shell之变量替换：临时替换

​${FILE:-word} 若变量为空，给变量FILE添加一个临时默认值word,FILE本身值并不变化
eg:

FILE1=${FILE:-word} 若FILE为空,则赋予FILE1值word

---

${FILE:+word} 若变量不为空，给变量添加一个临时默认值word,FILE本身值并不变化
eg:

FILE1=${FILE:+word} 若FILE不为空,则赋予FILE1值word

---

${FILE:=word} 若变量为空，给变量FILE添加一个永久默认值word
eg:

: ${FILE:=word} 若变量为空,则赋予FILE值word

---

${FILE:?word} 若变量为空，输出错误信息word

来自为知笔记(Wiz)