kill -9 pid &> /dev/null?????????? 屏蔽標準輸出和標準錯誤輸出
[root@localhost shell]# echo $BASH_VERSION
2.05b.0(1)-release
[root@localhost shell]# echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/huyi/bin
[root@localhost shell]# $PATH=$HOME:/usr/bin
bash: /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/huyi/bin=/root:/usr/bin: ????
[root@localhost shell]#
[root@localhost shell]# export PATH
[root@localhost shell]# hash
hits??? command
?? 1??? /bin/egrep
?? 1??? /bin/bash
?? 1??? /bin/cat
?? 1??? /bin/cp
?? 1??? /bin/stty
? 15??? /bin/ls
[root@localhost shell]# $!
[root@localhost shell]# echo ${newfruit:-apple}
apple
[root@localhost shell]# echo ${fruit:-peach}
orange
[root@localhost shell]# food=grapes
[root@localhost shell]# echo ${food:+pears}
pears
[root@localhost shell]# echo $food
grapes