运算符优先级决定术语的表达分组。这会影响一个表达式是如何进行计算。某些运算符的优先级高于其他运算符;例如，乘法运算符的优先级比所述加法运算高。
例如x= 7+ 3 *2;这里，x被赋值13，而不是20，因为运算符 * 优先级高于+，所以它首先被乘以 3 * 2，然后加上7。
这里，具有最高优先级的操作出现在表格上方，那些具有最低出现在底部。在一个表达式，更高的优先级运算符将首先计算。
| 分类 | 操作符 | 关联 |
|---|---|---|
| Unary | + - | Right to left |
| Multiplicative | * / % | Left to right |
| Additive | + - | Left to right |
| Shift | << >> | Left to right |
| Relational | < <= > >= | Left to right |
| Equality | == != | Left to right |
| Bitwise AND | & | Left to right |
| Bitwise XOR | ^ | Left to right |
| Bitwise OR | | | Left to right |
| Logical AND | && | Left to right |
| Logical OR | || | Left to right |
| Ternary | ?: | Right to left |
试试下面的例子来理解Tcl语言可供选择的运算符优先级：
#!/usr/bin/tclsh set a 20 set b 10 set c 15 set d 5 set e [expr [expr $a + $b] * $c / $d ] ;# ( 30 * 15 ) / 5 puts "Value of (a + b) * c / d is : $e\n" set e [expr [expr [expr $a + $b] * $c] / $d] ;# (30 * 15 ) / 5] puts "Value of ((a + b) * c) / d is : $e\n" set e [expr [expr $a + $b] * [expr $c / $d] ] ;# (30) * (15/5) puts "Value of (a + b) * (c / d) is : $e\n" set e [expr $a + [expr $b * $c ] / $d ] ;# 20 + (150/5) puts "Value of a + (b * c) / d is : $e\n"
当编译和执行上面的程序，会产生以下结果：
Value of (a + b) * c / d is : 90 Value of ((a + b) * c) / d is : 90 Value of (a + b) * (c / d) is : 90 Value of a + (b * c) / d is : 50
