The -f operator returns true only if the file exists.

只有在文件存在时，- f操作符才返回true。

youdao

They're usually made of fleece or wool and are only necessary if the temperature or wind chill is below 10 degrees f.

他们通常利用羊毛或者羊毛做成而且是唯一的必需品如果温度或者风寒冷在10度f下面。

youdao

Then T is an invertible linear operator preserving rank - partial ordering on Sn(F) if and only if there exists an invertible matrix (F) such that where .

刻画了在非负无零因子交换半环上强保持可逆矩阵的线性算子。

youdao

"If I'm a platoon with nobody behind, and I know that only that F-15 stands between me and the possibility of being overrun, then you're right I have an obligation to protect my soldiers or my Marines, " he says.

NPR: Commanders Look For New Strategy In Afghanistan

If they do need some comfort, they only need look at Ford (NYSE: F), who continue to believe in the power of Facebook, spending millions on advertising AND building a presence.

FORBES: GM Doesn't "Like" Facebook, Drops Ads