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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Counting down the days using bash)
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[#]: via: (https://www.networkworld.com/article/3487712/counting-down-the-days-using-bash.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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Counting down the days using bash
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======
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Need to know how many days there are before some important event? Let Linux bash and the date command help with that!
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Thinkstock
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With some pretty important holidays right around the corner, you might need to be reminded how much longer you have to prepare.
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Fortunately, you can get a lot of help from the **date** command. In this post, we’ll look at ways that **date** and bash scripts can tell you how many days there are between today and some event that you’re anticipating.
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[[Get regularly scheduled insights by signing up for Network World newsletters.]][1]
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First a couple hints at how this is going to work. The **date** command’s **%j** option is going to show you today’s date as a number between 1 and 366. January 1st, as you’d expect, will be displayed as 1 and December 31st will be 365 or 366 depending on whether it’s leap year. Go ahead and try it. You should see something like this:
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[][2]
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BrandPost Sponsored by HPE
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[Take the Intelligent Route with Consumption-Based Storage][2]
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Combine the agility and economics of HPE storage with HPE GreenLake and run your IT department with efficiency.
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```
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$ date +%j
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339
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```
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You can, however, get the **date** command to give you the day-of-the-year number for _any_ date by supplying it in a **date** command like this:
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```
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$ date -d "Mar 18" +%j
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077
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```
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One important thing to keep in mind is that this command will show you the date in the _current year_ even if that date is in the past. However, you can add a year to the command and fix that:
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```
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$ date -d "Apr 29" +%j
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119
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$ date -d "Apr 29 2020" +%j
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120
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```
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In a leap year, Apr 29th will be the 120th day of the year, not the 119th.
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If you want to count down the days until Christmas and don’t want to end up with fingerprints on your wall calendar, you can use a script like this:
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```
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#!/bin/sh
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XMAS=`date -d "Dec 25" +%j`
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TODAY=`date +%j`
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DAYS=$(($XMAS - $TODAY))
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case $DAYS in
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0) echo "It's today! Merry Christmas!";;
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[0-9]*) echo "$DAYS days remaining";;
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-[0-9]*) echo "Oops, you missed it";;
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esac
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```
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In this script, we get the day numbers for Dec 25th and today and then subtract one from the other. If the result is positive, we display the number of days remaining. If it’s zero, we issue a “Merry Christmas” message and, if it’s negative, we just tell the person running the script that they missed the holiday. Maybe they got carried away with the egg nog.
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The case statement is made up of ready-to-print statements for remaining-day counts that equal zero, those that that include any digits and those that start with a **-** sign (i.e., in the past).
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The same idea can be generalized for any date that someone wants to keep in focus. In fact, we can ask the person running our script to supply the date and then let them know how many days remain between now and then. This version of the script does just that.
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```
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#!/bin/sh
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echo -n "Enter event date (e.g., June 6): "
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read dt
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EVENT=`date -d "$dt" +%j`
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TODAY=`date +%j`
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DAYS=`expr $EVENT - $TODAY`
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case $DAYS in
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0) echo "It's today!";;
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[0-9]*) echo "$DAYS days remaining";;
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-[0-9]*) echo "Oops, you missed it";;
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esac
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```
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One problem you’ll have with this script is that, if the person running it is hoping to find out how many days must pass before they reach a special day the following year, they’ll be disappointed. Even if they supply a year when they enter the date, the date -d command will still only supply the day number in that year, not the days between now and then.
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Calculating the number of days between today and some date years from now can be a bit tricky. You’d need to include all the intervening years and pay attention to those that are leap years.
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### Using Unix (Epoch) time
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Another way to calculate the number of days between now and some special date is to take advantage of the way that Unix systems store dates. If you convert the number of seconds since the start of the day on Jan 1, 1970 to days, you can do this quite easily as in this script:
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```
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#!/bin/bash
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echo -n "Enter target date (e.g., Mar 18 2021)> "
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read target_date
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today=`echo $(($(date --utc --date "$1" +%s)/86400))`
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target=`echo $(($(date --utc --date "$target_date" +%s)/86400))`
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days=`expr $target - $today`
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echo "$days days until $target_date"
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```
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To explain, 86400 is the number of seconds in a day. Dividing the number of seconds since the epoch began by this number gives us the number of days.
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```
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$ ./countdown
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Enter target date (e.g., Mar 18 2021)> Mar 18 2020
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104 days until Mar 18 2020
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```
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Join the Network World communities on [Facebook][3] and [LinkedIn][4] to comment on topics that are top of mind.
