363 lines
8.4 KiB
Markdown
363 lines
8.4 KiB
Markdown
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# 原创
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： Shell流程控制：case 分支
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# Shell流程控制：case 分支
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## 流程控制：case 分支
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### case
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```
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case word in
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[pattern [| pattern]...) commands ;;]...
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esac
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```
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```
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#!/bin/bash
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# case-menu: a menu driven system information program
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clear
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echo "
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Please Select:
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1. Display System Information
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2. Display Disk Space
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3. Display Home Space Utilization
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0. Quit
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"
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read -p "Enter selection [0-3] > "
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case $REPLY in
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0) echo "Program terminated."
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exit
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;;
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1) echo "Hostname: $HOSTNAME"
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uptime
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;;
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2) df -h
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;;
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3) if [[ $(id -u) -eq 0 ]]; then
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echo "Home Space Utilization (All Users)"
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du -sh /home/*
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else
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echo "Home Space Utilization ($USER)"
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du -sh $HOME
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fi
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;;
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*) echo "Invalid entry" >&2
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exit 1
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;;
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esac
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```
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### 模式
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case 语句使用的模式和路径展开中使用的那些是一样的。模式以一个 “)” 为终止符。这里是一些有效的模式。
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|模式|描述
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|------
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|a)|若单词为 “a”，则匹配
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|[[:alpha:]])|若单词是一个字母字符，则匹配
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|???)|若单词只有3个字符，则匹配
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|*.txt)|若单词以 “.txt” 字符结尾，则匹配
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|*)|匹配任意单词。把这个模式做为 case 命令的最后一个模式，是一个很好的做法， 可以捕捉到任意一个与先前模式不匹配的数值；也就是说，捕捉到任何可能的无效值。
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```
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#!/bin/bash
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read -p "enter word > "
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case $REPLY in
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[[:alpha:]]) echo "is a single alphabetic character." ;;
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[ABC][0-9]) echo "is A, B, or C followed by a digit." ;;
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???) echo "is three characters long." ;;
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*.txt) echo "is a word ending in '.txt'" ;;
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*) echo "is something else." ;;
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esac
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```
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还可以使用竖线字符作为分隔符，把多个模式结合起来。这就创建了一个 “或” 条件模式。这对于处理诸如大小写字符很有用处。例如：
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```
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#!/bin/bash
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# case-menu: a menu driven system information program
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clear
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echo "
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Please Select:
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A. Display System Information
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B. Display Disk Space
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C. Display Home Space Utilization
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Q. Quit
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"
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read -p "Enter selection [A, B, C or Q] > "
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case $REPLY in
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q|Q) echo "Program terminated."
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exit
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;;
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a|A) echo "Hostname: $HOSTNAME"
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uptime
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;;
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b|B) df -h
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;;
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c|C) if [[ $(id -u) -eq 0 ]]; then
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echo "Home Space Utilization (All Users)"
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du -sh /home/*
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else
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echo "Home Space Utilization ($USER)"
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du -sh $HOME
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fi
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;;
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*) echo "Invalid entry" >&2
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exit 1
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;;
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esac
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```
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添加的 “;;&” 的语法允许 case 语句继续执行下一条测试，而不是简单地终止运行。
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```
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#!/bin/bash
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# case4-2: test a character
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read -n 1 -p "Type a character > "
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echo
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case $REPLY in
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[[:upper:]]) echo "'$REPLY' is upper case." ;;&
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[[:lower:]]) echo "'$REPLY' is lower case." ;;&
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[[:alpha:]]) echo "'$REPLY' is alphabetic." ;;&
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[[:digit:]]) echo "'$REPLY' is a digit." ;;&
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[[:graph:]]) echo "'$REPLY' is a visible character." ;;&
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[[:punct:]]) echo "'$REPLY' is a punctuation symbol." ;;&
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[[:space:]]) echo "'$REPLY' is a whitespace character." ;;&
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[[:xdigit:]]) echo "'$REPLY' is a hexadecimal digit." ;;&
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esac
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```
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## 位置参数
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### 访问命令行
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shell 提供了一个称为位置参数的变量集合，这个集合包含了命令行中所有独立的单词。这些变量按照从0到9给予命名。
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```
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#!/bin/bash
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# posit-param: script to view command line parameters
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echo "
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\$0 = $0
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\$1 = $1
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\$2 = $2
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\$3 = $3
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\$4 = $4
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\$5 = $5
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\$6 = $6
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\$7 = $7
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\$8 = $8
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\$9 = $9
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"
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```
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```
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[me@linuxbox ~]$ posit-param a b c d
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$0 = /home/me/bin/posit-param
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$1 = a
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$2 = b
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$3 = c
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$4 = d
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$5 =
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$6 =
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$7 =
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$8 =
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$9 =
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```
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### shift - 访问多个参数的利器
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执行一次 shift 命令， 就会导致所有的位置参数 “向下移动一个位置”。
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```
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#!/bin/bash
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# posit-param2: script to display all arguments
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# 只要参数个数不为零就会继续执行的 while 循环
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count=1
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while [[ $# -gt 0 ]]; do
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echo "Argument $count = $1"
