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Commitbd728e9

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#Overview
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-**Changes & Status**:
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- a file can be*tracked* or*untracked* which indicates whether the file is recognized by git as part of the repository.
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- a change includes*added*,*deleted*,*modified* at the minimum unit of lines.
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- staging meaning that you're selectively picking untracked files or changes into bucket, and waiting for**committing** them.
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-**Commit**: With staged changes, you're ready to generate a new snapshot, which is called*commit*.
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- commits constructs a acyclic graph representing all revisions of your repository.
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- you can rollback to one snapshot by*Reset*.
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- a branch is derived from a commit.
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#Commit
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Commits are snapshots for each phase of the repository, which allowing you to rollback to any instance of that commit.
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Git is able to compare base on two commits(usually the current and last commit), which is the diff.
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- Each commit stores a snapshot of current repository with a unique hash to be identified.
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- A commit can have**one or more** parent from which it was derived.
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- Each snapshot file or folder has a unique hash, and can inspect their content in some way.
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- the folder snapshot is called`tree`
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- the file snapshot is`blob`
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- a commit always shows the snapshot of root of the repository as entry of a commit
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>[!NOTE]
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>*Revision* is another synonym of*Commit* in git.
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>[!IMPORTANT]
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>**Commit** is the atomic identifier of how git manage the versions, it's the*true* and*real* concept while*branch* and*merge* are just*fake* concepts upon it.
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##Commit Identifier
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Each commit is identified by the uniquely generated SHA hash, you can reference the commit using the full hash or the leading 7 digit hash on git commands.
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Each snapshot instance of that particular commit could be found on`.git/objects/<first-2digits-of-hash>/<restof-hash>`
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>[!NOTE]
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>Those*identifier* of commits are called*refs* in git, you could find their representations in`.git/refs`
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1.**commit hash**: the most explicit identification of a commit.
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2.**branch name**: points to**latest** commit of the branch.
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3.**tag name**: points to the commit of that tag.
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4.`HEAD` or`@`: points to the**current** commit you're at.
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- for some other special identifier names, see documentation.
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5.`-` or`@{-1}`: previous commit you're at.
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-`-` is not a valid identifier in some scenarios, but you can use it in like`git checkout -` or`git merge -`.
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6.**commit expression**: see next section.
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###Commit Expression
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Commit expression is special syntax to reference commits backward from a given point.
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Three special symbols for traversing back in git:
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1.`@`: reference backward through***reflog***, you can see such symbol like`HEAD@{1}` in`git reflog` output.
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-`@{-<n>}`: go to nth previously checked out commit by`git checkout`
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-`<commit_identifier>@{<n>}`: go to previous nth commit
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-`@{<n>}` is equivalent to`<current_branch>@{<n>}`, this is only valid when you're on a branch.
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2.`~`: trace back to previous commits through**the first parent** only.
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-`<identifier>~3`: points to previous 3 commit from current branch.
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3.`^`: trace back to previous commits through a specified parent by its index.
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-`<identifier>^2`: points to previous commit through**second** parent of`<identifier>`.
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-`<identifier>^101`: if you're wild enough to merge from 100 branches, you might need to point to previous commit through the 101-th parent.
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>[!TIP]
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>`~1` and`^1` are always equivalent since they would pick the same first parent of the commit.
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>[!NOTE]
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>See`git help revisions` for more details
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>[!TIP]
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>You can expand the commit hash by special identifier using`git rev-parse <identifier>`
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>```console
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>$ git rev-parse @~1
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>42804017200559f28d12757e729f95dbd18f4998
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>$ git rev-parse main
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>e7bbe687097f56d152c2291190b67d91eed4cd57
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>```
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## Commit Inspection
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Commit inspection generally requires `git cat-file -p <hash>`, see `git help cat-file`.
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A commit can contain elements:
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- `tree`: the structure of current snapshot, including files and folders.
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- `parent`: the parent commit it was derived from, a commit can have multiple parents.
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- `author`: who did the changes.
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- `committer`: who made the commit, may differ from `author` when rebasing.
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- and more...
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```console
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$git cat-file -p 5eb5a747cfd9b17603b79d7ab64fc4ecee1751e3
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tree e060913d1a04d39082981dc116c5a5cd35ba8e52
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parent 02348ec70c2c901e9ecc4be5860330e6dcf911ff
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author sharpchen <rui.chen.sharp@gmail.com> 1749697268 +0800
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committer sharpchen <rui.chen.sharp@gmail.com> 1749697268 +0800
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```
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>[!TIP]
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>It's more general to use`git show <commit_identifier>` to inspect a commit.
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###Tree & Blob Inspection
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Tree in the inspected commit is a**recursive** structure and also has a hash that can be inspected by`git cat-file -p <hash>`.
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A`blob` represents a real file, a`tree` represents a folder as a container.
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So the tree printed from the commit is actually**the root folder of your repository**.
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```console
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$git cat-file -p e060913d1a04d39082981dc116c5a5cd35ba8e52
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100644 blob 397b4a7624e35fa60563a9c03b1213d93f7b6546 .gitignore
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100644 blob 67f24d19be001eebf7bcdde488d8ab594bcd969d PSScriptAnalyzerSettings.psd1
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100644 blob 4bd5bb6fcff0b514d0160f3cb8ff01b5de46eeb4 README.md
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100644 blob 207846dc4b8f93edd892df9b6058ded48f5355b8 dotfiles.ps1
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100755 blob fa9fe71978d17992a3f27eaf78f2619a25a1966d dotfiles.sh
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040000 tree 0199c6476a45a6dadadaf7baf70a136d7f8f217f dotfiles
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100644 blob b6e1531298fed5dc7a019f6c01018b5c1add3c99 flake.lock
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100644 blob 92d9549668557adb021f959f8b898ad96ee84efb flake.nix
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100644 blob bb166d071cb8541aac7df35b7c1f5dbf420dd70c home.nix
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100644 blob baa979aff962214ddb5e23502acdb211d8d32064 install.ps1
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040000 tree c1aef838a425cd6f209ecc01f28bef2208fd2d26 packages
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```
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You can either go deeper to the nested`tree` or inspect the snapshot content of a`blob` using the same way.
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```console
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$git cat-file -p 67f24d19be001eebf7bcdde488d8ab594bcd969d
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@{
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IncludeRules = @('PSAvoidTrailingWhitespace')
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}
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```
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##Checkout Commit
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#Git History & Reset
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-`git log`: commit logs that can be synchronized with remotes.
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-`git reflog`: local operation logs which is more detailed but limited to local repository.
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-**dynamic pointers** such as`HEAD` and branches have their dedicated reflog on their own.
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##Commit History Query
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-`-<n>`: limits count of commit logs
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-`--skip=<n>`: skip count
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- ..., see:`git help log` in*Commit Limiting* section
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###History Exclusion
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Use`^` to exclude commits from certain points, such as`git log ^main foo` to exclude commits from`main` but keep commits from`foo`.
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Example: show commits within branch`pwsh_es` but not in`master`:
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```console
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$git log ^master pwsh_es
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* e8284ebb96d8 (origin/pwsh_es, pwsh_es) maintainers: add sharpchen
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* c0c964d8ae68 powershell-editor-services: init at 4.1.0
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$git log master..pwsh_es
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* e8284ebb96d8 (origin/pwsh_es, pwsh_es) maintainers: add sharpchen
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* c0c964d8ae68 powershell-editor-services: init at 4.1.0
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```
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#Branching
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Each branch was deviated from certain commit of another branch, and could have its own`HEAD` when actual divergence was applied.
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So a branch besides the main branch is essentially a sequence of commits starting with a divergence from another branch.
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>[!IMPORTANT]
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>Branch is a concept upon commits, the branch is sometimes equivalent to the latest commit of that branch.
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>A branch is just a special commit points to latest commit of that branch and can trace back to the divergence.
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##Branch Identifier
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- branch name
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-`@{upstream}` or`@{u}`: refer to the local upstream
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##Branch Inspection
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A branch has its dedicated text file containing its latest commit hash at`./.git/refs/heads/<branch>`
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```console
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$cat ./.git/refs/heads/msbuid_ls
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4bfbc0c67e39f5615e0cdc4535611af1e71040d8
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```
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If the latest commit hash is the only thing the branch knows, how could it collect all of the commits when git requires to?
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The fact is, commits are chained with their parent and child, so git can track back along to the point where the branch was diverged.
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##Branch Manipulation
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>[!NOTE]
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>See`git help branch` and`tldr git-branch`.
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>Or`git help switch` and`tldr git-switch`.
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-`git branch`
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-`-d|--delete`: delete branch
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-`-m|--move`: rename branch
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-`-c|--copy`: copy branch
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##Merging
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###Fast-Forward & Three-way Merging
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‎docs/document/Skill/PowerShell/docs/Language/Array.md‎

