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Translating by MjSeven
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Advanced use of the less text file viewer in Linux
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======
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I recently read Scott Nesbitt's article "[Using less to view text files at the Linux command line][1]" and was inspired to share additional tips and tricks I use with `less`.
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### LESS env var
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If you have an environment variable `LESS` defined (e.g., in your `.bashrc`), `less` treats it as a list of options, as if passed on the command line.
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I use this:
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```
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LESS='-C -M -I -j 10 -# 4'
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```
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These mean:
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* `-C` – Make full-screen reprints faster by not scrolling from the bottom.
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* `-M` – Show more information from the last (status) line. You can customize the information shown with `-PM`, but I usually do not bother.
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* `-I` – Ignore letter case (upper/lower) in searches.
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* `-j 10` – Show search results in line 10 of the terminal, instead of the first line. This way you have 10 lines of context each time you press `n` (or `N`) to jump to the next (or previous) match.
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* `-# 4` – Jump four characters to the right or left when pressing the Right or Left arrow key. The default is to jump half of the screen, which I usually find to be too much. Generally speaking, `less` seems to be (at least partially) optimized to the environment it was initially developed in, with slow modems and low-bandwidth internet connections, when it made sense to jump half a screen.
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### PAGER env var
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Many programs show information using the command set in the `PAGER` environment variable (if it's set). So, you can set `PAGER=less` in your `.bashrc` and have your program run `less`. Check the man page environ(7) (`man 7 environ`) for other such variables.
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### -S
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`-S` tells `less` to chop long lines instead of wrapping them. I rarely find a need for this unless (and until) I've started viewing a file. Fortunately, you can type all command-line options inside `less` as if they were keyboard commands. So, if I want to chop long lines while I'm already in a file, I can simply type `-S`.
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The command-line optiontellsto chop long lines instead of wrapping them. I rarely find a need for this unless (and until) I've started viewing a file. Fortunately, you can type all command-line options insideas if they were keyboard commands. So, if I want to chop long lines while I'm already in a file, I can simply type
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Here's an example I use a lot:
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```
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su - postgres
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export PAGER=less # Because I didn't bother editing postgres' .bashrc on all the machines I use it on
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psql
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```
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Sometimes when I later view the output of a `SELECT` command with a very wide output, I type `-S` so it will be formatted nicely. If it jumps too far when I press the Right arrow to see more (because I didn't set `-#`), I can type `-#8`, then each Right arrow press will move eight characters to the right.
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Sometimes after typing `-S` too many times, I exit psql and run it again after entering:
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```
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export LESS=-S
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```
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### F
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The command `F` makes `less` work like `tail -f`—waiting until more data is added to the file before showing it. One advantage this has over `tail -f` is that highlighting search matches still works. So you can enter `less /var/log/logfile`, search for something—which will highlight all occurrences of it (unless you used `-g`)—and then press `F`. When more data is written to the log, `less` will show it and highlight the new matches.
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After you press `F`, you can press `Ctrl+C` to stop it from looking for new data (this will not kill it); go back into the file to see older stuff, search for other things, etc.; and then press `F` again to look at more new data.
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### Searching
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Searches use the system's regexp library, and this usually means you can use extended regular expressions. In particular, searching for `one|two|three` will find and highlight all occurrences of one, two, or three.
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Another pattern I use a lot, especially with wide log lines (e.g., ones that span more than one terminal line), is `.*something.*`, which highlights the entire line. This pattern makes it much easier to see where a line starts and finishes. I also combine these, such as: `.*one thing.*|.*another thing.*`, or `key: .*|.*marker.*` to see the contents of `key` (e.g., in a log file with a dump of some dictionary/hash) and highlight relevant marker lines (so I have a context), or even, if I know the value is surrounded by quotes:
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```
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key: '[^']*'|.*marker.*
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```
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`less` maintains a history of your search items and saves them to disk for future invocations. When you press `/` (or `?`), you can go through this history with the Up or Down arrow (as well as do basic line editing).
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I stumbled upon what seems to be a very useful feature when skimming through the `less` man page while writing this article: skipping uninteresting lines with `&!pattern`. For example, while looking for something in `/var/log/messages`, I used to iterate through this list of commands:
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```
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cat /var/log/messages | egrep -v 'systemd: Started Session' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session|User Slice' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session|User Slice|dbus' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session|User Slice|dbus|PackageKit Daemon' | less
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```
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But now I know how to do the same thing within `less`. For example, I can type `&!systemd: Started Session`, then decide I want to get rid of `systemd: Starting Session`, so I add it by typing `&!` and use the Up arrow to get the previous search from the history. Then I type `|systemd: Starting Session` and press `Enter`, continuing to add more items the same way until I filter out enough to see the more interesting stuff.
