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Part 12 - LFCS: How to Explore Linux with Installed Help Documentations and Tools
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==================================================================================
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Because of the changes in the LFCS exam requirements effective Feb. 2, 2016, we are adding the necessary topics to the [LFCS series][1] published here. To prepare for this exam, your are highly encouraged to use the [LFCE series][2] as well.
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>LFCS: Explore Linux with Installed Documentations and Tools – Part 12
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Once you get used to working with the command line and feel comfortable doing so, you realize that a regular Linux installation includes all the documentation you need to use and configure the system.
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Another good reason to become familiar with command line help tools is that in the [LFCS][3] and [LFCE][4] exams, those are the only sources of information you can use – no internet browsing and no googling. It’s just you and the command line.
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For that reason, in this article we will give you some tips to effectively use the installed docs and tools in order to prepare to pass the **Linux Foundation Certification** exams.
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### Linux Man Pages
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A man page, short for manual page, is nothing less and nothing more than what the word suggests: a manual for a given tool. It contains the list of options (with explanation) that the command supports, and some man pages even include usage examples as well.
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To open a man page, use the **man command** followed by the name of the tool you want to learn more about. For example:
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```
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# man diff
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```
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will open the manual page for `diff`, a tool used to compare text files line by line (to exit, simply hit the q key.).
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Let’s say we want to compare two text files named `file1` and `file2` in Linux. These files contain the list of packages that are installed in two Linux boxes with the same distribution and version.
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Doing a `diff` between `file1` and `file2` will tell us if there is a difference between those lists:
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```
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# diff file1 file2
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```
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>Compare Two Text Files in Linux
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where the `<` sign indicates lines missing in `file2`. If there were lines missing in `file1`, they would be indicated by the `>` sign instead.
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On the other hand, **7d6** means line **#7** in file should be deleted in order to match `file2` (same with **24d22** and **41d38**), and 65,67d61 tells us we need to remove lines **65** through **67** in file one. If we make these corrections, both files will then be identical.
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Alternatively, you can display both files side by side using the `-y` option, according to the man page. You may find this helpful to more easily identify missing lines in files:
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```
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# diff -y file1 file2
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```
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>Compare and List Difference of Two Files
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Also, you can use `diff` to compare two binary files. If they are identical, `diff` will exit silently without output. Otherwise, it will return the following message: “**Binary files X and Y differ**”.
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### The –help Option
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The `--help` option, available in many (if not all) commands, can be considered a short manual page for that specific command. Although it does not provide a comprehensive description of the tool, it is an easy way to obtain information on the usage of a program and a list of its available options at a quick glance.
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For example,
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```
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# sed --help
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```
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shows the usage of each option available in sed (the stream editor).
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One of the classic examples of using `sed` consists of replacing characters in files. Using the `-i` option (described as “**edit files in place**”), you can edit a file without opening it. If you want to make a backup of the original contents as well, use the `-i` option followed by a SUFFIX to create a separate file with the original contents.
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For example, to replace each occurrence of the word `Lorem` with `Tecmint` (case insensitive) in `lorem.txt` and create a new file with the original contents of the file, do:
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```
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# less lorem.txt | grep -i lorem
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# sed -i.orig 's/Lorem/Tecmint/gI' lorem.txt
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# less lorem.txt | grep -i lorem
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# less lorem.txt.orig | grep -i lorem
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```
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Please note that every occurrence of `Lorem` has been replaced with `Tecmint` in `lorem.txt`, and the original contents of `lorem.txt` has been saved to `lorem.txt.orig`.
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>Replace A String in Files
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### Installed Documentation in /usr/share/doc
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This is probably my favorite pick. If you go to `/usr/share/doc` and do a directory listing, you will see lots of directories with the names of the installed tools in your Linux system.
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According to the [Filesystem Hierarchy Standard][5], these directories contain useful information that might not be in the man pages, along with templates and configuration files to make configuration easier.
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For example, let’s consider `squid-3.3.8` (version may vary from distribution to distribution) for the popular HTTP proxy and [squid cache server][6].
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Let’s `cd` into that directory:
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```
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# cd /usr/share/doc/squid-3.3.8
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```
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and do a directory listing:
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```
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# ls
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```
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>Linux Directory Listing with ls Command
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You may want to pay special attention to `QUICKSTART` and `squid.conf.documented`. These files contain an extensive documentation about Squid and a heavily commented configuration file, respectively. For other packages, the exact names may differ (as **QuickRef** or **00QUICKSTART**, for example), but the principle is the same.
