シェルスクリプトサンプルコードでよく見かける typeset や declare ってなに？

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# 定義
$ VAR1=123

# 出力
$ echo "$VAR1"
123

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$ X=hello

$ echo "$X"
hello

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$ declare X=hello

$ echo "$X"
hello

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$ typeset X=hello

$ echo "$X"
hello

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$ type declare
declare is a shell builtin

$ type typeset
typeset is a shell builtin

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$ man bash
...(一部省略)...
       declare [-aAfFgilnrtux] [-p] [name[=value] ...]
       typeset [-aAfFgilnrtux] [-p] [name[=value] ...]
              Declare  variables  and/or give them attributes.  If no names are given then
              display the values of variables.  The -p option will display the  attributes
              and  values  of  each name.  When -p is used with name arguments, additional
              options, other than -f and -F, are ignored.  When  -p  is  supplied  without
              name  arguments,  it will display the attributes and values of all variables
              having the attributes specified by the  additional  options.   If  no  other
              options are supplied with -p, declare will display the attributes and values
              of all shell variables.  The -f option will restrict the  display  to  shell
              functions.  The -F option inhibits the display of function definitions; only
              the function name and attributes are printed.  If the extdebug shell  option
              is enabled using shopt, the source file name and line number where each name
              is defined are displayed as well.  The -F option implies -f.  The -g  option
              forces  variables  to  be created or modified at the global scope, even when
              declare is executed in a shell function.  It is ignored in all other  cases.
              The  following  options can be used to restrict output to variables with the
              specified attribute or to give variables attributes:
...(一部省略)...