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translating---geekpi
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Finding Files with mlocate
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============================================================
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In this series of articles, we’ll look at the mlocate tool and examine how to quickly and easily tune it to your own specifications.[Creative Commons Zero][1]Pixabay
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It’s not uncommon for a sysadmin to have to find needles buried deep inside haystacks. On a busy machine, there can be files in their hundreds of thousands present on your filesystems. What do you do when you need to make sure one particular configuration file is up to date, but you can’t remember where it is located?
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If you’ve used Unix-type machines for a while, then you’ve almost certainly come across the _find_ command before. It is unquestionably exceptionally sophisticated and highly functional. Here’s an example that just searches for links inside a directory, ignoring files:
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```
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# find . -lname "*"
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```
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You can do seemingly endless things with the find command; there’s no denying that. The find command is nice and succinct when it wants to be, but it can also get complex very quickly. This is not necessarily because of the find command itself, but coupled with “xargs” you can pass it all sorts of options to tune your output, and indeed delete those files which you have found.
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### Location, location, frustration
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There often comes a time when simplicity is the preferred route, however -- especially when a testy boss is leaning over your shoulder, chatting away about how time is of the essence. And, imagine trying to vaguely guess the path of the file you’ve never seen but that your boss is certain lives somewhere on the busy /var partition.
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Step forward _mlocate_ . You may be aware of one of its close relatives: _slocate_ , which securely (note the prepended letter s for secure) took note of the pertinent file permissions to prevent unprivileged users from seeing privileged files). Additionally, there is also the older, original locate command whence they came.
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The differences between mlocate and other members of its family (according to mlocate at least) is that, when scanning your filesystems, mlocate doesn’t need to continually rescan all your filesystem(s). Instead, it merges its findings (note the prepended m for merge) with any existing file lists, making it much more performant and less heavy on system caches.
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In this series of articles, we’ll look more closely at the mlocate tool (and simply refer to it as “locate” due to its popularity) and examine how to quickly and easily tune it to your heart’s content.
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### Compact and Bijou
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If you’re anything like me unless you reuse complex commands frequently then ultimately you forget them and need to look them up.The beauty of the locate command is that you can query entire filesystems very quickly and without worrying about top-level, root, paths with a simple command using locate.
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In the past, you might well have discovered that the _find _ command can be very stubborn and cause you lots of unwelcome head-scratching. You know, a missing semicolon here or a special character not being escaped properly there. Let’s leave the complicated find command alone now, relax, and have a look into the clever little command that is locate.
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You will most likely want to check that it’s on your system first by running these commands:
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For Red Hat derivatives:
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```
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# yum install mlocate
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```
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For Debian derivatives:
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```
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# apt-get install mlocate
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```
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There shouldn’t be any differences between distributions, but there are almost certainly subtle differences between versions; beware.
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Next, we’ll introduce a key component to the locate command, namely _updatedb_ . As you can probably guess, this is the command which _updates_ the locate command’s _db_ . Hardly counterintuitive.
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The db is the locate command’s file list, which I mentioned earlier. That list is held in a relatively simple and highly efficient database for performance. The updatedb runs periodically, usually at quiet times of the day, scheduled via a cron job. In Listing 1, we can see the innards of the file /etc/cron.daily/mlocate.cron (both the file’s path and its contents might possibly be distro and version dependent).
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```
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#!/bin/sh
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nodevs=$(< /proc/filesystems awk '$1 == "nodev" { print $2 }')
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renice +19 -p $$ >/dev/null 2>&1
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ionice -c2 -n7 -p $$ >/dev/null 2>&1
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/usr/bin/updatedb -f "$nodevs"
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```
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**Listing 1:** How the “updatedb” command is triggered every day.
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As you can see, the _mlocate.cron_ script makes careful use of the excellent _nice_ commands in order to have as little impact as possible on system performance. I haven’t explicitly stated that this command runs at a set time every day (although if my addled memory serves, the original locate command was associated with a slow-down-your-computer run scheduled at midnight). This is thanks to the fact that on some “cron” versions delays are now introduced into overnight start times.
