179 lines
4.3 KiB
Markdown
179 lines
4.3 KiB
Markdown
# 原创
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： Shell流程控制：while/until 循环
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# Shell流程控制：while/until 循环
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## 流程控制：while/until 循环
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### while
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```
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#!/bin/bash
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# while-count: display a series of numbers
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count=1
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while [ $count -le 5 ]; do
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echo $count
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count=$((count + 1))
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done
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echo "Finished."
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```
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优化上一节的菜单脚本：
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```
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#!/bin/bash
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# while-menu: a menu driven system information program
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DELAY=3 # Number of seconds to display results
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while true; do
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clear
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cat <<- _EOF_
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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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_EOF_
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read -p "Enter selection [0-3] > "
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if [[ $REPLY =~ ^[0-3]$ ]]; then
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if [[ $REPLY == 1 ]]; then
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echo "Hostname: $HOSTNAME"
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uptime
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sleep $DELAY
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continue
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fi
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if [[ $REPLY == 2 ]]; then
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df -h
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sleep $DELAY
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continue
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fi
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if [[ $REPLY == 3 ]]; then
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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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sleep $DELAY
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continue
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fi
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if [[ $REPLY == 0 ]]; then
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break
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fi
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else
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echo "Invalid entry."
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sleep $DELAY
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fi
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done
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echo "Program terminated."
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```
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shell中的循环支持`break和continue`
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### until
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```
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#!/bin/bash
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# until-count: display a series of numbers
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count=1
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until [ $count -gt 5 ]; do
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echo $count
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count=$((count + 1))
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done
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echo "Finished."
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```
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## 疑难排解
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```
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#!/bin/bash
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cd $dir_name
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rm *
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```
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可以这样改进：`cd $dir_name && rm *`，但是有可能未设置变量 dir_name 或其变量值为空，导致删除了用户家目录下面的所有文件。`&& cd $dir_name && rm *`：
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```
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if [[ -d $dir_name ]]; then
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if cd $dir_name; then
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rm *
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else
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echo "cannot cd to '$dir_name'" >&2
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exit 1
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fi
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else
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echo "no such directory: '$dir_name'" >&2
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exit 1
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fi
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```
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### 测试
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早发布，常发布：如果在开发周期的早期发现 bug，那么这些 bug 就越容易定位，而且越能低成本 的修复。比喻上述删除文件的操作非常危险，所以我们可以先这样进行测试，打印出要执行的语句就行：
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```
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if [[ -d $dir_name ]]; then
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if cd $dir_name; then
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echo rm * # TESTING
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else
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echo "cannot cd to '$dir_name'" >&2
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exit 1
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fi
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else
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echo "no such directory: '$dir_name'" >&2
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exit 1
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fi
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exit # TESTING
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```
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### 测试案例
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通过谨慎地选择输入数据或者运行边缘案例和极端案例来完成测试。比喻上述脚本，我们要测试：
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1. dir_name 包含一个已经存在的目录的名字1. dir_name 包含一个不存在的目录的名字1. dir_name 为空
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### 调试
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一个设计良好的脚本会对查找错误有帮助。设计良好的脚本应该具备防卫能力， 能够监测异常条件，并能为用户提供有用的反馈信息。
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### 找到问题区域
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隔离脚本中与出现的问题相关的代码区域对查找问题很有帮助。 隔离的代码区域并不总是真正的错误所在，但是隔离往往可以深入了解实际的错误原因。
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### 追踪
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添加提示信息追踪代码片段。把提示信息输出到标准错误输出，让其从标准输出中分离出来。bash 还提供了一种名为追踪的方法，这种方法可通过 -x 选项和 set 命令加上 -x 选项两种途径实现。
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```
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#!/bin/bash -x
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# trouble: script to demonstrate common errors
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number=1
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if [ $number = 1 ]; then
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echo "Number is equal to 1."
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else
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echo "Number is not equal to 1."
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fi
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or
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#!/bin/bash
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# trouble: script to demonstrate common errors
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number=1
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echo "number=$number" # DEBUG
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# 为脚本中的一块选择区域，而不是整个脚本启用追踪
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set -x # Turn on tracing
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if [ $number = 1 ]; then
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echo "Number is equal to 1."
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else
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echo "Number is not equal to 1."
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fi
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set +x # Turn off tracing
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```
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行首的加号表明追踪的迹象，使其与常规输出结果区分开来。使用单引号是为了防止变量展开。
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