vue 安裝成功，項目創建也成功了，執行npm run dev就失敗了，失敗結果

> [email protected] dev /Users/ann/test1

> webpack-dev-server --inline --progress --config build/webpack.dev.conf.js

sh: webpack-dev-server: command not found

npm ERR! file sh

npm ERR! code ELIFECYCLE

npm ERR! errno ENOENT

npm ERR! syscall spawn

npm ERR! [email protected] dev: `webpack-dev-server --inline --progress --config build/webpack.dev.conf.js`

npm ERR! spawn ENOENT

npm ERR!?

npm ERR! Failed at the [email protected] dev script.

npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm WARN Local package.json exists, but node_modules missing, did you mean to install?

npm ERR! A complete log of this run can be found in:

npm ERR! ? ? /Users/ann/.npm/_logs/2018-10-28T08_11_37_746Z-debug.log

，npm -v也顯示版本的。

有時候執行其他命令會出現command not found，比如我想用brew安裝node就會出現-bash: brew: command not found。

我是mac電腦，請求高手老師幫忙，困擾好多天了，萬分感謝