bash中的“ echo”和“！echo”有什么区别？

Question

What the difference between the command echo and !echo ?

I think that with !echo it concatenates with the previous command but I'm not sure.

Answer 1

The exclamation mark can have two meanings in bash:

1. history expansion

    !echo 

   would expand to the recent line starting with "echo".

2. negation (space after ! needed)

   ! echo ! echo would set $? to 0 if the echo call was unsuccessful, or to 1 if it was successful.