问题
What does an ampersand at the beginning of a line do in csh? It seems to be ignored (with no error message), but why?
回答1:
Found something interesting:
The semicolon (;) character separates successive commands on a single command line. For example,
% <command1> ; <command2>
executes <command1>, and when it finishes, <command2> gets executed.
The ampersand character (&) is similar to the semicolon (;) but does not wait for <command1> to finish.
Maybe it's treating it like an empty command?
回答2:
The best way to answer will be example. Taking idea of JoelFAn ahaed:
EXAMPLE 1
user$ date ; sleep 5s ; date
Thu Apr 5 10:46:45 IST 2012
Thu Apr 5 10:46:50 IST 2012
EXAMPLE 2
user$ date & sleep 5s & date
[1] 18371
[2] 18372
Thu Apr 5 10:47:09 IST 2012
Thu Apr 5 10:47:09 IST 2012
[1] - Done date
EXAMPLE3
user$ bg
[2] sleep 5s &
The examples are self explanatory.
Thanks
回答3:
I can't find anything in csh docs about it. However, I know in make it suppresses any output from the command (but still runs it). Perhaps that's what it is doing.
来源：https://stackoverflow.com/questions/2314726/ampersand-at-beginning-of-a-line-in-csh