关于cat用法的一点疑问？

[yang_hui1986527]

详见下面的代码，能否解释下cat的这种用法什么意思？

代码： 全选

cat <<MAYDAY
 ----------------------------------------------------------
User: $USER          Host:$THIS_HOST         DATE:$MYDATE
----------------------------------------------------------
	1:List files in current directory
	2:Use the vi editor
	3:See who is on the system
	H:Help screen
	Q:Exit Menu	
----------------------------------------------------------
MAYDAY

代码： 全选

#!/bin/bash
#menu
#set the date,user and hostname up
MYDATE=`date +%d/%m/%Y`
THIS_HOST=`hostname -s`
USER=`whoami`
#loop forever !
while :
do
 #clear the screen
 clear
 #here documents starts here 
 cat <<MAYDAY
 ----------------------------------------------------------
User: $USER          Host:$THIS_HOST         DATE:$MYDATE
----------------------------------------------------------
	1:List files in current directory
	2:Use the vi editor
	3:See who is on the system
	H:Help screen
	Q:Exit Menu	
----------------------------------------------------------
MAYDAY

 #here document finished
echo -e -n "\tEnter Your Choice [1,2,3,H,Q] >"
read CHOICE
 case $CHOICE in
 1) ls
  ;;
 2) vi
 ;;
 3) who
 ;;
 H|h) 
#use a here document for the help screen
cat <<MAYDAY
This is the help screen,nothing here yet to help you!
MAYDAY
 ;;
Q|q) exit 0
 ;;
 *) echo -e "\t\007unknown user response"
 ;;
esac
echo -e -n "\tHit the return key to continue"
read DUMMY
done

[BigSnake.NET]

不关 cat 的事
这里的意思是把两个 MAYDAY 之间的内容作为 cat 的标准输入

[hellojinjie]

那 <<< 三个连在一起呢?

[BigSnake.NET]

代码： 全选

   Here Documents
       This  type  of  redirection  instructs the shell to read input from the
       current source until a line containing  only  word  (with  no  trailing
       blanks)  is seen.  All of the lines read up to that point are then used
       as the standard input for a command.

       The format of here-documents is:

              <<[-]word
                      here-document
              delimiter

       No parameter expansion, command substitution, arithmetic expansion,  or
       pathname expansion is performed on word.  If any characters in word are
       quoted, the delimiter is the result of quote removal on word,  and  the
       lines  in the here-document are not expanded.  If word is unquoted, all
       lines of the here-document are subjected to parameter  expansion,  com-
       mand  substitution,  and arithmetic expansion.  In the latter case, the
       character sequence \<newline> is ignored, and \ must be used  to  quote
       the characters \, $, and `.

       If the redirection operator is <<-, then all leading tab characters are
       stripped from input lines and  the  line  containing  delimiter.   This
       allows  here-documents within shell scripts to be indented in a natural
       fashion.

   Here Strings
       A variant of here documents, the format is:

              <<<word

       The word is expanded and supplied to the command on its standard input.