1、变量使用（）与{ }的区别
$pwd、${pwd}：都为变量pwd值，加{ }会更安全
$(pwd)：执行命令，将命令的输出赋值 ，（）为执行命令与反引号效果相同 ` `
[root@localhost home]# cat b21430/pwd.sh
#!/bin/bash
set -x
pwd=$(cd $(dirname ${0});pwd)
echo "little pwd $(pwd)"
echo "none pwd $pwd"
echo "big pwd ${pwd}"
exit 0
[root@localhost home]#
[root@localhost home]# bash b21430/pwd.sh
+++ dirname b21430/pwd.sh
++ cd b21430
++ pwd
+ pwd=/home/b21430
++ pwd
+ echo 'little pwd /home'
little pwd /home
+ echo 'none pwd /home/b21430'
none pwd /home/b21430
+ echo 'big pwd /home/b21430'
big pwd /home/b21430
+ exit 0
2、一串命令使用()和{}括起的区别
()和{}都是对一串的命令进行执行,