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‎LPILinuxEssentials.md‎

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* [Getting CLI help](#cli-help)
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+ [Linux MAN pages](#man-pages)
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+ [Info pages](#info-pages)
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+ [More local documentation](#more-local-docs)
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+ [More local documentation](#more-local-docs)
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## <span id="package"></span>Package management
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### <span id="rpm"></span>RPM based
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#### Yum
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YUM repos are located in `/etc/yum.repos.d/`. Unlike APT, YUM has several repo files in the folder. Check out the man pages for YUM and the other package management commands.
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| command | purpose |
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| --- | --- |
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| `yum update` | updates the repos and gives you the option of updating the packages pending updates |
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| `yum search httpd` | searches for that package |
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| `yum install $package` | to install it |
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| `yum check-update $package` | to see if a package needs any updates |
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| `yum upgrade` | upgrade package |
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| `yum deplist $package` | check package’s list of dependencies |
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| `yum clean packages` | will remove dependencies that were left behind but are no longer needed |
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| `yum remove $package` | removes the package |
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| `yum list installed` | list all installed packages |
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#### RPM
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| command | purpose |
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| --- | --- |
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| `rpm -ipv package.rpm` | `i` means install, `p` means show progress, and `v` means verbose |
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| `rpm -q nano` | query the package for info (true file name) |
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| `rpm -qi nano` | query the package for more info |
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| `rpm -e nano` | uninstall the package |
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| `rpm -qR nano` | uninstall required packages |
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### <span id="debian"></span>Debian based: APT and dpkg
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#### APT
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Uses a sources list located in `etc/apt/sources.list`
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| command | purpose |
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| --- | --- |
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| `apt-get update` | searches the online repos and caches the list of packages for when we do a search via... |
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| `apt-cache search $package` | searches for a package in the APT cache |
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| `apt-get install nginx` | install package |
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| `apt-get remove nginx` | remove package |
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| `apt-get remove --purge nginx` | to get rid of config files and such |
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| `apt-get autoremove [$package]` | to remove unneeded packages. |
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| `apt-get upgrade` | upgrades packages |
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| `apt-get -f upgrade` | Imstalls dependencies that we’re flagged while attempting to install a Debian package |
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| `apt-get dist-upgrade` | upgrades the kernel and distribution packages |
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#### dpkg
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| command | purpose |
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| --- | --- |
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| `dpkg -i name.deb` | Installs Debian package |
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| `dpkg --get-selections` | shows all installed packages |
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| `dpkg --remove $package_name` | Removes Debian package |
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| `dpkg --purge $package_name` | Removes dependencies |
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[Back to table of contents](#toc)
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## <span id="cli-basics"></span>Command-line basics
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Shells are command-line interpreters that accept commands that are then sent to the OS kernel for processing. See list of popular shells I saved as an img on my iPad. You can use any shell installed on the computer by typing its name on the CLI.
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- An interactive shell is one in which you issue commands. A non-interactive shell is one that runs in the background.
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- You can have several CLI sessions, accessible via `Alt + F1` through `Alt + F6`.
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- You can `echo $SHELL`
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- You can set each user’s shell and see it in /etc/password
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- If using bash, You can see your command-line history in .bash_history and complete commands with `tab`
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+ `history`
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+ `history 20` show the last 20 commands.
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+ `!20` execute command # 20
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+ `!-2` execute the second-to-last command
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+ `!!` execute the previous command
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+ `!ssh` execute the last SSH command
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+ `!?search?` execute the last command with "search" somewhere in it
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+ `^original^replacement^` find latest command with `original` and replace that string with `replacement` on execution.
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* e.g.: `cat /etc/hots`, then `^hots^hosts^`
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### <span id="cli-syntax"></span>Command-line syntax
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Programs only run from inside folders indicated in the $PATH variable and not the working directory.
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- The letters after `-` are called flags.
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- `ls`:
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+ `-a` = all (show hidden files)
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+ `-l` = long listing (type and permissions, number of links the file has, owner, group, size in bytes, date modified)
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+ `-F` = Display a slash after a directory, an asterisk after an executable,
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+ `-S` = sort by file size, descending
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+ `-r` = reverse the sorting to ascending. E.g., `ls -lrS`
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an @ after each symbolic link.
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+ `-R` = recursively display contents of directories.