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Other packages, such as the Apache web server, provide configuration file templates inside `/usr/share/doc`, that will be helpful when you have to configure a standalone server or a virtual host, to name a few cases.
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### GNU info Documentation
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You can think of info documents as man pages on steroids. As such, they not only provide help for a specific tool, but also they do so with hyperlinks (yes, hyperlinks in the command line!) that allow you to navigate from a section to another using the arrow keys and Enter to confirm.
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Perhaps the most illustrative example is:
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```
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# info coreutils
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```
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Since coreutils contains the [basic file, shell and text manipulation utilities][7] which are expected to exist on every operating system, you can reasonably expect a detailed description for each one of those categories in info **coreutils**.
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>Info Coreutils
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As it is the case with man pages, you can exit an info document by pressing the `q` key.
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Additionally, GNU info can be used to display regular man pages as well when followed by the tool name. For example:
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```
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# info tune2fs
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```
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will return the man page of **tune2fs**, the ext2/3/4 filesystems management tool.
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And now that we’re at it, let’s review some of the uses of **tune2fs**:
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Display information about the filesystem on top of **/dev/mapper/vg00-vol_backups**:
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```
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# tune2fs -l /dev/mapper/vg00-vol_backups
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```
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Set a filesystem volume name (Backups in this case):
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```
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# tune2fs -L Backups /dev/mapper/vg00-vol_backups
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```
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Change the check intervals and `/` or mount counts (use the `-c` option to set a number of mount counts and `/` or the `-i` option to set a check interval, where **d=days, w=weeks, and m=months**).
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```
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# tune2fs -c 150 /dev/mapper/vg00-vol_backups # Check every 150 mounts
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# tune2fs -i 6w /dev/mapper/vg00-vol_backups # Check every 6 weeks
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```
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All of the above options can be listed with the `--help` option, or viewed in the man page.
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### Summary
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Regardless of the method that you choose to invoke help for a given tool, knowing that they exist and how to use them will certainly come in handy in the exam. Do you know of any other tools that can be used to look up documentation? Feel free to share with the Tecmint community using the form below.
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Questions and other comments are more than welcome as well.
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||||
--------------------------------------------------------------------------------
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||||
via: http://www.tecmint.com/linux-basic-shell-scripting-and-linux-filesystem-troubleshooting/
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||||
|
||||
作者：[Gabriel Cánepa][a]
|
||||
译者：[译者ID](https://github.com/译者ID)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]:http://www.tecmint.com/author/gacanepa/
|
||||
[1]: http://www.tecmint.com/sed-command-to-create-edit-and-manipulate-files-in-linux/
|
||||
[2]: http://www.tecmint.com/installing-network-services-and-configuring-services-at-system-boot/
|
||||
[3]: http://www.tecmint.com/sed-command-to-create-edit-and-manipulate-files-in-linux/
|
||||
[4]: http://www.tecmint.com/installing-network-services-and-configuring-services-at-system-boot/
|
||||
[5]: http://www.tecmint.com/linux-directory-structure-and-important-files-paths-explained/
|
||||
[6]: http://www.tecmint.com/configure-squid-server-in-linux/
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[7]: http://www.tecmint.com/sed-command-to-create-edit-and-manipulate-files-in-linux/
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[8]:
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@ -0,0 +1,185 @@
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Part 13 - LFCS: How to Configure and Troubleshoot Grand Unified Bootloader (GRUB)
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=====================================================================================
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Because of the changes in the LFCS exam requirements effective Feb. 2, 2016, we are adding the necessary topics to the [LFCS series][1] published here. To prepare for this exam, your are highly encouraged to use the [LFCE series][2] as well.
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>LFCS: Configure and Troubleshoot Grub Boot Loader – Part 13
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In this article we will introduce you to GRUB and explain why a boot loader is necessary, and how it adds versatility to the system.
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The [Linux boot process][3] from the time you press the power button of your computer until you get a fully-functional system follows this high-level sequence:
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* 1. A process known as **POST** (**Power-On Self Test**) performs an overall check on the hardware components of your computer.
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* 2. When **POST** completes, it passes the control over to the boot loader, which in turn loads the Linux kernel in memory (along with **initramfs**) and executes it. The most used boot loader in Linux is the **GRand Unified Boot loader**, or **GRUB** for short.
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* 3. The kernel checks and accesses the hardware, and then runs the initial process (mostly known by its generic name “**init**”) which in turn completes the system boot by starting services.
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In Part 7 of this series (“[SysVinit, Upstart, and Systemd][4]”) we introduced the [service management systems and tools][5] used by modern Linux distributions. You may want to review that article before proceeding further.