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### =
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The command `=` shows more information about the file and location, even more than `-M`. If the file is very long, and calculating `=` takes too long, you can press `Ctrl+C` and it will stop trying.
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If the content you're viewing is from a pipe rather than a file, `=` (and `-M`) will not show what it does not know, including the number of lines and bytes in the file. To see that data, if you know that `command` will finish quickly, you can jump to the end with `G`, and then `less` will start showing that information.
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If you press `G` and the command writing to the pipe takes longer than expected, you can press `Ctrl+C`, and the command will be killed. Pressing `Ctrl+C` will kill it even if you didn't press `G`, so be careful not to press `Ctrl+C` accidentally if you don't intend to kill it. For this reason, if the command does something (that is, it's not only showing information), it's usually safer to write its output to a file and view the file in a separate terminal, instead of using a pipe.
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### Why you need less
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`less` is a very powerful program, and contrary to newer contenders in this space, such as `most` and `moar`, you are likely to find it on almost all the systems you use, just like `vi`. So, even if you use GUI viewers or editors, it's worth investing some time going through the `less` man page, at least to get a feeling of what's available. This way, when you need to do something that might be covered by existing functionality, you'll know to search the manual page or the internet to find what you need.
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For more information, visit the [less home page][2]. The site has a nice FAQ with more tips and tricks.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/18/5/advanced-use-less-text-file-viewer
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作者：[Yedidyah Bar David][a]
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选题：[lujun9972](https://github.com/lujun9972)
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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/didib
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[1]:http://opensource.com/article/18/4/using-less-view-text-files-command-line
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[2]:http://www.greenwoodsoftware.com/less/
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@ -0,0 +1,111 @@
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Linux 中使用 less 查看文件的高级用法
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======
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最近，我正在拜读 Scott Nesbitt 的文章“[在 Linux 中使用 less 命令来查看文本文件][1]”，并受到了一些启发，所以我想分享一些使用 `less` 命令的技巧。
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### LESS 环境变量
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如果你定义了环境变量 `LESS`（例如在 `.bashrc` 中），那么 `less` 会将其视为一个选项列表，就像在命令行中传递给它一样。
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我这样定义：
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```
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LESS='-C -M -I -j 10 -# 4'
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```
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它的意思是：
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* `-C` – 通过不从底部滚动来加快全屏打印速度。
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* `-M` – 在最后一行（状态行）显示更多信息。你可以使用 `-PM` 来自定义显示的信息，但我通常只用 `-M`。
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* `-I` – 忽略搜索中的大小写。
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* `-j 10` – 在终端的第 10 行而不是第一行显示搜索结果。这样，每次按 `n` 或（`N`） 跳到下一个（或上一个）匹配项时，就会有 10 条上下文。
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* `-# 4` – 当按下向右或向左的箭头时，会向右或向左跳四个字符。默认情况时跳转半个屏幕，我觉得太多了。一般来说，`less` 似乎（至少部分）优化了最初开发时的环境，使用慢速调制解调器和低带宽的互联网连接，这样可以跳过半个屏幕。
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### PAGER 环境变量
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许多程序使用在 `PAGER` 环境变量中的命令来显示信息。因此，你可以在 `.bashrc` 中设置 `PAGER=less`，然后让程序运行 `less`。查看 man 页（`man 7 environ`）中是否有其它此类变量。·
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### -S
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`-S` 告诉 `less` 将长行隔断而不是将它们包起来。除非或直到我开始查看文件，否则我很少需要这样做。幸运的是，你可以在 `less` 中输入所有命令行选项，就像它们是键盘命令一样。因此，如果我想在文件已经打开的情况下隔断长行，我可以简单地输入 `-S`。
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这是我经常使用的一个例子：
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```