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3487712/counting-down-the-days-using-bash.html
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[b]: https://github.com/lujun9972
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[1]: https://www.networkworld.com/newsletters/signup.html
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[2]: https://www.networkworld.com/article/3440100/take-the-intelligent-route-with-consumption-based-storage.html?utm_source=IDG&utm_medium=promotions&utm_campaign=HPE20773&utm_content=sidebar ( Take the Intelligent Route with Consumption-Based Storage)
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[3]: https://www.facebook.com/NetworkWorld/
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[4]: https://www.linkedin.com/company/network-world
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124
translated/tech/20191209 Counting down the days using bash.md
Normal file
124
translated/tech/20191209 Counting down the days using bash.md
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@ -0,0 +1,124 @@
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Counting down the days using bash)
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[#]: via: (https://www.networkworld.com/article/3487712/counting-down-the-days-using-bash.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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用 bash 倒计时
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======
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需要知道重要事件发生前有多少天吗？让 Linux bash 和 date 命令可以帮助你！
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随着即将来临的重要假期，你可能需要提醒你还要准备多久。
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幸运的是，你可以从 **date** 命令获得很多帮助。在本篇中，我们将研究 **date** 和 bash 脚本如何告诉你从今天到你预期的事件之间有多少天。
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首先，有几个提示如何进行。**date** 命令的 **%j** 选项将以 1 至 366 之间的数字显示当前日期。如你所想的一样，1 月 1 日将显示为 1，12 月 31 日将显示为 365 或 366，这取决于是否是闰年。继续尝试。你应该会看到以下内容：
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```
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$ date +%j
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339
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```
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但是，你可以通过以下方式，在 **date** 命令中得到一年中_任何_一天的数字：
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```
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$ date -d "Mar 18" +%j
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077
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```
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要记住的是，即使该日期是过去的日期，此命令也会向你显示_当年_的日期。但是，你可以在命令中添加年来修复该问题：
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```
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$ date -d "Apr 29" +%j
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119
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$ date -d "Apr 29 2020" +%j
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120
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```
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在闰年中，4 月 29 日将是一年的 120 天，而不是 119 天。
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如果你想倒数圣诞节之前的日子并且不想在挂历上留下指纹，你可以使用以下脚本：
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```
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#!/bin/sh
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XMAS=`date -d "Dec 25" +%j`
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TODAY=`date +%j`
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DAYS=$(($XMAS - $TODAY))
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case $DAYS in
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0) echo "It's today! Merry Christmas!";;
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[0-9]*) echo "$DAYS days remaining";;
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-[0-9]*) echo "Oops, you missed it";;
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esac
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```
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在此脚本中，我们获取 12 月 25 日和今天的日期，然后相减。如果结果是正数，我们将显示剩余天数。如果为零，则发出 “Merry Christmas” 的消息，如果为负，那么仅告诉运行脚本的人他们错过了假期。也许他们沉迷在蛋酒中了。
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case 语句由可打印的语句组成，剩余时间等于 0，或任意数字或以 **-** 符号开头的数字（也就是过去）。
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对于人们想要关注的任何日期，都可以使用相同方法。实际上，我们可以要求运行脚本的人员提供日期，然后让他们知道从现在到那天还有多少天。这个脚本是这样的。
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```
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#!/bin/sh
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echo -n "Enter event date (e.g., June 6): "
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read dt
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EVENT=`date -d "$dt" +%j`
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TODAY=`date +%j`
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DAYS=`expr $EVENT - $TODAY`
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case $DAYS in
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0) echo "It's today!";;
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[0-9]*) echo "$DAYS days remaining";;
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-[0-9]*) echo "Oops, you missed it";;
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esac
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```
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使用此脚本会遇到的一个问题，如果运行该脚本的人希望知道到第二年这个特殊日子还有多少天，他们会感到失望。即使他们输入日期时提供了年，date -d 命令仍将仅提供金年中的天数，而不会提供从现在到那时的天数。
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计算从今天到某年的日期之间的天数可能有些棘手。你需要包括所有中间年份，并注意那些闰年。
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### 使用 Unix（Epoch）时间
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计算从现在到某个特殊日期之间的天数的另一种方法是利用 Unix 系统存储日期的方法。如果将自 1970 年 1 月 1 日开始的秒数转换为天数，那么就可以很容易地执行此操作，如下脚本所示：
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```
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#!/bin/bash
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echo -n "Enter target date (e.g., Mar 18 2021)> "
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read target_date
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today=`echo $(($(date --utc --date "$1" +%s)/86400))`
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target=`echo $(($(date --utc --date "$target_date" +%s)/86400))`
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days=`expr $target - $today`
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echo "$days days until $target_date"
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```
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解释一下，86400 是一天中的秒数。将自纪元开始以来的秒数除该数即为天数。
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```
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$ ./countdown
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Enter target date (e.g., Mar 18 2021)> Mar 18 2020
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104 days until Mar 18 2020
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```
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加入 [Facebook][3] 和 [LinkedIn][4] 上的 Network World 社区，评论热门主题。
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3487712/counting-down-the-days-using-bash.html
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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||||
译者：[geekpi](https://github.com/geekpi)
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||||
校对：[校对者ID](https://github.com/校对者ID)
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||||
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||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
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||||
[b]: https://github.com/lujun9972
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||||
[3]: https://www.facebook.com/NetworkWorld/
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||||
[4]: https://www.linkedin.com/company/network-world
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