首先说说fsck这个命令。该命令用来检查object database的完整性。例如是否所有commit均存在在object database中，是否有某个tree对象所引用到的blob对象丢失了，是否存在不被任何branch，tag所引用的"悬空"的commit。
几个常见参数有：
--root 报告当前repo的根节点，也即当前repo的第一个commit。
--tags 报告检查过的tags。
--no-reflog 将那些仅被reflog引用的commit视为unreachable。
--lost-found 根据对象的类型，将"悬空"对象写入到.git/lost-found/commit/ 或 .git/lost-found/other/目录中。
git fsck dangling blob 9daeafb9864cf43055ae93beb0afd6c7d144bfa4 #有一个blob对象，没有被任何commit引用到。但是依旧占用磁盘空间，需要清理。 git fsck --root --tags tagged commit b40c14f880c04fefcdce1328a2d6a537ed64b182 (test_tag1) in 00311b3f18af6306df2e5843d13a901681b677ef #一个tag tagged commit f62a94635be5b72c9778a04aea116f6b4349eb61 (test_tag2) in 4de836025102abda82c0fd544a8725c82c46752f #另一个tag root 63a763ead83edabcadac4f62a08b0d39b0d719b4 #当前repo的历史中第一次commit。 dangling blob 9daeafb9864cf43055ae93beb0afd6c7d144bfa4
上个示例中看到了有一个dangling blog，这种对象不被任何commit中的tree引用，但是却存在于object database中浪费磁盘空间，需要被清除。要实现类似object的清除，就需要用到第2个命令，git prune。
此命令没有太多的参数，直接git prune就可以了。如果在实际删除之前想要查看一下哪些object会被影响，可以使用git prune --dry-run来查看，该参数仅会输出将要被删除的object，并不会实际删除。
关于此命令，有一点需要注意：git prune只会删除unpacked object，如果一个对象已经被pack，那么git prune不会删除。
如果想要删除那些被pack的dangling文件要怎么样呢？第三个命令git repack就要出场了。
git repack -a -d git repack -A -d两者的区别在于：
-a 会直接删掉pack文件中的dangling对象。
-A 并不会立即删除，而是将pack文件中的dangling object重新以松散对象的形式存储在database中。也就是把dangling object从pack文件中"吐出来"。
关于3个命令的综合示例如下：
示例一
=> git log --oneline -3 b841691 Merge branch 'test' of github.scm.corp.ebay.com:gewang/TryOurInternalGithub ec55218 1 on test c1b13cb dd => touch test => echo test >> test => git add test => git commit -m "test git fsck" [master d2d0bfa] test git fsck 1 files changed, 1 insertions(+), 0 deletions(-) create mode 100644 test => git reset --hard HEAD^ #rollback到之前一个commit，制造一个dangling commit。 HEAD is now at b841691 Merge branch 'test' of github.scm.corp.ebay.com:gewang/TryOurInternalGithub => git fsck #因为刚刚的commit现在还可以通过reflog引用到，所以fsck并不认为它是unreachable。 => git fsck --no-reflog #忽略reflog就可以看到 dangling commit d2d0bfa2ee4dfe767862cb441595db9748d8beab => git reflog expire --expire=now --all #删除reflog => git fsck #成功制造了一个dangling commit。 dangling commit d2d0bfa2ee4dfe767862cb441595db9748d8beab => git prune #删掉它！ => git fsck
示例二
=> git log --oneline -3 b841691 Merge branch 'test' of github.scm.corp.ebay.com:gewang/TryOurInternalGithub ec55218 1 on test c1b13cb dd => touch test => echo test >> test => git add test => git commit -m "test fsck, prune and repack" [master 09f3015] test fsck, prune and repack 1 files changed, 1 insertions(+), 0 deletions(-) create mode 100644 test => git repack #首先将新创建的commit打包。 Counting objects: 3, done. Delta compression using up to 8 threads. Compressing objects: 100% (2/2), done. Writing objects: 100% (3/3), done. Total 3 (delta 0), reused 0 (delta 0) => git reset --hard HEAD^ HEAD is now at b841691 Merge branch 'test' of github.scm.corp.ebay.com:gewang/TryOurInternalGithub => git fsck => git fsck --no-reflog dangling commit 09f3015743b00805e100259b9b4b0c230236a891 => git reflog expire --expire=now --all => git fsck #制造了一个dangling commit。 dangling commit 09f3015743b00805e100259b9b4b0c230236a891 => git prune => git fsck #prune无法删除已经pack的object。 dangling commit 09f3015743b00805e100259b9b4b0c230236a891 => git repack -a -d #reapck -a -d 直接删除。 Counting objects: 173, done. Delta compression using up to 8 threads. Compressing objects: 100% (94/94), done. Writing objects: 100% (173/173), done. Total 173 (delta 59), reused 173 (delta 59) => git fsck