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count=$((count + 1))
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shift
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done
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```
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```
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[me@linuxbox ~]$ posit-param2 a b c d
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Argument 1 = a
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Argument 2 = b
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Argument 3 = c
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Argument 4 = d
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```
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### 处理集体位置参数
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shell 提供了两种特殊的参数，他们二者都能扩展成完整的位置参数列表，但以相当微妙的方式略有不同。
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|参数|描述
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|------
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|$*|展开成一个从1开始的位置参数列表。当它被用双引号引起来的时候，展开成一个由双引号引起来 的字符串，包含了所有的位置参数，每个位置参数由 shell 变量 IFS 的第一个字符（默认为一个空格）分隔开。
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|$@|展开成一个从1开始的位置参数列表。当它被用双引号引起来的时候， 它把每一个位置参数展开成一个由双引号引起来的分开的字符串。
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```
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#!/bin/bash
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# posit-params3 : script to demonstrate $* and $@
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print_params () {
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echo "\$1 = $1"
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echo "\$2 = $2"
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echo "\$3 = $3"
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echo "\$4 = $4"
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}
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pass_params () {
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echo -e "\n" '$* :'; print_params $*
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echo -e "\n" '"$*" :'; print_params "$*"
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echo -e "\n" '$@ :'; print_params $@
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echo -e "\n" '"$@" :'; print_params "$@"
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}
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pass_params "word" "words with spaces"
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```
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```
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[me@linuxbox ~]$ posit-param3
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$* :
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$1 = word
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$2 = words
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$3 = with
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$4 = spaces
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"$*" :
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$1 = word words with spaces
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$2 =
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$3 =
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$4 =
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$@ :
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$1 = word
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$2 = words
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$3 = with
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$4 = spaces
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"$@" :
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$1 = word
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$2 = words with spaces
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$3 =
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$4 =
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```
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### 一个更复杂的应用
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```
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#!/bin/bash
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# sys_info_page: program to output a system information page
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PROGNAME=$(basename $0)
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TITLE="System Information Report For $HOSTNAME"
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CURRENT_TIME=$(date +"%x %r %Z")
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TIMESTAMP="Generated $CURRENT_TIME, by $USER"
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report_uptime () {
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cat <<- _EOF_
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<H2>System Uptime</H2>
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<PRE>$(uptime)</PRE>
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_EOF_
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return
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}
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report_disk_space () {
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cat <<- _EOF_
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<H2>Disk Space Utilization</H2>
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<PRE>$(df -h)</PRE>
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_EOF_
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return
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}
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report_home_space () {
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if [[ $(id -u) -eq 0 ]]; then
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cat <<- _EOF_
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<H2>Home Space Utilization (All Users)</H2>
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<PRE>$(du -sh /home/*)</PRE>
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_EOF_
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else
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cat <<- _EOF_
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<H2>Home Space Utilization ($USER)</H2>
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<PRE>$(du -sh $HOME)</PRE>
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_EOF_
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fi
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return
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}
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usage () {
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echo "$PROGNAME: usage: $PROGNAME [-f file | -i]"
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return
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}
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write_html_page () {
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cat <<- _EOF_
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<HTML>
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<HEAD>
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<TITLE>$TITLE</TITLE>
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</HEAD>
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<BODY>
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<H1>$TITLE</H1>
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<P>$TIMESTAMP</P>
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$(report_uptime)
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$(report_disk_space)
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$(report_home_space)
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</BODY>
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</HTML>
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_EOF_
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return
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}
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# process command line options
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interactive=
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filename=
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while [[ -n $1 ]]; do
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case $1 in
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-f | --file) shift
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filename=$1
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;;
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-i | --interactive) interactive=1
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;;
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-h | --help) usage
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exit
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;;
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*) usage >&2
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exit 1
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;;
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esac
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shift
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done
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# interactive mode
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if [[ -n $interactive ]]; then
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while true; do
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read -p "Enter name of output file: " filename
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if [[ -e $filename ]]; then
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read -p "'$filename' exists. Overwrite? [y/n/q] > "
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case $REPLY in
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Y|y) break
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;;
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Q|q) echo "Program terminated."
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exit
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;;
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*) continue
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;;
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esac
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fi
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done
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fi
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# output html page
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if [[ -n $filename ]]; then
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if touch $filename && [[ -f $filename ]]; then
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write_html_page > $filename
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else
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echo "$PROGNAME: Cannot write file '$filename'" >&2
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exit 1
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fi
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else
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write_html_page
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fi
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```
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