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This is probably because of the so-called Thundering Herd of Hippos problem. Imagine lots of computers (or hungry animals) waking up at the same time to demand food (or resources) from a single or limited source. This can happen when all your hippos set their wristwatches using NTP (okay, this allegory is getting stretched too far, but bear with me). Imagine that exactly every five minutes (just as a “cron job” might) they all demand access to food or something otherwise being served.
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If you don’t believe me then have a quick look at the config from -- a version of cron called _anacron_ , in Listing 2, which is the guts of the file _/etc/anacrontab._
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```
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# /etc/anacrontab: configuration file for anacron
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# See anacron(8) and anacrontab(5) for details.
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SHELL=/bin/sh
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PATH=/sbin:/bin:/usr/sbin:/usr/bin
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MAILTO=root
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# the maximal random delay added to the base delay of the jobs
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RANDOM_DELAY=45
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# the jobs will be started during the following hours only
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START_HOURS_RANGE=3-22
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#period in days delay in minutes job-identifier command
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1 5 cron.daily nice run-parts /etc/cron.daily
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7 25 cron.weekly nice run-parts /etc/cron.weekly
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@monthly 45 cron.monthly nice run-parts /etc/cron.monthly
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```
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**Listing 2:** How delays are introduced into when “cron” jobs are run.
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From Listing 2, you have hopefully spotted both “RANDOM_DELAY” and the “delay in minutes” column. If this aspect of cron is new to you, then you can find out more here:
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```
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# man anacrontab
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```
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Failing that, you can introduce a delay yourself if you’d like. An [excellent web page][3] (now more than a decade old) discusses this issue in a perfectly sensible way. This website discusses using sleep to introduce a level of randomality, as seen in Listing 3.
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```
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#!/bin/sh
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# Grab a random value between 0-240.
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value=$RANDOM
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while [ $value -gt 240 ] ; do
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value=$RANDOM
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done
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# Sleep for that time.
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sleep $value
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# Syncronize.
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/usr/bin/rsync -aqzC --delete --delete-after masterhost::master /some/dir/
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```
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**Listing 3: **A shell script to introduce random delays before triggering an event, to avoid a [Thundering Herd of Hippos.][4]
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The aim in mentioning these (potentially surprising) delays was to point you at the file _/etc/crontab_ , or the root user’s own crontab file. If you want to change the time of when the locate command runs specifically because of disk access slowdowns, then it’s not too tricky. There may be a more graceful way of achieving this result, but you can also just move the file _/etc/cron.daily/mlocate.cron_ somewhere else (I’ll use the _/usr/local/etc_ directory), and as the root user add an entry into the root user’s crontab with this command and then paste the content as below:
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```
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# crontab -e
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33 3 * * * /usr/local/etc/mlocate.cron
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```
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Rather than traipse through _/var/log/cron_ and its older, rotated, versions, you can quickly tell the last time your cron.daily jobs were fired, in the case of anacron at least, with:
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```
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# ls -hal /var/spool/anacron
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```
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Next time, we’ll look at more ways to use locate, updatedb, and other tools for finding files.