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+ `-t` = sort by date modified, desc
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### <span id="basic-commands"></span>Basic commands
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| Command | Purpose |
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| :--- | :--- |
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| `cd` | change directory. By itself, takes you to your home directory. |
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| `env` | display current user’s environment variables |
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| `halt` or `init 0` | shutdown. Note that `init` works but is deprecated |
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| `ifconfig` or `ip addr` | shows NIC configs |
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| `netstat` | status of the network |
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| `reboot` or `init 1` | restart. Note that `init` works but is deprecated |
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| `route` | view routing table |
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| `shutdown` | `-H`= halt; `-P`= poweroff; `c` = cancel pending shutdown; `r` = reboot |
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| `su` | substitute user or super user. E.g., `su josue` or `su -` to become root |
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| `top` | list of running apps/processes; `top -h` gives usage info |
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| `uname` | print OS name. `-n` = hostname; `-r` = kernel’s release; `-v` = kernel’s version number; `-m` 32- or 64-bit; `-p` = processor info; `-o` = full official name of the OS; `-a` = all info above |
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| `which $program` | full path of the application |
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| `whoami` | current user |
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### <span id="cmd-history"></span>Command history and completion
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- A user's command history is kept in **~/.bash_history**
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- The `$HISTFILESIZE` env variable shows how many lines will be saved in the history file. a value of 0 means save nothing.
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- The `$HISTCONTROL` env variable shows Bash's history behavior
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- `history` shows a numbered list of the commands. Rerun a command with `!<num>`
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- You can use the `tab` key to complete a partially-typed command
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### <span id="shell-config-files"></span>shell configuration files
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Different shell use different configuration files. Make sure you know which files your Linux distro uses. A system without a GUI puts you in the login shell. It’s important to know which shell your in so you know which configuration file will be used for it.
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- login shell: Shell you're presented with when you log in remotely (e.g., SSH)
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+ First file executed is `/etc/profile`. This file sets default variables for all users.
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+ Then these files are executed in order. Once there's a match the others are ignored even if they exist:
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* `~/.bash_profile`
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* `~/.bash_login`
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* `~/.profile`
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* `~/.bashrc`
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* `/etc/bashrc`
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+ `~/.bash_logout` is executed when the user logs out
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- non-login shell: Shell you're presented with when you use the terminal application or when you run a script
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+ Executes `~/.bashrc`, which calls `/etc/bashrc`
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- bash_profile: Login shell that stores user-specific shell preferences
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- bashrc: Non-login shell that stores user-specific functions and aliases
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- /etc/profile: (check exact name) affects all users
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### <span id="variables"></span>Environment / shell variables
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- Variables are placeholders for another value. They can be used in scripts.
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- User defined variables: Created by the user
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- Environment variables: Created by the OS to configure the system environment. E.g., `echo $HOME`
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- You can view all environment variables with `env` (not alphabetized) and `set` (alphabetized)
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- Change or create a variable (note there is no space on either side of equal sign:
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+ via `VAR=VALUE`. Example, `PATH=$PATH:/var/opt/`.
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+ `export $PATH` to make that new value available to users in other shells
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+ Configure your bash config files to make this happen every time you start your system.
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#### Common environment variables
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| variable | description |
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| --- | --- |
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| LOGNAME | username of current user |
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| OLDPWD | previous working directory |
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| OSTYPE | duh |
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| PATH | distro dependent |
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| USER and USERNAME | username of current user |
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| HOST and HOSTNAME | system hostname |
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| ENV | you can type `env` or `set` |
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| EUID | UID number of current user |
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| HISTFILE | full path of file |
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| HISTSIZE | size history can grow to |
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### <span id="user-def-vars"></span>User-defined variables
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- Variables cannot start with a number. They can contain `-` amd `_`
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- Convention is to make variables upper-case.
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- Example: `THEDUDE="Jeff Bridges" ; export THEDUDE`
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### <span id="globbing"></span>Globbing
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Globbing is the process of using wildcards to expand a search. Globbing stands for **global command**.
