shell命令中調試的set -x
-x選項可以用來跟蹤腳本的執行，使shell在執行腳本的過程中
把它實際執行的每一個命令行顯示出來，並且在行首顯示一個"+",
"+"后面顯示的是經過了變量替換后的命令行內容，有助於分析
實際執行的命令
將-x與trap捕捉DEBUG信號相結合對trapdebug.sh腳本進行調試
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