Linux+ Study guide
Chapter 1
Shell Facts
The Linux shell is the Command Line Interface (CLI) or Text User Interface (TUI) that administrators use to control a Linux operating system. Users and programs use the shell to send commands to the system. A shell might be opened inside a Graphical User Interface (GUI), or might be the sole method used to run the computer.
Although most Linux distributions now include a graphical interface, and many administration tools have been converted to a graphical format, many tasks are best performed from the command prompt. In addition, while graphical elements vary between distributions, shell commands are more likely to be consistent between distributions.
The following table describes many common shell types: Shell Type | Description | bash | The Bourne-again shell (bash) is the standard shell used in most Linux computers. It uses commands similar to a UNIX shell. Bash includes features such as: * Command completion when pressing the tab key * Command history * Improved arithmetic functions | sh | The Bourne shell is an earlier version of the Bash shell, and is similar in many ways. Sh is the original shell created by Steve Bourne. | ksh | The Korn shell was developed by David Korn. Ksh has scripting features not found in bash. | csh | The C-shell uses syntax similar to syntax used in the C programming language. | tcsh | The tcsh shell is an improved version of csh. It offers command line editing and completion features not available with csh. |
Despite their differences, all shells share some common characteristics: * A Linux system can use multiple shells at the same time. * A list of shells is stored in the /etc/shells file. * All shells are interfaces with the kernel, separate and distinct from it. * Shells are run both interactively by end users and automatically by the computer's processes. * Shells can run within one another either interactively when a user starts a second shell from the first shell's command line, or automatically by scripts or programs. * Shells use configuration files to establish their operating environments.

Shell Commands Introduction
The Linux shell is the command line interface (CLI) or text user interface (TUI) that administrators use to control a Linux computer. While many Linux distributions offer a Graphical User Interface (GUI) that can perform the same tasks, the Linux+ exam tests only for command line tasks.
Keep in mind the following tips for working in the shell: * The tilde symbol (~) in the prompt indicates the present working directory is the home directory of the current user. * The command path is the set of directories in which Linux searches to find executable files for the commands typed into the shell prompt: * The path does not include the present working directory. * To see the path, type echo $PATH. * To add a directory to a path, type PATH=$PATH:directory_path. * To execute programs: * Type the executable name in the present working directory if the program is in the command path. * Type ./filename to run an executable in the present working directory. (./ indicates the present working directory.) * Type the full pathname to the file to run an executable from anywhere. * File names and paths are case-sensitive. * Type the beginning of the name, then press Tab to complete a file, command, or directory name. (Press Tab twice if there is more than one possible file, command, or directory name.) * Press the up arrow at the command prompt to print the last command. Press it again to scroll through all previous commands.
The following table describes several common commands used from the shell. Use... | To... | pwd | Show the present working directory. | whoami | Display the current username. | uname | Print system information. Be aware of the following uname options: * -a prints all system information. * -o prints the operating system. * -p prints the processor's architecture type. | su | Switch users in the shell prompt. Be aware of the following su options: * su -l user_name switches to the specified user in a login shell. * su user_name (without the dash, but with the username) switches to the user. * su - user_name (with the dash and username) switches to the user and loads the user's environmental variables. * su - (with the dash, but no username) switches to the root user and loads the root user's environmental variables. * The root user account is the Linux system superuser. * The root user can perform any task; some utilities do not work if the administrator is not logged in as the root user. * su (no dash or username) switches to the root user and but does not load the root user's environmental variables. Linux uses different prompt symbols to indicate the current user: * # indicates the current user is the root user. * $ indicates the current user is a normal user.Note: su requires the password of the user except when switching from root to a normal user. | exit | To exit the login shell or back to the original user after using the su command. | exec | Execute an executable and to replace the shell with the new process created by the executable file. | cd | Change directories. For example when the /usr directory is the current directory: * cd bin changes to the bin directory in the current directory. * cd /usr/bin changes to the bin directory from anywhere in the file system. | ls | Show names of files and directories in the current directory. The ls command commonly uses the following options: * -a shows all files and directories, including hidden files. * -l shows information about files including size, permissions, owner, and modified date. * -d displays only directories. * -s sorts files by size. * -X sorts by extension.Many distributions have a color scheme for different file types: * Directories are blue. * Text files are white. * Links are cyan. * Image files are magenta. * Compressed files are red. | history | See all commands in the history queue. The -c option clears the history list.
Note: History command queues are separate for each user. A command typed as one user cannot be used after using the su command to switch to another user. | clear | Clear the shell screen. | chsh | Change the default shell. Be aware of the chsh options: * -s changes to a different installed shell. The command prompts for a password. * -l lists all installed shells.For example, chsh -s /bin/ksh changes the default shell for the user to the Korn shell if it is installed on the computer |

