npm debug log missing when home directory is read‑only in CI

Direct answer

When npm cannot open its debug‑log file for writing, it writes an error message to stderr (e.g., "Failed to write debug log: EACCES: permission denied, open …") and continues the command. It does not silently skip log creation nor fall back to another location.

Confirmed facts

• npm determines the debug‑log path from the config value logfile (default: ~/.npm/_logs/.log for npm ≥ 7, npm-debug.log in the current working directory for npm < 6).
• If the target directory or file cannot be opened for writing, npm emits the permission‑denied warning to stderr and proceeds without creating the log.
• The log location can be overridden at runtime by setting the environment variable npm_config_logfile or by running npm config set logfile before the failing command.

Likely explanation

In a CI runner where $HOME is mounted read‑only, the default log directory (~/.npm/_logs) is unwritable. npm therefore fails to open the log file, prints the warning to stderr, and no debug log is produced unless the logfile config is explicitly redirected to a writable location.

Steps to obtain a usable debug log in CI

1. Check the npm version in use (run npm --version) – this determines the default path.
2. Export a writable log file location, for example:

   export npm_config_logfile=/tmp/npm-debug.log

3. Run the failing npm command as usual; stderr will no longer contain the permission‑denied warning.
4. After the command finishes, inspect the log at the path you set (/tmp/npm-debug.log) for the debug output.

To tailor the recommendation to your environment, please confirm the npm version you are running (output of npm --version).