如果仅有if，标准语法：if [ 表达式 ]; then... fi
#!/bin/bash
if [ "$#" -gt 0 ]
then
echo "There's Beans"
fi
if [ "$1" = "cool" ]
then
echo "Cool Beans"
fi
如果需要else：
#!/bin/bash
if [ "$1" = "cool" ]
then
echo "Cool Beans"
else
echo "Not Cool Beans"
fi
更一进步的，添加else if：
#!/bin/bash
if [ "$1" = "cool" ]
then
echo "Cool Beans"
elif [ "$1" = "neat" ]
then
echo "Neato cool"
else
echo "Not Cool Beans"
fi
if 语句所使用的条件表达式总结如下：
| String Comparison | Description |
|---|---|
| Str1 = Str2 | Returns true if the strings are equal |
| Str1 != Str2 | Returns true if the strings are not equal |
| -n Str1 | Returns true if the string is not null |
| -z Str1 | Returns true if the string is null |
| Numeric Comparison | Description |
|---|---|
| expr1 -eq expr2 | Returns true if the expressions are equal |
| expr1 -ne expr2 | Returns true if the expressions are not equal |
| expr1 -gt expr2 | Returns true if expr1 is greater than expr2 |
| expr1 -ge expr2 | Returns true if expr1 is greater than or equal to expr2 |
| expr1 -lt expr2 | Returns true if expr1 is less than expr2 |
| expr1 -le expr2 | Returns true if expr1 is less than or equal to expr2 |
| !expr1 | Negates the result of expression |
| File Conditionals | Description |
|---|---|
| -d file | True if the file is a directory |
| -e file | True if the file exists (note that this is not particularly portable, thus -f is generally used) |
| -f file | True if the provided string is a file |
| -g file | True if the group id is set on a file |
| -r file | True if the file is readable |
| -s file | True if the file has a non-zero size |
| -u | True if the user id is set on a file |
| -w | True if the file is writable |
| -x | True if the file is an executable |
