命令行解释器

do nothing, successfully

AND and OR lists are sequences of one of more pipelines separated by the &&  and  ||  control  operators,
respectively.  AND and OR lists are executed with left associativity.  An AND list has the form

       command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

       command1 || command2

command2  is  executed  if and only if command1 returns a non-zero exit status.  The return status of AND
and OR lists is the exit status of the last command executed in the list.

{ list; }
       list is simply executed in the current shell environment.  list must be terminated with a  newline
       or  semicolon.   This  is known as a group command.  The return status is the exit status of list.
       Note that unlike the metacharacters ( and ), { and } are reserved words and  must  occur  where  a
       reserved  word  is permitted to be recognized.  Since they do not cause a word break, they must be
       separated from list by whitespace or another shell metacharacter.

display a line of text

Echo the STRING(s) to standard output.

Commands separated  by  a ; are executed sequentially; the shell waits for each command to terminate in turn.  The
return status is the exit status of the last command executed.