Gulp cannot be used after installation

  node.js, question

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Gulp has been installed before, but today I met such a problem. No matter whether it is a global or a local installation, gulp cannot be run after the installation, and the installation is not wrong.
The same is true of cnpm. After the installation is completed, it cannot run and does not report an error.
Can you tell me what causes this?

The global directory of your npm is not in the PATH environment variable.