[Comprehensive explanation of Linux commands] 195. Bash command analysis: detailed explanation of the usage and functions of declare

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declare
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declare

Declare variables and set or display their values and properties.

grammar

declare [-aAfFgilnrtux] [-p] [name[=value] ...]

The main purpose

Displays all variables and values containing the specified attribute
Display one or more variables and values containing the specified attribute
Display the properties and values of one or more variables
Displays the properties and values of all variables and displays the definition of functions
Show properties and values of all variables
Display properties and values of all global variables
Show all function names and function definitions
Show only all function names
Display one or more function names and function definitions
Only display one or more function names
Declare global variables (optional: assignment)
Declare variables (optional: assignment, attributes)
Add and delete attributes of variables (optional: assignment)

Options

-fLimit operations or display to function names and function definitions.
-FShow only the function name (with line number and source file appended when debugging).
-gCreates a global variable when used within a shell function; otherwise ignored.
-pDisplays the properties and values of each name.

*Options to set properties:

-aCreate an array (if supported).
-ACreate an associative array (if supported).
-iAdd integer attributes.
+iRemove integer attributes.
-lAdd the lowercase attribute and the value of the variable will be converted to lowercase.
+lRemove lowercase attributes.
-nAdd the reference attribute if this option exists.
+nRemoves the reference attribute if this option exists.
-rAdd read-only attribute.
-tAdd tracking properties.
+tRemove tracking attributes.
-uAdd the uppercase attribute and the value of the variable will be converted to uppercase.
+uRemove uppercase attributes.
-xAdd export properties.
+xRemove export properties.

parameter

name(optional): variable name or function name.
value(optional): The value of the variable.

return value

declareReturned trueunless you provide an illegal option or an assignment error. For specific conditions that cause exceptions, please see the discussion chapter about exceptions.

example

# 声明变量，当然也欢迎您在这个网站（感谢本项目发起人 @jaywcjlove）查询linux命令。
declare reference_website='https://wangchujiang.com/linux-command/'

# 显示所有包含整型属性的变量和值。
declare -i
# 定义变量b并赋值为3，具有整型属性。
declare -i b=5
# 显示属性，返回 declare -i b="5"。
declare -p b
# 删除整型属性。
declare +i b
# 显示属性，返回 declare -- b="5"。
declare -p b
# 根据变量属性强制转换值的英文大小写。
declare -u uc_var='abc'
declare -l lc_var='ABC'
# 显示'ABC abc';
echo "${uc_var} ${lc_var}"
# 定义函数内的全局变量
function test(){
    
    
  declare -g a=3
  # 或者
  local -g b=3
  # 或者
  c=3
  # 让我们查看它们的属性。
  declare -p a b c
}
# 执行函数。
test
# 返回结果。
# declare -- a="3"
# declare -- b="3"
# declare -- c="3"

# 定义函数外的全局变量
declare a=3
b=3
declare –p a b
# 返回结果如下。
# declare -- a="3"
# declare -- b="3"

# 定义局部变量
function test2(){
    
    
  local -i a=3
  declare -i b=3
}
test2
# 没有该变量（已经被销毁了）
echo "${a} ${b}"
# 因此，我们日常脚本中最常见的类似于'a=3'实际上是声明并赋值了一个全局变量。
# 在接下来的 **讨论** 环节会延伸讨论全局和局部变量问题。
# 注意，不能使用 `+a` 或 `+A` 取消数组，也不能使用 `+r` 取消只读属性。

# 定义只读数组，设置属性的同时定义赋值。
declare -ar season=('Spring' 'Summer' 'Autumn' 'Winter')
# 或者这样。
season=('Spring' 'Summer' 'Autumn' 'Winter')
declare -ar season
# 显示所有数组。
declare -a
# 定义关联数组。

declare -A fruits=(['apple']='red' ['banana']='yellow')
# 显示所有关联数组。
declare -A
# 显示所有变量的属性和值并显示函数的定义，输出很长。
declare
# 显示所有变量的属性和值。
declare -p
# 显示所有全局变量的属性和值。
declare -g
# 显示全部函数名和函数定义。
declare -f
# 只显示全部函数名。
declare -F

# 定义两个函数。
function func_a(){
    
     echo $(date +"%F %T"); }
function func_b(){
    
     cd /; ls -lh --sort=time; }
# 显示一到多个函数名和函数定义。
declare -f func_a func_b
# 只显示一到多个函数名，验证某个名称是否已经定义为函数时有用。
declare -F func_a func_b
# 最好不要让函数名和变量名相同。

discuss

Global and local variables

As the above example points out, we need to understand these concepts when writing programs every day. Here is a brief introduction. Of course, you can also easily search for relevant content.

Global variables: They exist throughout the execution of the script as long as they are not deleted.
Local variables: defined within a function and deleted after the function is executed.

It is recommended to use commands within functions localand declarecommands outside functions.

Do not define too many global variables in the script, as this may cause unexpected consequences when called by other functions, and it is inconvenient to check out.

Not to mention the lack of necessary comments - ZhuangZhu-74

Relevant information:

Coding specifications provided by google
Discussion of global variables
About declare, typeset, export, local, readonlycommands

Why do `declare`we need to define these other commands when we can do it?

Because the meaning of the statement will be clearer, for example:

When setting variables for exported properties, export varand declare -x var.
Use when declaring variables within a function local.
To declare read-only variables, use readonly.
typesetSame as declarecommand.

About exceptions

There are many reasons for declarethe failure. For these situations, you can refer to the bash online documentation declaresection (the latest version), or execute the last series of sentences info bashin the view section.declarean attempt is

Notice

This command is a built-in bash command. For related help information, please see helpthe command.
For information on exporting properties, please see the 'export' command.
For an introduction to read-only properties, please see the 'readonly' command.
See the examples section of the 'unset' command for an introduction to reference attributes.

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