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/intro-to-linux/2017/11/finding-files-mlocate
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作者：[CHRIS BINNIE][a]
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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.linux.com/users/chrisbinnie
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[1]:https://www.linux.com/licenses/category/creative-commons-zero
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[2]:https://www.linux.com/files/images/question-markjpg
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[3]:http://www.moundalexis.com/archives/000076.php
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[4]:http://www.moundalexis.com/archives/000076.php
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translated/tech/20171106 Finding Files with mlocate.md
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使用 mlocate 查找文件
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============================================================
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在这一系列的文章中，我们将来看下 mlocate，并检验如何快速、轻松地调整到你自己的规格。[Creative Commons Zero][1]Pixabay
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对于一个系统管理员来说, 要找到埋在干草深处的针头并不少见。在一台繁忙的机器上，文件系统中可能存在数十万个文件。当你需要确定一个特定的配置文件是最新的，但是你不记得它在哪里时怎么办？
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如果你已经使用了一些类 Unix 机器，那么你几乎肯定会遇到 _find_ 命令。毫无疑问，它是非常复杂和功能强大的。以下是一个只搜索目录中的链接，忽略文件的例子：
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```
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# find . -lname "*"
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```
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你可以用 find 命令做似乎所有的事情。这是不容否认的。find 命令在想要的时候是很好且简洁的，但是它也可以很快地变得复杂。这不一定是因为 find 命令本身，而是与 “xargs” 结合，你可以传递各种选项来调整你的输出，并删除你找到的那些文件。
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### 位置、位置、沮丧
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然而，通常情况下简单是最好的选择，特别是当一个脾气暴躁的老板靠在你的肩膀上时，闲聊着时间的重要。而且，试想模糊地猜测你从来没有见过的文件的路径，但是你的老板肯定在繁忙的 /var 分区的某处。
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来看 _mlocate_。你可能会注意到它的一个近亲：_slocate_，它安全地（注意前面的字母 s 代表安全）记录了相关的文件权限，以防止非特权用户看到特权文件）。此外，还有一个更老的，原始的定位命令，它们来了。
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mlocate 与其家族的其他成员（至少根据 mlocate）的不同之处在于，在扫描文件系统时，mlocate 不需要持续重新扫描所有的文件系统。相反，它将其发现（注意前面的 m 代表合并）与任何现有的文件列表合并在一起，使其在系统缓存中性能更高、更轻量级。
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在本系列文章中，我们将更仔细地看 mlocate （因为 “locate” 的流行，简单地将其称为 “locate”），并研究如何快速轻松地将其调整到你理想的状态。
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### 小巧和宝石
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如果你像我一样，除非你经常重复使用复杂的命令，那么最终你会忘记它们，并且需要查看它们。locate 命令的优点是可以快速查询整个文件系统，而不用担心顶层、根、路径，只需要使用简单的 locate 命令。
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过去，你可能已经发现 _find_ 命令可能非常固执，并让你经常抓耳挠腮。你知道，在这里丢失了一个分号或一个特殊的字符不能在那里正确地转义。现在让我们离开这个复杂的 find 命令，放松一下，看一下这个聪明的小命令。
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你可能会首先通过运行以下命令来检查它是否在你的系统上：
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对于 Red Hat 的衍生产品：
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```
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# yum install mlocate
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```
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对于 Debian 的衍生产品：
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```
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# apt-get install mlocate
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```
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发行版之间不应该有任何区别，但版本之间几乎肯定有细微差别。小心。
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接下来，我们将介绍 locate 命令的一个关键组件，名为 _updatedb_。正如你可能猜到的那样，这是_更新_ locate 命令的_数据库_的命令。非常符合直觉。
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数据库是我之前提到的 locate 命令的文件列表。该列表被保存在一个相对简单而高效的数据库中。updatedb 定期运行，通过 cron 计划，通常在一天的安静时间运行。在清单 1 中，我们可以看到文件 /etc/cron.daily/mlocate.cron 的内部（文件的路径及其内容可能都是依赖发行版的）。
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```
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#!/bin/sh
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nodevs=$(< /proc/filesystems awk '$1 == "nodev" { print $2 }')
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renice +19 -p $$ >/dev/null 2>&1
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ionice -c2 -n7 -p $$ >/dev/null 2>&1
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/usr/bin/updatedb -f "$nodevs"
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```
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**清单 1：** 每天如何触发 “updatedb” 命令。
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如你所见，_mlocate.cron_ 脚本使用了优秀的 _nice_ 命令来尽可能少地影响系统性能。我还没有明确表示这个命令每天都在设定的时间运行（但如果我没有记错的话，原始的 locate 命令与你在午夜时的计算机减速有关）。这是因为, 在一些 “cron” 版本上, 延迟现在被引入到隔夜开始时间。。
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这可能是因为所谓的 “Thundering Herd of Hippos” （河马的雷鸣）问题。想象许多计算机（或饥饿的动物）同时醒来从单一或有限的来源要求食物（或资源）。当所有的河马都使用 NTP 设置他们的手表时，这可能会发生（好吧，这个寓言扯多了，但忍受着我）。想象一下，正好每五分钟（就像一个 “cron 任务”），他们都要求获得食物或其他东西。
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如果你不相信我，请看下配置文件 - 清单 2 中名为 _anacron_ 的 cron 版本，这是文件 _/etc/anacrontab_ 的内容。
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```
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# /etc/anacrontab: configuration file for anacron
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# See anacron(8) and anacrontab(5) for details.