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- `*` = match __0+__ of any character
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- `?` = match __1__ of any character
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- `[Aabc]` = match any single character in list
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- `[^abc]` = exclude characters in list
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- Examples:
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+ `ls -l ????.txt` search for a four-character text file
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+ `ls -l [F]*.txt` search for all text files beginning with capital F
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+ `ls -l f[igh][lfz]e*.txt` what you'd expect from regex, except that `*` matches anything 0+ times
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+ `ls -l [Rr]eport201[0-9]`
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### <span id="quoting"></span>Quoting
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| Character | Description | Example |
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| :--- | :--- | :--- |
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| `"` | allows variable interpolation. | `echo "The path is $PATH"` |
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| `'` | __does not__ allow variable interpolation | `echo 'The path is $PATH'` |
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| `\` | Escapes special chars | `echo "You owe \$5.00"` |
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### <span id="formatting"></span>Formatting commands
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- Commands tend to be lower-case
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- Spacing doesn't matter (2 spaces or a tab is OK)
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- You can wrap long commands along several lines but will need to escape it. E.g., `ls \{enter key} -lah`
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### <span id="options"></span>Working with options
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- Command = what to do; options = how to do it; arguments = what to do it with.
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- Parameters with a leading `-` are called __options__ and switch certain parts of the command on/off. `ls -la` = `ls -l -a`
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- Paramters with no leading `-` are called arguments.
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### <span id="locate-find-whereis"></span>Locate, find, whereis
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#### Locate
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Searches its file database for files or directories the user has access to. Faster than `find` but doesn't allow you to indicate the directory.
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locate passwd
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#### Find
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- Syntax is `find $dir [$dir2] {-name | -iname | -size | -mtime | -atime | -ctime}`
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- You can glob using `*`, `?`, and `[]`.
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- __If you glob, you must use single quotes.__
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- This command will search the directory recursively.
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+ You can exclude a directory with `-or -iname "$dir_name" prune`
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Examples:
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find . -iname '*keyword*' # Match keyword
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find / -size +1024 # greater than size in bytes
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find . -mtime -1 # modified time less than 1 day
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find . -atime -1 # accessed time less than 1 day
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find . -ctime -1 # created time less than 1 day
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find . -iname '*.txt' -or -iname "implementations" -prune
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#### whereis
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Searches for executables and man page files
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whereis cd
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[Back to table of contents](#toc)
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## <span id="cli-help"></span>Getting CLI help
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### <span id="man-pages"></span>Linux MAN pages
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- Meant as a quick reference for people who already know a command and need to learn certain options. Not meant to be a tutorial.
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- Quality can vary significantly from one page to the next
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- Man pages have 9 sections. You'll mostly use section 1, executable programs and shell commands. To see section 5, for example, type `man 5 $command`
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- Use `whatis $command` to search for man page entries matching that command. E.g., `whatis passwd`.
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- `apropos $keyword` search man pages for entries containing the keyword.
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- You can use `/` inside a man page to search forward or `?` to search backwards
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- Man pages are organized like this:
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+ Name
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+ Synopsis: Brief description of how the command is used, incl. optional parameters (in brackets) and required paramters (underlined). `...` means multiple parameters of that type. E.g., `ls -la file1 file2`
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+ Description
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+ Options
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+ Files
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+ See Also
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+ Bugs
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+ History
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+ Author
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### <span id="info-pages"></span>Info pages
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- Similar to man pages, but has hyperlinks (denoted by asterisk.
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- Programs from the Free Software Foundation use info pages instead of man pages.
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- `info $topic`
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### <span id="more-local-docs"></span>More local documentation
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- other ways to get help include an application's readme file.
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- Locations for readme files include:
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+ `/usr/doc/packagename`
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+ `/usr/share/doc/packagename`
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+ `/usr/share/doc/packages/packagename`
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- Many programs have help files in PostScript, PDF, or HTML format.
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- Configuration files are typically in the `/etc` directory.
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- For RPM packages, try `rpm -ql passwd | grep doc` or `rpm -ql yum | grep README`
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#### Reading different file formats
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| file ext | program used to read them |
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| :--- | :--- |
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| `.1` - `.9` | man, info, less |
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| `.gz` or `.bz2` | gunzip or bunzip2 to decompress, then less to read |
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| `.txt` | any text editor |
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| `.htm`, `.html` | any web browser, often less |
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| `.odt` | LibreOffice, OpenOffice.org, any word processor |
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| `.pdf` | `.xpdf`, Adobe Reader |
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| `.tif`, `.png`., `.jpg` | Gimp

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