shell命令中调试的set -x
-x选项可以用来跟踪脚本的执行，使shell在执行脚本的过程中
把它实际执行的每一个命令行显示出来，并且在行首显示一个"+",
"+"后面显示的是经过了变量替换后的命令行内容，有助于分析
实际执行的命令
将-x与trap捕捉DEBUG信号相结合对trapdebug.sh脚本进行调试
sh -x trapdebug.sh
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2
trapdebug.sh: 14: trapdebug.sh: let: not found
+ let b=b*2
trapdebug.sh: 15: trapdebug.sh: let: not found
+ let c=c-10
trapdebug.sh: 16: trapdebug.sh: let: not found
+ :
+ a 10
trapdebug.sh: 10: trapdebug.sh: a: not found
+ let a=a+2