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>
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>$(1,2,3).Length # 3 this has different semantic but with same result
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>
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>(,(1,2,3)).Length # 3 # does spread items # [!code highlight]
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>(,(1,2,3)).Length # 3 # does spread items # [!code highlight]
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>
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>(,@(1,2,3)).Length # 1 # does not spread items # [!code highlight]
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>(,@(1,2,3)).Length # 1 # does not spread items # [!code highlight]
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>
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>((gci), (gci ..)).Length # 2 # [!code highlight]
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>((gci), (gci ..)).Length # 2 # [!code highlight]
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> ```
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### From Expression and Statement
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Age = 18
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}
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@($foo.GetEnumerator()).Length # 2, System.Collections.DictionaryEntry from the HashTable # [!code highlight]
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@($foo.GetEnumerator()).Length # 2, System.Collections.DictionaryEntry from the HashTable # [!code highlight]
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```
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###From Range
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Range operator can create array from integer range and character range inclusively from both sides.
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>[!NOTE]
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>character range was added in PowerShell 6
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>character range was added in PowerShell 6
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```ps1
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1..10 # 1 to 10 inclusively
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((1,2,3) * 3).Length # 9
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```
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Apratical usage of repetition is initialization with same value to the whole array.
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Apractical usage of repetition is initialization with same value to the whole array.
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```ps1
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@(255) * 100 # Fill up array sized 100 with 255 to all elements
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(1..10)[0..2+4..5+7]
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```
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##Substration
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##Subtraction
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Tosubstract a collection from another collection, you can certainly use`LINQ` or use a simple pipeline.
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Tosubtract a collection from another collection, you can certainly use`LINQ` or use a simple pipeline.
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```ps1
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@(1,2,3) | where { @(1, 2) -notcontains $_ } # 3
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$arr -eq $null # empty array
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$null -eq $arr # False, the result we expected # [!code highlight]
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$null -eq $arr # False, the result we expected # [!code highlight]
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```
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```ps1
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$a, $b, $c = 1,2,3 # $a = 1, $b = 2, $c = 3
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$a, $b = 1,2,3 # $a = 1, $b = 3 This might not be expected # [!code warning]
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$a, $b = 1,2,3 # $a = 1, $b = 3 This might not be expected # [!code warning]
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```
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>[!WARNING]

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