Reading an external file failed with dimension related errors

The text file you create contains TeX commands, which are present in the descriptions of some of the packages. Thus you need to detokenize somewhere. If you do it after reading, TeX will insert spaces after the control sequences. So I'd read as strings, and then deal with the i and end-of-line as 'other' characters in your tidy-up macros:

\documentclass{article} \usepackage[T1]{fontenc} \begingroup \catcode`\^^M=12\relax% \expandafter\gdef\expandafter\trimtok\detokenize{i}#1^^M{#1}% \gdef\trimmer#1{\expandafter\trimtok #1}% \endgroup \newread\reader \immediate\write18{tlmgr list --only-installed > installed-packages.txt} \begin{document} \makeatletter \begin{enumerate} \openin\reader=installed-packages.txt\relax \loop \readline\reader to \data \unless\ifeof\reader \item \trimmer{\data} \repeat \closein\reader \end{enumerate} \makeatother \end{document} 

The actual error you get is related to the first 'problematic' line

i addlines: A user-friendly wrapper around \enlargethispage. 

which tries to execute \enlargethispage without a suitable argument!