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UBA build fails with Psych::SyntaxError due to backslashes in script path

Updated

This article describes a Unity Build Automation (UBA) build failure caused by an internal YAML syntax error that occurred, triggered after backend and runner updates. The error appeared as a Psych::SyntaxError referencing an unexpected hexadecimal number while parsing a quoted scalar. The root cause was a backslash (\) used in the path of the pre-build script field, which became invalid under stricter YAML parsing. Updating the path to use forward slashes (/) resolved the issue and restored successful builds.

 

Symptoms:

  • UBA builds suddenly start failing after previously working for weeks.
  • Build logs show errors similar to:
did not find expected hexdecimal number while parsing a quoted scalar at line 26 column 22 (Psych::SyntaxError)
  • Failures persist even after triggering clean builds.

Causes:

  • Recent backend and build runner updates introduced stricter YAML parsing via the Psych library.
  • In YAML, when values are double-quoted, the backslash (\) is treated as an escape character and must form a valid escape sequence.
  • The pre-build script field in the Advanced Settings contained a Windows-style path with backslashes (for example, \ucb_scripts\pre_build.sh), which Psych interpreted as invalid escape sequences, causing a Psych::SyntaxError.

Resolution:

  1. Open your Configurations section in the Unity Dashboard and navigate to the affected build target (for example, main).
  2. Go to Advanced Settings.
  3. Locate the 'Pre-build script' (or similar) field.
  4. Replace any backslashes (\) in the script path with forward slashes (/). For example:
    • Change: \ucb_scripts\pre_build.sh
    • To: ./ucb_scripts/pre_build.sh (or ucb_scripts/pre_build.sh, depending on your setup).
  5. Save the configuration.
  6. Trigger a new clean build for the affected target.
  7. Confirm that the build completes successfully and the Psych::SyntaxError no longer appears in the logs.

More Information:

For more information about pre and post-build scripts, please review the Run custom scripts during the build process documentation entry.

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