get-tar-commit-id Git Command Guide

The git get-tar-commit-id command reads a tar archive created by git archive from standard input and extracts the commit ID stored in it. It only reads the first 1024 bytes, making it efficient for large archives.

git get-tar-commit-id Syntax:

git get-tar-commit-id [<input-file>]

Options:

Option	Description
	File to read archive from (default: stdin)
-h, —help	Display help information

Parameters:

Parameter	Description
	Optional tar archive file to read from (reads from stdin if not provided)

Caution

Only reads first 1024 bytes of input, regardless of archive size. Returns exit code 1 if no commit ID is found (archive not created by git archive or created with tree ID instead of commit/tag).

git get-tar-commit-id Command Samples:

Extract commit ID from standard input archive

cat archive.tar | git get-tar-commit-id

Processes tar archive from stdin and outputs the commit ID.

Extract from file directly

git get-tar-commit-id archive.tar

Reads archive from specified file and extracts commit ID.

Used with git archive in pipeline

git archive --prefix=project/ HEAD | git get-tar-commit-id

Create archive and immediately extract its commit ID.

Verify archive integrity in scripts

tar -xzf repo.tar.gz && cd repo && git get-tar-commit-id ../repo.tar.gz

Extract and use commit ID for verification in deployment scripts.

Check multiple archives

for archive in *.tar.gz; do echo "$archive: $(git get-tar-commit-id < "$archive")"; done

Batch process multiple archives to check their commit IDs.

Integrate with build system

COMMIT_ID=$(git archive --prefix=build/ HEAD | tee build.tar | git get-tar-commit-id)
echo "Built from commit: $COMMIT_ID"

Create numbered builds with commit tracking.

Tip

Only works with archives created by git archive containing commit IDs. For tree-based archives, the command exits quietly with status 1. Use git archive --prefix=<path> <commit> to ensure commit ID is embedded.

What does get-tar-commit-id do exactly?

To understand what get-tar-commit-id does, know it parses tar header metadata to find the commit identifier embedded by git archive. It reads only the initial portion to remain efficient on large archives.

How is the commit ID embedded in the archive?

Git archive creates PaxHeaders containing extended attributes including the original commit identifier when archiving from commits or tags. The tar format’s Pax headers store this metadata separately from the file content.

What’s the difference between commit and tree archives?

Archives created from commits (e.g., HEAD, tags) contain commit metadata including the ID, while tree archives (tree IDs directly) lack this information because trees don’t have unique identifiers in the same way.

Can get-tar-commit-id handle compressed archives?

Yes, it works with compressed tar archives (.tar.gz, .tar.bz2, .tar.xz) because compression is at the tar level. The tool operates on the uncompressed tar stream through pipe decompression via tools like gunzip.

Why limit to first 1024 bytes?

The commit ID is stored in tar header extensions which appear very early in the archive. Limiting reading prevents processing large archives unnecessarily when the required metadata is always in the initial portion.

How do I use this in CI/CD pipelines?

Common usage extracts commit IDs for reproducible builds: ARCHIVE_COMMIT=$(cat release.tar | git get-tar-commit-id) then use that ID for tagging, logging, or verification purposes.

What formats does get-tar-commit-id support?

Supports all tar formats that git archive generates, including basic ustar and extended formats with PaxHeaders (PAX format). Modern git uses PAX extensions to store additional metadata.

How does this relate to git describe?

While git describe gives human-readable references, get-tar-commit-id extracts the exact commit hash used to create the archive, providing definitive provenance information.

Can this be used to verify archive authenticity?

While it provides provenance (which commit created the archive), it doesn’t verify cryptographic signatures. For authenticity, combine with signed commits and external verification methods.

Why exit with code 1 when no ID found?

The exit code allows scripts to detect invalid/non-git-archived input. Shell scripts can use this to distinguish between archive format errors and successful extraction.

Applications of the git get-tar-commit-id command

1. Build System Integration: Extract commit IDs from deployment archives for reproducible builds
2. Deployment Verification: Confirm which code version was packaged for rollback debugging
3. Archive Management: Catalog collections of tarballs by their originating commits
4. CI/CD Pipeline Tracing: Track exact commit versions through deployment pipelines
5. Release Management: Associate binary distributions with exact source code versions
6. Forensic Analysis: Determine commit origins from recovered or backup archives