Undefined control sequence in .bbl file during bibliography rendering

Direct answer

BibTeX inserts LaTeX control sequences into the generated .bbl according to the rules baked into the bibliography style (.bst) you selected. When that style emits a macro your preamble does not define—such as \newblock, \url, \href, or a custom command—LaTeX raises Undefined control sequence while reading the .bbl on the final pdflatex pass. This assumes a traditional BibTeX workflow using a .bst style, not biblatex, which generates its own .bbl with different internals.

Confirmed facts

• The error occurs in LaTeX, not BibTeX; the BibTeX log (.blg) will not name the macro.
• The exact macro name and the line number in the .bbl are recorded in the LaTeX log (.log).
• Any manual edit to the .bbl is lost the next time you run bibtex.

Likely explanation

The .bst file assumes a particular LaTeX environment. Standard styles often expect natbib (for \newblock and citation hooks) or url/hyperref (for \url and \href). Publisher or custom styles may assume non-standard macros, or even biblatex internals such as \bibinfo and \bibfield that are undefined under traditional BibTeX.

Identify the command without editing the .bbl

Search the LaTeX log for the error and the associated line number:

grep -n "Undefined control sequence" main.log

The output names the offending macro. You can open the .bbl at the reported line to confirm the context, but the correction belongs in your document preamble.

Apply the fix

1. If the macro is \newblock, add \usepackage{natbib}.
2. If it is \url, \href, or \path, add \usepackage{url} or \usepackage{hyperref}. Load hyperref near the end of the preamble.
3. If the macro is non-standard, either switch to a compatible .bst or add a minimal \providecommand in the preamble. Redefining a macro without knowing its arguments can silently corrupt bibliography formatting.

Rerun the full build: pdflatex → bibtex → pdflatex → pdflatex.

One missing diagnostic

What is the exact undefined control sequence reported in your .log? If it is a standard macro, the fix is a known package. If it is a custom or biblatex macro, the recommendation shifts to changing the bibliography style or providing a compatible definition.