Article directory
let
Simple calculator that performs arithmetic expressions.
overview
let arg [arg ...]
The main purpose
Execute one or more arithmetic expressions.
parameter
arg: Arithmetic expression
return value
Returns 1 when letthe last executed expression evaluates to 0, otherwise returns 0. When letthe divisor of the executed expression is 0, 1 is returned and an error is reported.
Operator precedence decreasing table
| operator | describe |
|---|---|
id++, id-- |
Increment after variable, decrement after variable |
++id,--id |
Variable pre-increment, variable pre-decrement |
-,+ |
Positive sign, negative sign |
!,~ |
Logical No, bitwise negation |
** |
Exponentiation |
*, /, % |
Multiplication, division, remainder |
+,- |
addition, subtraction |
<<,>> |
Bitwise shift left and right |
<=, >=, <, > |
Compare |
==,!= |
equal to, not equal to |
& |
bitwise AND |
^ |
bitwise XOR |
| ` | ` |
&& |
logic and |
| ` | |
expr ? expr : expr |
Conditional operator (ternary operator) |
=, *=, /=, %=, +=, -=, <<=, >>=, &=, ^=, ` |
=` |
example
# 尝试直接在终端中执行算术表达式（就像在python的IDLE）。
3+4
bash：3+4：command not found...
# 换一种方式。
3 + 4
bash：3：command not found...
# 看来不行。
# let命令赋值。
let a=3**4
echo ${a}
# 显示81。
# ((...))和let命令等效。
((a=3**4))
# let常用于变量赋值，而外部命令expr可直接返回表达式的值。
let 3+4
# 没有显示7。
# 执行后显示7，注意空格。
expr 3 + 4
# 条件表达式。
if ((8>4)); then
echo '8 is greater than 4.'
else
echo 'error'
fi
# 注意空格。
if [[ 12 -le 10 ]]; then
echo 'error'
else
echo '12 is greater than 10.'
fi
# 可以通过declare命令设置整型属性的方法来进行算术运算。
# local命令与此类似。
# 没有指定整型属性，输出为字符串'a+b'。
declare a=3 b=4 c
c=a+b
echo ${c}
# 不过可以使用以下方式赋值。
c=$((a+b))
echo ${c}
# 显示7
# 设置了整型属性就可以直接加了。
declare -i a=3 b=4 c
c=a+b
echo ${c}
# 同上。
declare -i a
a=2*3
echo ${a}
# 显示6。
Notice
This command is a built-in bash command. For related help information, please see helpcommand.
In addition to commands for performing arithmetic calculations let, there are also external commands expr, bcetc.
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