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su - postgres
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export PAGER=less # 因为我没在所有的机器上编辑 postgres .bashrc，而是在 psql 使用
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```
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有时当我查看一个 `SELECT` 命令的输出非常宽，我会输入 `-S` 以便将其格式化。如果当我按下右箭头想查看更多内容时它跳得太远（因为我没有设置 `-#`），则可以输入 `-#8`，那么每次按下右箭头都会向右移动八个字符。
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有时在多次输入 `-S` 之后，我会退出 psql 并设置环境变量后再次运行它：
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```
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export LESS=-S
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```
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### F
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命令 `F` 使 `less` 像 `tail -f` 一样工作，等待更多的数据被添加到文件后再显示。与 `tail -f` 相比，它的一个优点是，高亮显示搜索匹配仍然有效。因此，你可以输入 `less /var/log/logfile`，搜索某些内容时，它将高亮显示所有出现的内容（除非你使用了 -g），然后按下 `F` 键，当更多数据写入到日志时，`less` 将显示它并高亮新的匹配项。
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按下 `F` 后，可以按 `Ctrl+C` 来停止其查找新数据（这不会干掉它），这样你可以返回文件查看旧内容，搜索其它内容等，然后再次按 `F` 键来查看更多新数据。
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### 搜索
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搜索使用系统的 regexp 库，这通常意味着你可以使用扩展正则表达式。特别是，搜索 `one|two|three` 将找到并高亮所有的 one、two 或 three。
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我经常使用的另一种模式是 `.*someting.*`，特别是对于一些很长的日志行（例如，跨越多个终端行），它会高亮整行。这种模式使查看一行的起始和结束位置变得更加容易。我还会结合其它内容，例如 `.*one thing.*|.*another thing.*`，或者使用 `key: .*|.*marker.*` 来查看 `key` 的内容。例如，一个日志文件中包含一些字典/哈希的转储。它会高亮相关的标记行，这样我就有上下文了，甚至，如果我知道这个值被引号引起来的话，就可以：
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```
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key: '[^']*'|.*marker.*
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```
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`less` 会保留你搜索项的历史纪录，并将其保存到磁盘中以备将来调用。当你按下 `/` 或 `?` 时，可以使用向上或向下箭头浏览历史记录（以及基本的行编辑）。
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在撰写本文时，我无意间看了下 less 手册页，发现了一个非常有用的功能：使用 `&!pattern` 跳过无趣的行。例如，当我在 `/var/log/messages` 中寻找内容时，经常会遍历以下命令：
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```
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cat /var/log/messages | egrep -v 'systemd: Started Session' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session|User Slice' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session|User Slice|dbus' | less
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cat /var/log/messages | egrep -v 'systemd: Started Session|systemd: Starting Session|User Slice|dbus|PackageKit Daemon' | less
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```
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但是现在我知道如何在 `less` 中做同样的事情。例如，我可以输入 `&!systemd: Started Session`，然后决定删除 `systemd: Starting Session`，所以我输入 `&!`，并使用向上箭头从历史记录中获得上一次搜索的结果。然后我输入 `|systemd: Starting Session` 并按下 `Enter`，继续以相同的方式添加更多条目，直到我过滤掉足够多的条目，看到更有趣的内容。
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### =
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命令 `=` 显示有关文件和位置的更多信息，甚至比 `-M` 更多。如果文件非常大，计算 `=` 花费的时间太长，可以按下 `Ctrl+C`，它将停止尝试。
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如果你正在查看的内容来自管道而不是文件，则 `=` （和 `-M`）不会显示未知内容，包括文件中的行数和字节数。要查看这些数据，如果你知道管道命令将很快结束，则可以使用 `G` 跳到最后，然后 `less` 将开始显示这些信息。
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如果按下 `G` 并且写入管道的命令花费的时间比预期的长，你可以按下 `Ctrl+C`，该命令将被终止。即使你没有按 `G`，`Ctrl+C` 键也会杀死它。因此，如果你不想杀死它，请不要意外按下 `Ctrl+C`。出于这个原因，如果命令执行了某些操作（不仅是显示信息），通常更安全的做法是将其输出写入文件并在单独的终端中查看文件，而不是使用管道。
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### 为什么你需要 less
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`less` 是一个非常强大的程序，与该领域中较新的竞争者（例如 most 和 moar）不同，你可能会在几乎所有的系统上找到它，就像 `vi` 一样。因此，即使你使用 GUI 查看器或编辑器，花一些时间浏览 `less` 手册页也是值得的，至少可以了解以下可用的内容。这样，当你需要做一些现有功能可能涉及的工作时，就会知道如何要搜索手册页或互联网来找到所需的内容。
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有关更多信息，访问 [less 主页][2]。网站有一个不错的常见问题解答，其中包含更多提示和技巧。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/18/5/advanced-use-less-text-file-viewer
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作者：[Yedidyah Bar David][a]
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选题：[lujun9972](https://github.com/lujun9972)
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译者：[译者ID](https://github.com/MjSeven)
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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/didib
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[1]:http://opensource.com/article/18/4/using-less-view-text-files-command-line
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[2]:http://www.greenwoodsoftware.com/less/
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