示例三
=> git log --oneline -3 b841691 Merge branch 'test' of github.scm.corp.ebay.com:gewang/TryOurInternalGithub ec55218 1 on test c1b13cb dd => touch test => echo test >> test => git add test => git commit -m "test fsck, prune and repack -A" [master 8df4fa5] test fsck, prune and repack -A 1 files changed, 1 insertions(+), 0 deletions(-) create mode 100644 test => git repack Counting objects: 3, done. Delta compression using up to 8 threads. Compressing objects: 100% (2/2), done. Writing objects: 100% (3/3), done. Total 3 (delta 0), reused 0 (delta 0) => find .git/objects/ # 当前database中的文件。 .git/objects/ .git/objects//8d .git/objects//8d/f4fa5b683a6c2614d0bb5f6a9bbd0df6e76ebb .git/objects//9d .git/objects//9d/aeafb9864cf43055ae93beb0afd6c7d144bfa4 .git/objects//e6 .git/objects//e6/a38484a68f1d818090516266a1fe5115bedb2d .git/objects//info .git/objects//info/packs .git/objects//pack .git/objects//pack/pack-f82243c549f6e21d59eb4b3cd90c1f7ea4bc2b2f.idx .git/objects//pack/pack-f82243c549f6e21d59eb4b3cd90c1f7ea4bc2b2f.pack .git/objects//pack/pack-ff6a31c9b76b7a762bf76eb2500048a55e0e076d.idx .git/objects//pack/pack-ff6a31c9b76b7a762bf76eb2500048a55e0e076d.pack => git gc # 清理一下。 Counting objects: 176, done. Delta compression using up to 8 threads. Compressing objects: 100% (96/96), done. Writing objects: 100% (176/176), done. Total 176 (delta 60), reused 175 (delta 59) => find .git/objects/ #现在干净多了。 .git/objects/ .git/objects//info .git/objects//info/packs .git/objects//pack .git/objects//pack/pack-25320b3227fb4b00d05585542643651019b1a041.idx .git/objects//pack/pack-25320b3227fb4b00d05585542643651019b1a041.pack => git reset --hard HEAD^ HEAD is now at b841691 Merge branch 'test' of github.scm.corp.ebay.com:gewang/TryOurInternalGithub => git fsck => git fsck --no-reflog dangling commit 8df4fa5b683a6c2614d0bb5f6a9bbd0df6e76ebb => git reflog expire --expire=now --all => git fsck #制造dangling commit。 dangling commit 8df4fa5b683a6c2614d0bb5f6a9bbd0df6e76ebb => git prune => git fsck dangling commit 8df4fa5b683a6c2614d0bb5f6a9bbd0df6e76ebb => git repack -A -d #这次使用-A Counting objects: 173, done. Delta compression using up to 8 threads. Compressing objects: 100% (94/94), done. Writing objects: 100% (173/173), done. Total 173 (delta 59), reused 172 (delta 59) => git fsck #这货还在 dangling commit 8df4fa5b683a6c2614d0bb5f6a9bbd0df6e76ebb => find .git/objects/ #对应的松散对象也出现啦。 .git/objects/ .git/objects//8d .git/objects//8d/f4fa5b683a6c2614d0bb5f6a9bbd0df6e76ebb .git/objects//9d .git/objects//9d/aeafb9864cf43055ae93beb0afd6c7d144bfa4 .git/objects//e6 .git/objects//e6/a38484a68f1d818090516266a1fe5115bedb2d .git/objects//info .git/objects//info/packs .git/objects//pack .git/objects//pack/pack-ff6a31c9b76b7a762bf76eb2500048a55e0e076d.idx .git/objects//pack/pack-ff6a31c9b76b7a762bf76eb2500048a55e0e076d.pack
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