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125 lines
4.9 KiB
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[#]: subject: "Watch commands and tasks with the Linux watch command"
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[#]: via: "https://opensource.com/article/21/9/linux-watch-command"
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[#]: author: "Moshe Zadka https://opensource.com/users/moshez"
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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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Watch commands and tasks with the Linux watch command
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======
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See how the watch command can let you know when a task has been
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completed or a command has been executed.
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![Clock, pen, and notepad on a desk][1]
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There are many times when you need to wait for something to finish, such as:
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* A file download.
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* Creating or extracting a [tar][2] file.
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* An [Ansible][3] job.
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Some of these processes have some sort of progress indication, but sometimes the process is run through a layer of abstraction, and the only way to measure the progress is through its side effects. Some of these might be:
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* A file being downloaded keeps growing.
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* A directory extracted from a tarball fills up with files.
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* The Ansible job builds a [container][4].
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You can query all of these things with commands like these:
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```
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$ ls -l downloaded-file
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$ find . | wc -l
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$ podman ps
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$ docker ps
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```
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But running these commands over and over, even if it is with the convenience of [Bash history][5] and the **Up Arrow**, is tedious.
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Another approach is to write a little Bash script to automate these commands for you:
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```
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while :
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do
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docker ps
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sleep 2
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done
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```
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But such scripts can also become tedious to write. You could write a little generic script and package it, so it's always available to you. Luckily, other open source developers have already been there and done that.
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The result is the command `watch`.
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### Installing watch
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The `watch` command is part of the [`procps-ng` package][6], so if you're on Linux, you already have it installed.
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On macOS, install `watch` using [MacPorts][7] or [Homebrew][8]. On Windows, use [Chocolatey][9].
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### Using watch
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The `watch` command periodically runs a command and shows its output. It has some text-terminal niceties, so only the latest output is on the screen.
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The simplest usage is: `watch <command>`.
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For example, prefixing the `docker ps` command with `watch` works like this:
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```
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`$ watch docker ps`
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```
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The `watch` command, and a few creative Unix command-line tricks, can generate ad-hoc dashboards. For example, to count audit events:
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```
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`$ watch 'grep audit: /var/log/kern.log |wc -l'`
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```
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In the last example, it is probably useful if there's a visual indication that the number of audit events changed. If change is expected, but you want something to look "different," `watch --differences` works well. It highlights any differences from the last run. This works especially well if you are grepping in multiple files, so you can easily see which one changed.
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If changes are not expected, you can ask for them to be highlighted "permanently" to know which ones to investigate by using `watch --differences=permanent`. This is often more useful.
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### Controlling frequency
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Finally, sometimes the command might be resource-intensive and should not be run too frequently. The `-n` parameter controls the frequency. Watch uses two seconds by default, but `watch -n 10` might be appropriate for something more resource-intensive, like grepping for a pattern in any file in a subdirectory:
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```
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`$ watch -n 10 'find . -type f | xargs grep suspicious-pattern'`
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```
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### Watch a command with watch
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The `watch` command is useful for many ad-hoc system administration tasks where you need to wait for some time-consuming step, without a progress bar, before moving on to the next one. Though this is not a great situation to be in, `watch` can make it slightly better—and give you time to start working on those notes for the retrospective! Download the **[cheat sheet][10] **to keep helpful syntax and options close at hand.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/9/linux-watch-command
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作者：[Moshe Zadka][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://opensource.com/users/moshez
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/desk_clock_job_work.jpg?itok=Nj4fuhl6 (Clock, pen, and notepad on a desk)
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[2]: https://opensource.com/article/17/7/how-unzip-targz-file
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[3]: https://opensource.com/resources/what-ansible
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[4]: https://opensource.com/resources/what-docker
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[5]: https://opensource.com/article/20/6/bash-history-control
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[6]: https://opensource.com/article/21/8/linux-procps-ng
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[7]: https://opensource.com/article/20/11/macports
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[8]: https://opensource.com/article/20/6/homebrew-mac
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[9]: https://opensource.com/article/20/3/chocolatey
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[10]: https://opensource.com/downloads/watch-cheat-sheet
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