linux - What does “” exactly mean in “echo temp.txt”?

Non existing file

$ ls file_not_exists.txt
ls: cannot access file_not_exists.txt: No such file or directory
$ echo <> file_not_exists.txt

$ ls file_not_exists.txt 
file_not_exists.txt
$ cat file_not_exists.txt
$

File with content

$ cat temp.txt 
asdf
$ echo temp.txt 
temp.txt
$ echo <> temp.txt 

$ cat temp.txt 
asdf 

If the file does not exist, echo <> file_not_exists.txt will create a new file. So I think > works (redirecting the empty output into a newly created file). But if there is something in the file (like temp.txt), why it is not emptied by echo <> temp.txt?

Answer

From the Advanced Bash Scripting Guide

[j]<>filename
  #  Open file "filename" for reading and writing,
  #+ and assign file descriptor "j" to it.
  #  If "filename" does not exist, create it.
  #  If file descriptor "j" is not specified, default to fd 0, stdin.
  #
  #  An application of this is writing at a specified place in a file. 
  echo 1234567890 > File    # Write string to "File".
  exec 3<> File             # Open "File" and assign fd 3 to it.
  read -n 4 <&3             # Read only 4 characters.
  echo -n . >&3             # Write a decimal point there.
  exec 3>&-                 # Close fd 3.
  cat File                  # ==> 1234.67890
  #  Random access, by golly.

So,

echo <> temp.txt

Will create temp.txt if it does not exist, and print an empty line. That is all. It is equivalent to:

touch temp.txt && echo

---

Note, most programs will not expect the STDIN file descriptor (0) to be open for writing, so in most cases, the following will be roughly equivalent:

command <> file
command 0<> file
touch file && command < file

And since most programs will not expect STDOUT to be open for reading, the following are usually roughly equivalent:

command 1<> file
command > file

And for STDERR:

command 2<> file
command &2> file