Linux Bash Shell Learning (11): Process Control-case

This article is part 3 of the fifth chapter of "Learning the bash Shell" 3rd Edition, Flow Control, but we will not limit this article. Flow control is a very common part of any programming language, including case. Here, we will continue to learn about them.

Case checks whether the string style matches and the number is equal to each other for different processing. Modify the if-then-else structure to a better reading mode. The format is as follows:

CaseExpression
In

Pattern1)

Statements
;;

Pattern2)

Statements
;;

...

Esac

Looking at the end esac and the fi IN if/else are both written in reverse, which may be a characteristic of bash. Note that statement is followed by two semicolons. Any style can contain multiple styles, which are separated by "|". If one of them is matched, statement is executed. Case will be checked in sequence until matching. The following is an example of processing different image files with different suffixes. We do not want to take specific measures, but mainly look at the use of case.

For filename in "$ @"; do

Case
$ FilenameIn

*. Jpg)
Exit 0;;

*. Tga)
<Do statement>;;

*. Xpm)
<Do statement>

;;

*. Pcx)
<Do statement>

;;

*. Tif)
<Do statement>

;;

*. Gif)
<Do statement>

;;

*)
Echo "procfile: $ filename is an unknown graphics file ."

Exit 1;;

Esac

Done

In other advanced languages, such as C language and common switch methods, they all have a default option, indicating the processing method when none of the above matches, in bash, the same processing can be performed at the end. Here is another example.

Cd ()

{

# Check the number of write but parameters, and process 0 or 1 parameters, 2 parameters, and others respectively.

Case
"$ #"In

# If there are one or zero parameters, use the original cd processing method. Note | the use of this parameter indicates that either of these two conditions can be matched.

0 | 1)
Builtin cd $1 ;;

# For the two parameters, replace all strings matching parameter 1 in the current path with style 2 and jump to the new path.

2)
Newdir =$ {PWD // $1/$2}

# Here is an example of case nesting. If the change path is the same as the original one, that is, there is no change in the processing, and other (that is, there is a change) processing.


Case "$ newdir" in

$ PWD) echo "bash: cd: bad substitution"> & 2;

Return 1 ;;

*) Builtin cd "$ newdir ";;

Esac ;;

# Process two or more parameters.

*) Echo "bash: cd: wrong arg count" 1> & 2; return 1 ;;

Esac

}

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