In my case, the directory tree is following
[peter@CentOS6 a]$ tree
.
├── 2.txt
└── b
└── 1.txt
1 directory, 2 files
why the following two command does only get 2.txt?
[peter@CentOS6 a]$ ls -R *.txt 2.txt [peter@CentOS6 a]$ ls -R | grep *.txt 2.txt
It isn't clear if you are asking "why" meaning "explain the output" or "how should it be done". Steephen has already answered the latter, this is an answer to the former.
The reason for that is called "shell expansion". When you type *.txt in the command line, the program doesn't get it as a parameter, but rather the shell expands it and then passes the results.
*.txt expands to be "all files in the current directory with arbitrarily many symbols in the beginning, ending with '.txt' and not starting with '.'".
This means that when you type "ls -R *.txt" the command that actually executes is "ls -R 2.txt"; and when you do "ls -R | grep *.txt" it actually executes "ls -R | grep 2.txt".
This is the exact reason why Steephen has put quotation marks around the wildcard in the answer provided. It is necessary to stop this expansion. In fact you could also do so with single quotes or by placing a slash before any special character. Thus any of the following will work:
find . -name "*.txt"
or
find . -name '*.txt'
or
find . -name \*.txt