Piping a Tar Stream Directly into Unrar

Piping a tar stream directly into the unrar utility will result in an immediate failure, because unrar cannot parse or extract the tar archive format. When you attempt this operation, unrar expects proprietary RAR file headers and structures, encounters the incompatible headers of a TAR byte stream instead, and terminates execution with an error indicating an unrecognized or corrupt archive.

Format Incompatibility

The core reason the operation fails is a structural mismatch between archive specifications. The tar utility creates an uncompressed, continuous stream of 512-byte blocks, containing file headers and sequential file data. Conversely, unrar requires files formatted strictly according to the RAR specification, which begins with a distinct magic byte signature (52 61 72 21 1A 07 for standard RAR archives) followed by specific block checksums and metadata tables. Because the incoming byte stream from tar lacks these signatures, unrar rejects the input data immediately.

Handling of Standard Input (stdin)

Most official builds of the non-free unrar binary—as well as the open-source variants—are designed to work primarily with random-access file paths on disk rather than continuous standard input streams. While some Unix utilities accept a hyphen (-) to represent standard input, standard unrar commands such as unrar x - often treat - literally as a filename or reject it as an invalid syntax. Even when redirected or piped into an environment that feeds stdin directly to the process, unrar cannot seek backward or forward through a non-seekable pipe, which is typically required to traverse RAR headers.

Expected Error Messages

When running a command such as tar -cf - my_folder | unrar x -, you will typically see one of the following terminal errors:

  • Cannot open -: The utility attempts to find a literal file named - on the filesystem rather than reading standard input.
  • Corrupt archive or unrecognized format: If an implementation does read the stream, it reads the initial bytes, detects that the RAR signature is absent, and aborts.
  • No files to extract: The utility exits immediately with a non-zero exit status code.

The Correct Approach

To extract or manipulate a stream generated by tar, you must use tools designed to read the TAR format:

  • Direct TAR extraction: Pipe directly into tar using tar -cf - source/ | tar -xf - -C destination/.
  • Standard compression: If compression is required during streaming, use standard compatible tools such as gzip, bzip2, or xz (e.g., tar -czf - source/ | tar -xzf -).
  • Multi-format extractors: If using a universal tool like bsdtar, it can automatically detect and extract TAR formats from stdin, but standard unrar remains exclusively bound to RAR archives.