1.1 The Shell

Question 2

What is a shell?

Answer 2

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.

Question 3

What are the common shells?

Answer 3

• bash - Bourne-Again Shell
• sh - Bourne Shell
• ksh - Korn Shell
• csh - C Shell
• tcsh - TC Shell

Question 4

Features of bash

Answer 4

• *bash** - The Bourne-again shell - 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

Question 5

Features of sh

Answer 5

sh - The Bourne shell - An earlier version of the Bash shell, and is similar in many ways. Sh is the original shell created by Steve Bourne.

Question 6

Features of ksh

Answer 6

ksh - The Korn shell - Developed by David Korn. Ksh has scripting features not found in bash.

Question 7

Features of csh

Answer 7

csh The C-shell - Uses syntax similar to syntax used in the C programming language.

Question 8

Features of tcsh

Answer 8

tcsh - The TC shell - An improved version of csh. It offers command line editing and completion features not available with csh.

Question 9

Common characteristics of all shells

Answer 9

• 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.

Question 10

How do you get into the shell?

Answer 10

• Boot into it at run level 3 (no GUI)
• Access it through a terminal window using a terminal application within the GUI

Question 11

What is the difference between how a Linux shell recognizes the path to executable programs compared to how a DOS environment recognizes the path?

Answer 11

• In DOS or Windows, the shell searches for your command in the current directory first, followed by the directories in the PATH variable.
• In Linux, the shell only searches the directories in the PATH variable for your command. It does not search the current directory.

Question 12

How can you execute commands that are not in the PATH environment variable?

Answer 12

• Use an absolute path
• Use a relative path
• Copy or move the command file to a directory that is in the path (like ~/bin or /bin)
• Add the directory that contains the command file to the PATH variable

Question 13

How can you execute commands in the current working directory?

Answer 13

Prefix the command with a “dot-slash” (./)

Question 14

What types of items can Tab complete be used to complete once you start typing the entities’ name?

Answer 14

Files and directories.

Question 15

What is tab-completion?

Answer 15

A shell feature that gives you the ability to type part of a filename and then have the shell complete the filename (or provide a list of matching suggestions) by hitting the tab key.

Question 16

How does command history work?

Answer 16

Every time you enter a command, that command is written to a hidden file in your home directory. You can access previous commands with the up- and down-arrow keys.

Question 17

What common commands are used to navigate through shells?

Answer 17

• ‘pwd’
• ‘ls’
• ‘cd’

Question 18

Which file contains the history of shell commands?

Answer 18

~/.bash_history

Question 19

What does the tilde symbol (~) in the command prompt represent?

Answer 19

The tilde symbol (~) in the prompt indicates the present working directory is the home directory of the current user.

Question 20

What is the command path?

Answer 20

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:.

Question 21

How do you execute programs in the shell?

Answer 21

• 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.

Question 22

Are filenames and paths case-sensitive?

Answer 22

Yes

Question 23

How does tab-completion work?

Answer 23

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.)

Question 24

How do you recall the previous command and scroll through all previous commands?

Answer 24

Press the up arrow at the command prompt to print the last command. Press it again to scroll through all previous commands.

Question 25

What does the pwd command do?

Answer 25

Shows the present working directory.

Question 26

What does the **whoami** command do?

Answer 26

Displays the current username.

Question 27

What does the **uname** command do?

Answer 27

**uname** prints system information.

Question 28

What does the command **uname -a** do?

Answer 28

**uname -a** prints all system information.

Question 29

What does the command **uname -o** do?

Answer 29

**uname -o** prints all system information.

Question 30

What does the command **uname -p** do?

Answer 30

**uname -p** prints all system information.

Question 31

What does the **su** command do?

Answer 31

**su** switches users. With no arguments, switches to the root user.

Question 32

What does the command **su -l user\_name** do?

Answer 32

**su -l user\_name** switches to the specified user in a login shell.

Question 33

What does the command **su user\_name** do?

Answer 33

**su user\_name** (without the dash, but with the username) switches to the user.

Question 34

What does the command **su - user\_name** do?

Answer 34

**su - user\_name** (with the dash and username) switches to the user and loads the user's environmental variables.

Question 35

What does the command **su -** do?

Answer 35

**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. It can perform any task; some utilities do not work if the administrator is not logged in as the root user.

Question 36

What are the different shell prompt symbols and what do they represent?

