Shell condition limit

Conditional Criterion

Just talk about 『Program", Then the conditional criterion, that is, the criterion" If then ", must be learned! Because in many cases, we have to determine how the program should proceed based on certain data.

If... then

This if... then is the most common condition criterion ~ To put it simply, when a condition is met, a job is performed. The judgment of this if... then has many levels! We will introduce them as follows:

 

Single-layer, simple conditional

If you only have one sequence type, we can simply look at it as follows:

If[Conditional criterion];ThenThe command work content that can be executed when the condition criterion is set;Fi<= SetIfIn turn, it becomesFiNow! EndIfMeaning!
 

Example:

[Root @ WWW Scripts] # CP Sh06. Sh Sh06- 2 . Sh <= It's faster to use it! [Root @ WWW Scripts] # vi sh06 - 2 . Sh  # ! /Bin/ Bash # program: # This program shows the user  '  S choice  # History :#  2005 / 08 / 25  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport pathread -P "  Please input (y/N ):  "  YN If [ "  $ YN  " = "  Y  " ] | [ "  $ YN  " = "  Y  " ]; Then      Echo   " OK, continue  "  Exit  0  Fi  If [ "  $ YN  " = "  N  " ] | [ "  $ YN  " = "  N " ]; Then      Echo   "  Oh, interrupt!  "  Exit  0  Fi  Echo   "  I don't know what your choice is  " & Exit 0 

 

Multiple and complex condition types

Simple syntax

# One condition is used to determine whether the score is successful or fails (Else)If[Conditional criterion];ThenThe command work content that can be executed when the condition criterion is set;ElseCommand work content that can be executed when the conditional statement is invalid;Fi
 

In more complex cases, you can use this syntax:

# Multiple condition judgment (If...Elif...Elif...Else) Run in different situationsIf[Condition criterion 1];ThenThe command work that can be performed when the conditional statement is created;Elif[Condition method 2];ThenThe command work content that can be executed when the condition is set to condition 2;ElseCommand work content that can be executed when neither of condition 1 nor 2 is true;Fi
 

Example

[Root @ WWW Scripts] # CP Sh06- 2 . Sh Sh06-3 . Sh  [Root @ WWW Scripts] # vi sh06 - 3 . Sh  # ! /Bin/ Bash # program: # This program shows the user  '  S choice  # History :#  2005 / 08 / 25  Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport pathread -P "  Please input (y/N ):  "  YN  If [ "  $ YN  " = "  Y  " ] | [ "  $ YN  " ="  Y  " ]; Then      Echo   "  OK, continue  "  Elif [ "  $ YN  " = "  N  " ] | [ "  $ YN " = "  N  " ]; Then      Echo   "  Oh, interrupt!  "  Else      Echo   "  I don't know what your choice is  "  Fi 

 

Use Case... esac to judge

The syntax is as follows:

  case  $ variable name  in  <= Keyword:  case  , there is also a rich front variable font size  "   content of the first variable  " ) <=  we recommend that you enclose each variable in double quotation marks, and enclose the keyword in parentheses.) program section;  <=  each category ends with two consecutive semicolons!   "  content of the second variable  "  ) program segment;  *) <= The Last variable will use *  to indicate that all other program programs whose values do not contain the content of the first variable and the content of the second variable exit   1  ;;  esac  <= final  case  end! "Write in turn" Think about it! 

Example

[Root @ WWW Scripts] # vi sh09- 2 . Sh  # ! /Bin/ Bash # program: # Show  "  Hello  " From $ 1 ... By using Case .... Esac  # History :#  2005 / 08 / 29 Vbird first releasepath =/Bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin:/usr/local/sbin :~ / Binexport path  Case $ 1   In    "  Hello  "  )  Echo   "  Hello, how are you?  "  ;;  "" )  Echo   "  You must input parameters, ex> {$0 someword}  "  ;; *) # Actually, it is equivalent to bytes, 0 ~ Infinite number of arbitrary bytes!  Echo   "  Usage $0 {Hello}  "  ;;  Esac 

 

Self-http://vbird.dic.ksu.edu.tw/linux_basic/0340bashshell-scripts_4.php#sh06