1. Workbook 6 – Part 1 The Bash Shell
RH030 Linux Computing Essentials

2. Linux+ Guide to Linux Certification, 2e
Workbook 6 Objectives
This workbook is all about the shell.
Understanding shell functionality
what a shell is and does
Understanding what the term ‘subshells’ refers too.
Overview of shell features Metacharachers, redirection, piping
History
shell variables
Shell variables
Local verse Environmental
Creating new variables
Exporting variables
Identifying common shell environment variables
$LOGNAME, $SHELL,$HOME, $PATH
Manipulating common shell environment variables
Startup & Login environment files – covered next week
global & local environment files.
Used to create permanent aliases/variables
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3. Chapter 1 – Introduction to Bash
Key Concepts
The default shell in Red Hat Enterprise Linux is the bash shell.
The bash shell can be used interactively, or as a powerful scripting language.
bash allows users to customize their shell & local environment.
All shells have functionality – such as history.
The bash shell maintains a history of the command lines that it executes.
Command lines can be retrieved from the history using various history expansions that begin with "!".
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4. Understanding the Shell
A shell is just a program:
This program acts as an interface between the user and the system.
It is run from an executable file.
The executable files for shells are stored in /bin
The /etc/shell configuration file determines what different shells’s are valid for this system.
How the shell is used:
A shell can be used interactively, or as a powerful scripting language.
Interactively it:
Provides the user a command-line interface to the system.
Interpreting the commands as they are run.
Manipulating the commands input and output
BASH is the most commonly used.
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5. There are many different Shells available.
Different shells have different functionality.
Bourne /bin/sh
original shell program
developed by Stephen Bourne for AT&T
Very limited functionality
Korn /bin/ksh
superset of the Bourne shell
added features such as aliasing and history
once most widely used shell
Bash /bin/bash
superset of the Bourne & Korn shell
Now most widely used shell
C Shell /bin/csh
based on the C programming language
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6. Interactively Processing commands
The shell interactively manages commands as they are run.
Interpreting the contents of the commands.
Manipulating the commands input and output
A command is really just a utility program which the shell runs.
When a program is run it becomes a process that the kernel carries out.
For every actions performed there are 2 possible results.
2>
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7. It uses the standard I/O devices on the system to handle the commands input/output.
The three common file descriptors
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8. Shells allow us to use redirection
The shell provides a way to redirect flow of information to and from a command away from these standard devices.
It also allows you to separate the error outputs.
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9. Redirection can get very complicated.
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10. Shells allow us to use command piping
A pipe passes the output from one command to become the input of another command.
Figure 8-2: Piping information from one command to another
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11. Shells allow us to use command piping
Figure 8-2: Piping information from one command to another
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12. Piping can get very complicated.
Command piping passes the standard output of one command to the standard input of another.
Commonly used to reduce or filter the amount of information displayed on terminal screen
Example of pipe & redirection: ls –l/etc| sort +n2 | head 12 > savefile
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13. Using Shell History Function
As a convenience to users of interactive shells, the bash shell keeps a history of each command entered by the user, and provides a variety of ways to make commands from this history available at the finger tips.
cat ~/.bash_history
The easiest way to view your current history is to use history command.
Substitution
As an alternative to the arrow keys, the bash shell also performs "history substitution", which is triggered by the exclamation point.
!! Previous command
!n Command number
n !-n The nth most recent command
!cmd The most recent command that began cmd
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14. Linux+ Guide to Linux Certification, 2e
~/.bashrc
bash executes commands found in the ~/.bashrc file.
Used upon login and every time a new shell (subshell) is started.
Thus allowing users to customize their shell & local environment.
Use vi to modify ~/.bashrc
# infront of the line = comment line
date >> .bash_timestamps
Or use echo >> to append to the file
echo umask 777 >> ~/.bashrc
echo date >> .bash_timestamps >> ~./.bashrc
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15. Chapter 2 – command lists & scripts
These examples are being used to help develop your understanding of the use of subshells.
Example 1 - Multiple commands can be separated with a ;
Note - When using a “;” to run multiple commands the shell's current working directory is changed as the commands are run.
elvis]$ cd /etc/X11; ls
applnk prefdm sysconfig xorg.conf.backup xkb
desktop-menus proxymngr twm xorg.conf.wbx Xmodmap
fs rstart X xorg.conf.works Xresources
gdm serverconfig xdm XftConfig.README-OBSOLETE xserver
lbxproxy starthere xorg.conf xinit xsm
X11]$
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16. Running commands in a sub-shell
Here they use a sub-shell in which to run the commands
When using “parenthesis” to run multiple commands the shell's current working directory is left unchanged.
When bash encounters parenthesis on the command line, it spawns an entirely new child bash process (called a subshell).
And runs the commands within the subshell.
After running the commands, the subshell exits, BUT the user is left in the original (unchanged) shell.
Example 2 - commands are wrapped in parenthesis
elvis]$ (cd /etc/X11; ls)
applnk prefdm sysconfig xorg.conf.backup xkb
desktop-menus proxymngr twm xorg.conf.wbx Xmodmap
fs rstart X xorg.conf.works Xresources
gdm serverconfig xdm XftConfig.README-OBSOLETE xserver
lbxproxy starthere xorg.conf xinit xsm
elvis]$
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17. Linux+ Guide to Linux Certification, 2e
Upon exiting, every command returns an integer to its parent called a return value.
What is a return value
It is to tell the system whether the command worked or failed.
Held in the shell variable $?
This can be used to display the returned value of previously executed command.
