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#### 条件表达式
```
# 文件比较
-f文件存在, -d目录存在, -e,文件或目录存在，-s文件存在且非空, -r文件存在且可读, -w, -x
# 字符串比较
==, =, !=, < , >, -n字符串非空, -z字符串为空
# 数字比较
-eq, -ne, -lt, -le, -gt, -ge
# 逻辑运算
-a且，-o或，&&且, ||或
#### 参考资料
1. [bash手册](http://www.gnu.org/software/bash/manual/bash.html)
2. `man bash`