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### Introducing GRUB Boot Loader
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Two major **GRUB** versions (**v1** sometimes called **GRUB Legacy** and **v2**) can be found in modern systems, although most distributions use **v2** by default in their latest versions. Only **Red Hat Enterprise Linux 6** and its derivatives still use **v1** today.
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Thus, we will focus primarily on the features of **v2** in this guide.
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Regardless of the **GRUB** version, a boot loader allows the user to:
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* 1). modify the way the system behaves by specifying different kernels to use,
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* 2). choose between alternate operating systems to boot, and
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* 3). add or edit configuration stanzas to change boot options, among other things.
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Today, **GRUB** is maintained by the **GNU** project and is well documented in their website. You are encouraged to use the [GNU official documentation][6] while going through this guide.
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When the system boots you are presented with the following **GRUB** screen in the main console. Initially, you are prompted to choose between alternate kernels (by default, the system will boot using the latest kernel) and are allowed to enter a **GRUB** command line (with `c`) or edit the boot options (by pressing the `e` key).
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>GRUB Boot Screen
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One of the reasons why you would consider booting with an older kernel is a hardware device that used to work properly and has started “acting up” after an upgrade (refer to [this link][7] in the AskUbuntu forums for an example).
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The **GRUB v2** configuration is read on boot from `/boot/grub/grub.cfg` or `/boot/grub2/grub.cfg`, whereas `/boot/grub/grub.conf` or `/boot/grub/menu.lst` are used in **v1**. These files are NOT to be edited by hand, but are modified based on the contents of `/etc/default/grub` and the files found inside `/etc/grub.d`.
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In a **CentOS 7**, here’s the configuration file that is created when the system is first installed:
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```
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GRUB_TIMEOUT=5
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GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
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GRUB_DEFAULT=saved
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GRUB_DISABLE_SUBMENU=true
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GRUB_TERMINAL_OUTPUT="console"
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GRUB_CMDLINE_LINUX="vconsole.keymap=la-latin1 rd.lvm.lv=centos_centos7-2/swap crashkernel=auto vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos_centos7-2/root rhgb quiet"
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GRUB_DISABLE_RECOVERY="true"
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```
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In addition to the online documentation, you can also find the GNU GRUB manual using info as follows:
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```
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# info grub
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```
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If you’re interested specifically in the options available for /etc/default/grub, you can invoke the configuration section directly:
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```
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# info -f grub -n 'Simple configuration'
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```
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Using the command above you will find out that `GRUB_TIMEOUT` sets the time between the moment when the initial screen appears and the system automatic booting begins unless interrupted by the user. When this variable is set to `-1`, boot will not be started until the user makes a selection.
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When multiple operating systems or kernels are installed in the same machine, `GRUB_DEFAULT` requires an integer value that indicates which OS or kernel entry in the GRUB initial screen should be selected to boot by default. The list of entries can be viewed not only in the splash screen shown above, but also using the following command:
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### In CentOS and openSUSE:
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```
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# awk -F\' '$1=="menuentry " {print $2}' /boot/grub2/grub.cfg
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```
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### In Ubuntu:
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```
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# awk -F\' '$1=="menuentry " {print $2}' /boot/grub/grub.cfg
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```
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In the example shown in the below image, if we wish to boot with the kernel version **3.10.0-123.el7.x86_64** (4th entry), we need to set `GRUB_DEFAULT` to `3` (entries are internally numbered beginning with zero) as follows:
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```
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GRUB_DEFAULT=3
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```
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>Boot System with Old Kernel Version
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One final GRUB configuration variable that is of special interest is `GRUB_CMDLINE_LINUX`, which is used to pass options to the kernel. The options that can be passed through GRUB to the kernel are well documented in the [Kernel Parameters file][8] and in [man 7 bootparam][9].
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Current options in my **CentOS 7** server are:
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```
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GRUB_CMDLINE_LINUX="vconsole.keymap=la-latin1 rd.lvm.lv=centos_centos7-2/swap crashkernel=auto vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos_centos7-2/root rhgb quiet"
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```
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Why would you want to modify the default kernel parameters or pass extra options? In simple terms, there may be times when you need to tell the kernel certain hardware parameters that it may not be able to determine on its own, or to override the values that it would detect.
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This happened to me not too long ago when I tried **Vector Linux**, a derivative of **Slackware**, on my 10-year old laptop. After installation it did not detect the right settings for my video card so I had to modify the kernel options passed through GRUB in order to make it work.