This example shows how the ls command succeeded.
elvis]$ ls -l /etc/passwd
-rw-r--r-- 1 root root 3694 Aug 15 16:26 /etc/passwd
elvis]$ echo $? 0
In contrast, the following example shows how the ls command failed
elvis]$ ls -l /etc/password ls: /etc/password:
No such file or directory
elvis]$ echo $? 1
echo
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18. Running Multiple Command Conditionally
The bash shell uses && and || to join two commands conditionally.
When commands are conditionally joined, the first will always execute.
The second command may execute or not.
Depending on the return value of the first command.
For example
By coupling two commands with &&, the second command will only run if the first command succeeded
(i.e., had a return value of 0).
elvis]$ mkdir /tmp/boring && mv numbers.txt /tmp/boring
elvis]$ ls
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19. Linux+ Guide to Linux Certification, 2e
Difference of && or ||&
Wth || bash will execute the second command only if the first command "fails"
(i.e., had a return value of 1).
elvis]$ mkdir /tmp/boring || mv numbers.txt /tmp/boring
elvis]$ ls
In the above example, if the mkdir command succeeded,
Will the file be moved?
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20. Chapter 3 – Bash Variables
Key Concepts
Shell variables are assigned using an A=apple syntax.
Variables are examined with the $ character, as in echo $A.
At the kernel level, every process (such as a subshell) has a collection of environment variables, which are inherited by child processes.
The export command converts a shell variable into an environment variable.
The set and env commands list shell variables and environment variables, respectively.
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21. Linux+ Guide to Linux Certification, 2e
What is a variable?
A variable is a placeholder for information required by a program or processes so that it can function properly.
A variable has a name which holds a value.
<variable-name>=value
sheila=teacher
It contains accessible information that can be changed.
Changing the value of a variable is called setting the variable.
Displaying the value of a variable:
To view contents of a specified variable you must always use a $ sign infront of the variable name.
This tells the shell you are referring to the value of the variable
NOT the variable itself.
echo $sheila
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22. The shell manages the variables
There are 2 types of variables:
User-defined variables:
Commonly called local variables
Created and defined by the local user
A user can create their own local variables in any shell.
Environment variables:
Contains system information or properties which system needs to run.
These are used by the system and programs in the running of the overall of the system and accessed regularly.
Such as required for each user profile.
Such as your unique login id.
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23. Creating a new local variables
Creating or Changing value of a variable:
To create a new variable name you use the following format.
<new-variable-name> equal sign (=) and new value
Sheila=teacher
Double quotes should surround an entire string.
Sheila= “Sheila is the teacher”
To display the value of any variable using the echo command.
echo $Sheila
teacher
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24. All operating systems use variables
Configuration scripts use environmental variables
They are commonly used to store and refer to any changeable information within system configuration files or scripts .
Setting or modifying information used in the multiuser environment
They allow the administrator to change the information being used within the script without having to actually edit the system configuration files or script.
As it is the value of the variable which is being used the in system configuration files and scripts that changes.
The variable name never changes and it is this name which is written into system configuration files and scripts.
$ USER=Sheila
$ echo “Welcome to the system $USER”
$ Welcome to the system Sheila
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25. Environment Variables
Most variables available in the system are global environment variables.
They are used to allow a networking or multiuser environment.
They are commonly referred to as system wide or global variables
These are automatically setup for each user as you login.
They are read from global or system wide environment startup files
/etc/profile /etc/bashrc
And then from the local shell’s environment startup files.
~/.bash_profile ~/.bashrc
Each user can often modify these values to suit their specific needs in their own local environment
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26. Examples of Some Frequently Used Environmental Variables
Name Description
USER Sets the unique id by which this user will be identified
HOME Sets the directory which will be this users home directory.
PATH Specifies the directories that the shell is to look through
to find an executable command.
These directories are searched in the order of the path.
SHELL Sets your default shell.
HOSTNAME Sets the name of your system
MAIL Location of the users mailbox.
PS Sets how your prompt will be displayed.
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27. Displaying Variables & their values
Displaying all currently available variables & their current values
set command:
Displaying the value of a specific variable:
To view contents of a specified variable you must always use a $ sign infront of the variable name.
This tells the shell you are referring to the value of the variable
NOT the variable itself.
echo $HOME
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28. Examples - Environment Variables
Table 8-3: Common BASH environment variables
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29. Examples - Environment Variables
Table 8-3 (continued): Common BASH environment variables
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30. Linux+ Guide to Linux Certification, 2e
Local Variables
User-defined variables
A user can create their own local variables in any shell.
But it will only be available in the current shell.
Subshell = A new shell opened by another shell
Most commands run in a subshell.
Variables created in the current shell are not automatically available to it’s subshells
Unless you export them.
This make them available to any new subshells
They will still only be temporary
To make them available permanently you will need to put them into your shell’s local script startup environment files.
More on this next week.
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31. Linux+ Guide to Linux Certification, 2e
Exporting Variables
Changes how the system sees them.
It makes them into an environmental variable
And the entire system becomes aware of them.
For a variable to be automatically available to all subshells it needs to be exported or declared to the entire system that they are available for use.
The export command Is used to do this.
It declares the existence of defined variables within the system.
This makes them available to all subshells.
export
lists all the currently exported / declared available variables
export teacher
Used with an existing variable it will export/declare that variable.
Linux+ Guide to Linux Certification, 2e

32. Modifying the values of variables
$PATH Environment Variable
Displays the current settings on the system path.
echo $PATH
Backup the current path settings into a new variable
OLDPATH=$PATH
Add your new directory ensuring you don’t overwrite the existing settings already being used on the system path.
PATH=$PATH: newpathname to add
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33. Linux+ Guide to Linux Certification, 2e
Summary
bash
echo $?
&& ||
set env export
$ PATH $HOME $SHELL
/etc/profile
/etc/bashrc
~/.bash_profile
~/.bashrc
Linux+ Guide to Linux Certification, 2e