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SHELL=/bin/sh
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PATH=/sbin:/bin:/usr/sbin:/usr/bin
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MAILTO=root
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# the maximal random delay added to the base delay of the jobs
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RANDOM_DELAY=45
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# the jobs will be started during the following hours only
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START_HOURS_RANGE=3-22
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#period in days delay in minutes job-identifier command
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1 5 cron.daily nice run-parts /etc/cron.daily
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7 25 cron.weekly nice run-parts /etc/cron.weekly
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@monthly 45 cron.monthly nice run-parts /etc/cron.monthly
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```
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**清单 2：** 运行 “cron” 任务时如何引入延迟。
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从清单 2 可以看到 “RANDOM_DELAY” 和 “delay in minutes” 列。如果 cron 的这个方面对你来说是新的，那么你可以在这找到更多的东西：
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```
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# man anacrontab
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```
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否则，如果你愿意，你可以自己推迟一下。一个[很棒的网页][3]（现在已有十多年的历史）以非常合理的方式讨论了这个问题。本网站讨论如何使用 sleep 来引入一个随机性，如清单 3 所示。
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```
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#!/bin/sh
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# Grab a random value between 0-240.
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value=$RANDOM
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while [ $value -gt 240 ] ; do
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value=$RANDOM
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done
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# Sleep for that time.
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sleep $value
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# Syncronize.
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/usr/bin/rsync -aqzC --delete --delete-after masterhost::master /some/dir/
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```
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**清单 3：**在触发事件之前引入随机延迟的 shell 脚本，以避免[河马的雷鸣][4]。
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在提到这些 (可能令人惊讶的) 延迟时，是指 _/etc/crontab_ 或 root 用户自己的 crontab 文件。如果你想改变 locate 命令运行的时间，特别是由于磁盘访问速度减慢，那么它不是太棘手。实现它可能会有更优雅的方式，但是你也可以把文件 _/etc/cron.daily/mlocate.cron_ 移到别的地方（我使用_/usr/local/etc 目录），使用 root 用户添加一条记录到 root 用户的 crontab，粘贴以下内容：
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```
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# crontab -e
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33 3 * * * /usr/local/etc/mlocate.cron
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```
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不是通过 _/var/log/cron_ 以及它的旧的、轮转的版本，使用 anacron，你可以快速地告诉它上次 cron.daily 任务被触发的时间：
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```
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# ls -hal /var/spool/anacron
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```
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下一次，我们会看更多的使用 locate、updatedb 和其他工具来查找文件的方法。
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/learn/intro-to-linux/2017/11/finding-files-mlocate
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作者：[CHRIS BINNIE][a]
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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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||||
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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|
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[a]:https://www.linux.com/users/chrisbinnie
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[1]:https://www.linux.com/licenses/category/creative-commons-zero
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||||
[2]:https://www.linux.com/files/images/question-markjpg
|
||||
[3]:http://www.moundalexis.com/archives/000076.php
|
||||
[4]:http://www.moundalexis.com/archives/000076.php
|
||||
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Reference in New Issue
Block a user