh',

t/meson.build

@@ -591,6 +591,7 @@ integration_tests = [
   't5004-archive-corner-cases.sh',
   't5100-mailinfo.sh',
   't5150-request-pull.sh',
+  't5151-son.sh',
   't5200-update-server-info.sh',
   't5300-pack-object.sh',
   't5301-sliding-window.sh',

t/t5151-son.sh

@@ -0,0 +1,63 @@
+#!/bin/sh
+
+test_description='Test git son command.'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+
+test_expect_success 'setup parent repository' '
+	echo "parent content" >file.txt &&
+	git add file.txt &&
+	git commit -m "Initial parent commit"
+'
+
+test_expect_success 'son creates child repository' '
+	git son my-child &&
+	test -d my-child &&
+	test -d my-child/.git
+'
+
+test_expect_success 'son sets parent remote in child' '
+	(
+		cd my-child &&
+		git remote get-url parent
+	)
+'
+
+test_expect_success 'son adds child to parent .gitignore' '
+	grep "my-child/" .gitignore
+'
+
+test_expect_success 'son child has initial commit' '
+	(
+		cd my-child &&
+		test $(git log --oneline | wc -l) -eq 1
+	)
+'
+
+test_expect_success 'son fails if target already exists' '
+	test_must_fail git son my-child
+'
+
+test_expect_success 'son with --branch requires --inherit' '
+	test_must_fail git son --branch main branch-child
+'
+
+test_expect_success 'son with --branch leaves no directory on failure' '
+	! test -e branch-child
+'
+
+test_expect_success 'son with --inherit fetches parent history' '
+	git init --bare "$TRASH_DIRECTORY/parent.git" &&
+	git push "$TRASH_DIRECTORY/parent.git" main &&
+	git remote add origin "file://$TRASH_DIRECTORY/parent.git" &&
+	git son --inherit inherited-child &&
+	(
+		cd inherited-child &&
+		git log --oneline parent/main
+	)
+'
+
+test_done