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Another example is when you need to bring the system to single-user mode to perform maintenance tasks. You can do this by appending the word single to `GRUB_CMDLINE_LINUX` and rebooting:
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```
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GRUB_CMDLINE_LINUX="vconsole.keymap=la-latin1 rd.lvm.lv=centos_centos7-2/swap crashkernel=auto vconsole.font=latarcyrheb-sun16 rd.lvm.lv=centos_centos7-2/root rhgb quiet single"
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```
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After editing `/etc/defalt/grub`, you will need to run `update-grub` (Ubuntu) or `grub2-mkconfig -o /boot/grub2/grub.cfg` (**CentOS** and **openSUSE**) afterwards to update `grub.cfg` (otherwise, changes will be lost upon boot).
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This command will process the boot configuration files mentioned earlier to update `grub.cfg`. This method ensures changes are permanent, while options passed through GRUB at boot time will only last during the current session.
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### Fixing Linux GRUB Issues
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If you install a second operating system or if your GRUB configuration file gets corrupted due to human error, there are ways you can get your system back on its feet and be able to boot again.
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In the initial screen, press `c` to get a GRUB command line (remember that you can also press `e` to edit the default boot options), and use help to bring the available commands in the GRUB prompt:
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>Fix Grub Configuration Issues in Linux
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We will focus on **ls**, which will list the installed devices and filesystems, and we will examine what it finds. In the image below we can see that there are 4 hard drives (`hd0` through `hd3`).
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Only `hd0` seems to have been partitioned (as evidenced by msdos1 and msdos2, where 1 and 2 are the partition numbers and msdos is the partitioning scheme).
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Let’s now examine the first partition on `hd0` (**msdos1**) to see if we can find GRUB there. This approach will allow us to boot Linux and there use other high level tools to repair the configuration file or reinstall GRUB altogether if it is needed:
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```
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# ls (hd0,msdos1)/
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```
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|
||||
As we can see in the highlighted area, we found the `grub2` directory in this partition:
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>Find Grub Configuration
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||||
Once we are sure that GRUB resides in (**hd0,msdos1**), let’s tell GRUB where to find its configuration file and then instruct it to attempt to launch its menu:
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||||
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```
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set prefix=(hd0,msdos1)/grub2
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set root=(hd0,msdos1)
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||||
insmod normal
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||||
normal
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||||
```
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>Find and Launch Grub Menu
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||||
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||||
Then in the GRUB menu, choose an entry and press **Enter** to boot using it. Once the system has booted you can issue the `grub2-install /dev/sdX` command (change `sdX` with the device you want to install GRUB on). The boot information will then be updated and all related files be restored.
|
||||
|
||||
```
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# grub2-install /dev/sdX
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||||
```
|
||||
|
||||
Other more complex scenarios are documented, along with their suggested fixes, in the [Ubuntu GRUB2 Troubleshooting guide][10]. The concepts explained there are valid for other distributions as well.
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||||
|
||||
### Summary
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||||
|
||||
In this article we have introduced you to GRUB, indicated where you can find documentation both online and offline, and explained how to approach an scenario where a system has stopped booting properly due to a bootloader-related issue.
|
||||
|
||||
Fortunately, GRUB is one of the tools that is best documented and you can easily find help either in the installed docs or online using the resources we have shared in this article.
|
||||
|
||||
Do you have questions or comments? Don’t hesitate to let us know using the comment form below. We look forward to hearing from you!
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: http://www.tecmint.com/linux-basic-shell-scripting-and-linux-filesystem-troubleshooting/
|
||||
|
||||
作者：[Gabriel Cánepa][a]
|
||||
译者：[译者ID](https://github.com/译者ID)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: http://www.tecmint.com/author/gacanepa/
|
||||
[1]: http://www.tecmint.com/sed-command-to-create-edit-and-manipulate-files-in-linux/
|
||||
[2]: http://www.tecmint.com/installing-network-services-and-configuring-services-at-system-boot/
|
||||
[3]: http://www.tecmint.com/linux-boot-process/
|
||||
[4]: http://www.tecmint.com/linux-boot-process-and-manage-services/
|
||||
[5]: http://www.tecmint.com/best-linux-log-monitoring-and-management-tools/
|
||||
[6]: http://www.gnu.org/software/grub/manual/
|
||||
[7]: http://askubuntu.com/questions/82140/how-can-i-boot-with-an-older-kernel-version
|
||||
[8]: https://www.kernel.org/doc/Documentation/kernel-parameters.txt
|
||||
[9]: http://man7.org/linux/man-pages/man7/bootparam.7.html
|
||||
[10]: https://help.ubuntu.com/community/Grub2/Troubleshooting
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