Shell Configuration Facts
Shell configuration files are scripts that execute when a shell starts. The shell type determines which shell configuration files are executed. Shell types include: * Login shells run when the system starts and is only using the Text User Interface (TUI) as the user interface. * Non-login shells run when the system boots into a Graphical User Interface (GUI) and a user starts a terminal session. The following are the names of the files used when the shell starts: Configuration File | Description | Used by shell type | ~/.bashrc | ~/.bashrc stores shell preferences for individual users. | non-login
(login on some distributions) | /etc/profile | /etc/profile stores system-wide configuration commands and is used primarily to set environment variables. | login | ~/.bash_profile | ~/.bash_profile stores shell preferences for individual users. | login | ~/.bash_login | ~/.bash_login stores commands that execute when a user logs in. | login | ~/.profile | ~/.profile stores configuration preferences similar to /etc/profile, but for individual users. | login | ~/.bash_logout | ~/.bash_logout stores commands that execute when a user logs out. | login |
Be aware of the following: * Login shells execute the configuration scripts they use in the following order: 1. /etc/profile 2. ~/.bash_profile (If this file is found, the shell does not look for additional configuration script files) 3. ~/.bash_login (If this file is found, the shell does not look for additional configuration script files) 4. ~/.profile (This file only executes in the absence of the preceding two) * The su -l command switches to a user into a login shell; however, without the -l option, a non-login shell is started.

Help Facts
Help pages are part of every Linux distribution. They provide information about options and uses for the nearly 1000 commands that are available on a Linux computer. The following table describes help options available for a Linux system. Option | Description | Examples | Man page | A manual (man) page is text-based help file for a specific command stored on the computer. A man page shows the command's syntax, options, and related files and commands. Be aware of the following details: * The man command opens the corresponding file in the command prompt window using the default paging program. * Man pages are typically stored in the /usr/man or /usr/share/man directory. Subdirectories store man pages for different types of commands or languages. * The MANPATH environment variable can be altered to specify a different location for man pages. * Some distributions set this variable using the /etc/man.config script, with corresponding scripts in each user's home directory to allow variation for individual users. * Press Q to exit the man page. * Use the -k option to search the man pages for a command. | man userdel shows the man pages for the userdel command. man man shows the man pages for the man command. man -k user lists all the commands with user in the command name or description. | Info pages | For GNU software, info pages are the primary documentation source. Info pages use hypertext links to navigate the pages. Use the following keys to navigate through an info page: * h shows a navigation help screen. * Tab moves to the next hyperlink. * Home moves to the beginning of a node. * Enter follows the selected hyperlink. * u moves up a node level. * Space moves to the next screen. * Del moves to the previous screen. * q exits an info page. | info mkfifo opens the info page for the mkfifo command. info man opens the info page for the man command. | On-screen help | Many commands include on-screen help as one of the options for the command. In most cases, help displays an abbreviated list that shows of the command syntax and available options with brief descriptions. If the command is typed incorrectly, many commands display the help information automatically.To view the help available for a command, type: * command --help * command -hAlthough some commands support both switches, the --help switch is more common. If necessary, use the more command to scroll through the on-screen help. | jobs --help for help with the jobs command. echo -h for help with the echo command. | whatis database | The whatis database is an index of the man pages on the system. Use the whatis database to find man pages that contain a specific word (i.e., search string). * Run /usr/sbin/makewhatis to create the whatis database. The database must be created first. * Use makewhatis -u to update the database after changes have been made to the man pages. The following commands will return a list of the man pages containing the search string in the whatis database: * apropos searchstring * whatis searchstring | makewhatis -u updates the database. apropos grep returns all man pages containing the term grep. |

