Use of case in shell

caseStatements are used to execute different code blocks based on the value of an expression.

Basic structure

case expression in
    model1)
        Code block1
        ;;
    model2)
        Code block2
        ;;
    ...
    *)
        默认Code block
        ;;
esac

Detailed explanation

• case expression in：

  • caseStart a keywordcaseSentence.
  • expressionis the value to match, which can be a variable or a fixed value.
  • inKeywords indicate the beginning of the pattern list.

• model：

  • Each pattern must be followed by a close bracket.), mark the end of the mode.
  • The mode can be a single value or multiple values|Separate, indicating that multiple values ​​can match the same code block.

• Code block：

  • The code block corresponding to each pattern is a series of commands that are executed when the match is successful.
  • Code blocks;;End, indicating the end of the branch.

• *)：

  • *)is the default branch, used to handle all cases that do not match any previous pattern.
  • The default branch is also optional, but it is generally recommended to include default branches to handle unexpected situations.

• esac：

  • esacKeyword ends the wholecaseSentence.

Example

Example 1: Basic usage

#!/bin/bash

# Get user inputread -p "Please enter a number (1-7): " num

# output the corresponding week according to the input numbercase $num in
    1)
        echo "Monday"
        ;;
    2)
        echo "Tuesday"
        ;;
    3)
        echo "Wednesday"
        ;;
    4)
        echo "Thursday"
        ;;
    5)
        echo "Friday"
        ;;
    6)
        echo "Saturday"
        ;;
    7)
        echo "Sunday"
        ;;
    *)
        echo "Invalid input"
        ;;
esac

Example 2: Multi-value Matching

#!/bin/bash

# Get user inputread -p "Please enter a letter (a-z): " letter

# Output the corresponding information according to the input letterscase $letter in
    a|e|i|o|u)
        echo "This is a vowel"
        ;;
    b|c|d|f|g|h|j|k|l|m|n|p|q|r|s|t|v|w|x|y|z)
        echo "This is a consonant letter"
        ;;
    *)
        echo "Invalid input"
        ;;
esac

Example 3: Range Matching

#!/bin/bash

# Get user inputread -p "Please enter a number (1-100): " num

# The corresponding range according to the input digital outputcase $num in
    [1-9])
        echo "The numbers are in 1 arrive 9 between"
        ;;
    [1-9][0-9])
        echo "The numbers are in 10 arrive 99 between"
        ;;
    100)
        echo "The number is 100"
        ;;
    *)
        echo "Invalid input"
        ;;
esac

Special usage

• Wildcard：

  • Wildcards can be used*and?Perform fuzzy matching.
  • *Match any character of any length.
  • ?Match a single arbitrary character.

• Compound mode：

  • Brackets can be used()Contains multiple modes to form a composite mode.

Example 4: Wildcard Match

#!/bin/bash

# Get user inputread -p "Please enter a file name: " filename

# Output the corresponding information according to the entered file namecase $filename in
    *.txt)
        echo "This is a text file"
        ;;
    *.jpg|*.jpeg|*.png)
        echo "This is a picture file"
        ;;
    *)
        echo "Unknown type of file"
        ;;
esac

Summarize

caseStatements are a powerful conditional control structure that is suitable for a variety of scenarios, especially when different operations need to be performed based on multiple possible values. By using pattern matching and default branches rationally, you can write concise and easy-to-maintain scripts.

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