What are commit-ish and tree-ish in Git?
Note for non-native English [sic!] speakers: "-ish" is a suffix that can be applied to an adjective in order to indicate "having qualities like" or "slightly" - see http://chambers.co.uk/search/?query=ish&title=21st
Hence "tree-ish" - like a "tree" .... "commit-ish" - like a "commit"
eg "Mars appears like a reddish star" ("d" is doubled !); "the food on the plate was not hot, but warmish"
I believe that this helps explain "what are ..." better, in that it explains the language usage.
The Short Answer (TL;DR)
Here's a complete list of commit-ish and tree-ish identifiers (from the Git revisions documentation):
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| Commit-ish/Tree-ish | Examples
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| 1. <sha1> | dae86e1950b1277e545cee180551750029cfe735
| 2. <describeOutput> | v1.7.4.2-679-g3bee7fb
| 3. <refname> | master, heads/master, refs/heads/master
| 4. <refname>@{<date>} | master@{yesterday}, HEAD@{5 minutes ago}
| 5. <refname>@{<n>} | master@{1}
| 6. @{<n>} | @{1}
| 7. @{-<n>} | @{-1}
| 8. <refname>@{upstream} | master@{upstream}, @{u}
| 9. <rev>^ | HEAD^, v1.5.1^0
| 10. <rev>~<n> | master~3
| 11. <rev>^{<type>} | v0.99.8^{commit}
| 12. <rev>^{} | v0.99.8^{}
| 13. <rev>^{/<text>} | HEAD^{/fix nasty bug}
| 14. :/<text> | :/fix nasty bug
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| Tree-ish only | Examples
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| 15. <rev>:<path> | HEAD:README.txt, master:sub-directory/
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| Tree-ish? | Examples
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| 16. :<n>:<path> | :0:README, :README
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Identifiers #1-14 are all "commit-ish", because they all lead to commits, but because commits also point to directory trees, they all ultimately lead to (sub)directory tree objects, and can therefore also be used as "tree-ish".
#15 can also be used as tree-ish when it refers to a (sub)directory, but it can also be used to identify specific files. When it refers to files, I'm not sure if it's still considered "tree-ish", or if acts more like "blob-ish" (Git refers to files as "blobs").
The Long Answer
Commits and Directory Trees in Git
At its lowest levels, Git keeps track of source code using four fundamental objects:
- Annotated tags, which point to commits.
- Commits, which point to the root directory tree of your project.
- Trees, which are directories and subdirectories.
- Blobs, which are files.
Each of these objects has its own sha1 hash ID, since Linus Torvalds designed Git like an content- addressable filesystem, i.e. files can be retrieved based on their content (sha1 IDs are generated from file content). The Pro Git book gives this example diagram:
Commit-ish vs Tree-ish
Many Git commands can accept special identifiers for commits and (sub)directory trees:
"Commit-ish" are identifiers that ultimately lead to a commit object. For example,
tag -> commit"Tree-ish" are identifiers that ultimately lead to tree (i.e. directory) objects.
tag -> commit -> project-root-directory
Because commit objects always point to a directory tree object (the root directory of your project), any identifier that is "commit-ish" is, by definition, also "tree-ish". In other words, any identifier that leads to a commit object can also be used to lead to a (sub)directory tree object.
But since directory tree objects never point to commits in Git's versioning system, not every identifier that points to a (sub)directory tree can also be used to point to a commit. In other words, the set of "commit-ish" identifiers is a strict subset of the set of "tree-ish" identifiers.
The set of tree-ish identifiers that cannot be used as commit-ish are
<rev>:<path>, which leads directly to directory trees, not commit objects. For example,HEAD:subdirectory.Sha1 identifiers of directory tree objects.