Shell之declare定义变量

Posted 2020-10-20

实验环境

 ~]# cat /etc/redhat-release 
CentOS Linux release 7.3.1611 (Core) 

命令说明

declare 与 typeset 命令都是bash的内建命令（builtin commands），两者所实现的功能完全一样，用来设置变量值和属性。
typeset现已弃用，由declare进行替代，可查看帮助手册：

~]# help typeset
typeset: typeset [-aAfFgilrtux] [-p] name[=value] ...
    Set variable values and attributes.    
    Obsolete.  See `help declare‘.

~]# help declare
declare: declare [-aAfFgilrtux] [-p] [name[=value] ...]
    Set variable values and attributes.

命令选项

typeset 和 declare的选项参数是通用的，下面以declare进行说明：

Declare variables and give them attributes. If no NAMEs are given, display the attributes and values of all variables.
declare [-aAfFgilrtux] [-p] [name[=value] ...]

选项：

-f [name]：列出之前由用户在脚本中定义的函数名称和函数体；
-F [name]：仅列出自定义函数名称；
-g name：在shell函数中可创建全局变量；
-p [name]：显示指定变量的属性和值；
-a name：声明变量为普通数组；
-A name：声明变量为关联数组（支持索引下标为字符串）；
-i name ：将变量定义为整数型（求值结果仅为整数，否则显示为0）；
-r [name[=value]] 或 readonly name：将变量定义为只读（不可修改和删除）；
-x name[=value] 或 export name[=value]：将变量设置为环境变量；

PS：使用 + 可取消定义的变量类型，如取消整数变量定义declare +i name。

unset name：取消变量的属性和值，只读变量除外。
Unset values and attributes of shell variables and functions.

使用示例

#!/bin/bash


echo "Set a custom function - func1"
echo
func1 ()
{
  echo This is a function.
}


echo "Lists the function body."
echo "============================="
declare -f        
echo
echo "Lists the function name."
echo "============================="
declare -F       


echo


declare -i var1   # var1 is an integer.
var1=2367
echo "var1 declared as $var1"
var1=var1+1       # Integer declaration eliminates the need for ‘let‘.
echo "var1 incremented by 1 is $var1."
# Attempt to change variable declared as integer.
echo "Attempting to change var1 to floating point value, 2367.1."
var1=2367.1       # Results in error message, with no change to variable.
echo "var1 is still $var1"


echo


declare -r var2=13.36         # ‘declare‘ permits setting a variable property
                              #+ and simultaneously assigning it a value.
echo "var2 declared as $var2" # Attempt to change readonly variable.
echo
echo "Change the var2‘s values to 13.37"
var2=13.37                    # Generates error message, and exit from script.


echo "var2 is still $var2"    # This line will not execute.


exit 0                        # Script will not exit here.

参考链接

declare Advanced Bash-Scripting Guide