Environment Variable Facts
An environment variable is a setting that the operating system or programs working in the operating system access. Environment variables make up the user environment. Be aware of the following details: * The standard for writing variables names (called variable identifiers) is to use upper case (e.g., SHELL and EUID) * Changing environmental variables from the defaults result in user-defined variables. * A user-defined variable applies only to the current session; export the user-defined variables so they apply to child sessions. * Add user-defined variables to the shell configuration files to make them persistent.
The table below lists common environment variables: Variable | Description | BASH | The location of the bash executable file | SHELL | The user's login shell. | CPU | The type of CPU. | DISPLAY | Location where X Windows output goes. | ENV | The location of the configuration file for the current shell. | EUID | The ID number of the current user. | HISTFILE | The filename where past commands are stored. | HISTSIZE | The number of past commands that HISTFILE stores for the current session. | HISTFILESIZE | The number of past commands that HISTFILE stores for the multiple sessions. | HOME | The absolute path of the user's home directory. | HOST | The name of the computer. | HOSTNAME | HOSTNAME is identical to HOST, but used on certain distributions. | INFODIR | The path to the computer's information pages. | LOGNAME | The user name of the current user. | MAIL | The path to the current user's mailbox file. | MANPATH | The path to the computer's man pages. | OLDPWD | The path of the directory the user was in prior to the current path. | OSTYPE | The type of operating system. Usually this is Linux. | PATH | The directory prefixes used to search for programs and files. * Use a colon to separate entries in the PATH variable. * Do not include a period (.) in the PATH variable. A period indicates that the working directory is in the path, and this poses a security risk. | PS1 | The characters the shell uses to indicate normal user ($), root user (#) and similar items. | PWD | The path of the current working directory. | LANG | The language the operating system uses. | PAGER | Used by the man command to specify the program in which to display man pages. |
The table below lists the most common environment variable commands: Use... | To... | Examples | echo $variable | View the variable's value. | echo $SHELL displays the current shell's path. | env | Display the values for environment variables applied to child sessions. | | set | Set shell environment variables. Without options, set displays the set environment variables for the system. | | unset variable | Remove an environment variable. | unset HOMEDIR removes the HOMEDIR variable. | VARIABLE=value | Create a user-defined environment variable.
Note: To append information to an environment variable, put the current variable in the command. For example, PATH=$PATH:/bin/additionalpath. | HOMEDIR=/projects gives the HOMEDIR variable a value of /projects. | export variable | Export a user-defined variable to make it available to child sessions. | export HOMEDIR makes the HOMEDIR user-defined variable available to child sessions.
PATH=$PATH:/bin/special ; export PATH appends a directory to PATH and immediately exports the variable. |

Alias Facts
An alias is a custom command that performs a specific action. Most distributions have aliases that are invoked at startup; however, an alias can be invoked from the shell. Be aware of the following: * Aliases defined with the alias command are not persistent across reboots. * Add the alias to /etc/profile or home/user/.bashrc to make them persistent across reboots.
The following table describes the commands that create and remove aliases. Use... | To... | Example | alias | Display the currently defined aliases on the system. | | alias name | Create a custom command that: * Adds additional functionality to an existing command. * Performs multiple functions.Note: When creating the alias, encapsulate the command(s) with quotation marks or apostrophes. | alias ls='ls --ignore=*.elf' prevents the ls command from displaying .elf files even if ls -a is used. alias securebackup='cp ./*.* /dev/st0/*.*;shred -fuvz ./*' creates a command that copies all files to the storage tape, then shreds the original files. alias forcelogout="killall /usr/bin/Xorg" creates a shortcut kills all Xserver processes. | unalias name | Remove an alias. | unalias ls removes all aliases specified for the ls command and places it back in its original state. unalias forcelogout deletes the forcelogout alias if it exists. |