Answer 36

* '#' indicates the current user is the root user. * '$' indicates the current user is a normal user.

Question 37

What does the **exit** command do?

Answer 37

Exits the login shell or exits back to the original user after using the su command.

Question 38

What does the **exec** command do?

Answer 38

Runs an executable and replaces the shell with the new process created by the executable file.

Question 39

What does the **cd** command do?

Answer 39

Changes directories.

Question 40

What does the **ls** command do?

Answer 40

Shows names of files and directories in the current directory.

Question 41

What are some common **ls** options and what do they do?

Answer 41

* **ls -a** shows all files and directories, including hidden files. * **ls -l** shows information about files including size, permissions, owner, and modified date. * **ls -d** displays only directories. * **ls -s** sorts files by size. * **ls -X** sorts by extension.

Question 42

What are some common **ls** file colors and what do they represent?

Answer 42

* **Blue**: directories * **White**: text files * **Cyan**: symbolic links * **Magenta**: image files * **Red**: compressed files

Question 43

What does the **history** command do and what does the **-c** option do?

Answer 43

Shows all commands in the current user's history queue. The -c option clears the history list.

Question 44

What does the **clear** command do?

Answer 44

Clears the shell screen.

Question 45

What does the **chsh** command do and what do the options **-s** and **-l** do?

Answer 45

* *chsh** changes the default shell. * **chsh -s** changes to a different installed shell. The command prompts for a password. * **chsh -l** lists all installed shells.

Question 46

What is a **login** shell used?

Answer 46

A shell that is started when a user first logs onto a system - such as when your system is configured to boot directly into a shell environment (like **bash**).

Question 47

What is a **non-login** shell?

Answer 47

A shell that is started from within a running system by a user who has already logged onto that system - such as when you start a terminal program from within a GUI.

Question 48

What is the effect of a leading period (.) before a filename?

Answer 48

The file is hidden and is not displayed by the 'ls' command unless the '-a' switch is used.

Question 49

Which shell configuration files are executed by non-login shells?

Answer 49

Only **.bashrc**

Question 50

Which shell configuration files are used by RedHat, CentOS, and Fedora Linux (in addition to **/etc/profile**)?

Answer 50

* **~/.bashrc** * **~/.bash\_profile** * **~/.bash\_logout**

Question 51

Which shell configuration files are used by SUSE Linux (in addition to **/etc/profile**)?

Answer 51

* **~/.bashrc** * **~/.profile**

Question 52

How do you list hidden files in addition to regular files?

Answer 52

**ls -a**

Question 53

How do some distributions run the **~/.bashrc** shell configuration file in login shells?

Answer 53

By calling (sourcing) the script from within the **.bash\_profile** script.

Question 54

How do you append a path onto the end of the current **PATH** variable?

Answer 54

**PATH=$PATH:*new\_path***

Question 55

What are shell configuration files?

Answer 55

Scripts that execute when a shell starts.

Question 56

What determines which shell configuration files are executed?

Answer 56

The shell type.

Question 57

What are the two different types of shells?

Answer 57

* **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.

Question 58

How is **~/.bashrc** used?

Answer 58

Shell configuration file. Stores shell preferences for individual users. Executed by non-login shells (and by login shells for some distributions).

Question 59

How is **/etc/profile** used?

Answer 59

Shell configuration file. Stores system-wide configuration commands and is used primarily to set environment variables. Executed by login shells.

Question 60

How is **~/.bash\_profile** used?

Answer 60

Shell configuration file. Stores shell preferences for individual users. Executed by login shells.

Question 61

How is **~/.bash\_login** used?

Answer 61

Shell configuration file. Stores commands that execute when a user logs in. Executed by login shells. (Not executed if **~/.bash\_profile** exists.)

Question 62

How is **~/.profile** used?

Answer 62

Shell configuration file. Stores configuration preferences similar to /etc/profile, but for individual users. Executed by login shells. (Not executed if **~/.bash\_profile** or **~/.bash\_login** exist.)

Question 63

How is **~/.bash\_logout** used?

Answer 63

Shell configuration file. Stores commands that execute when a user logs out. Executed by login shells. Executed by login shells.

Question 64

Which shell configuration files are executed by login shells and in what order?

Answer 64

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) 5. **~/.bashrc** (Depending on the distribution)

Question 65

Which command switches to a user using a login shell?

Answer 65

**su -l**