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

Workbook 6 – Part 1The Bash Shell

RH030 Linux Computing Essentials

Linux+ Guide to Linux Certification, 2e

2

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


3

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 "!".

Chapter 1 – Introduction to Bash


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.


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 added features such as aliasing and history Now most widely used shell

C Shell /bin/csh based on the C programming language


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>


7

It uses the standard I/O devices on the system to handle the commands input/output.

The three common file descriptors


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.


9

Redirection can get very complicated.


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


11

Shells allow us to use command piping

Figure 8-2: Piping information from one command to another


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


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


14

~/.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


15

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@station elvis]$ cd /etc/X11; lsapplnk prefdm sysconfig xorg.conf.backup xkbdesktop-menus proxymngr twm xorg.conf.wbx Xmodmapfs rstart X xorg.conf.works Xresourcesgdm serverconfig xdm XftConfig.README-OBSOLETE xserverlbxproxy starthere xorg.conf xinit xsm

[elvis@station X11]$

Chapter 2 – command lists & scripts


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@station elvis]$ (cd /etc/X11; ls)applnk prefdm sysconfig xorg.conf.backup xkbdesktop-menus proxymngr twm xorg.conf.wbx Xmodmapfs rstart X xorg.conf.works Xresourcesgdm serverconfig xdm XftConfig.README-OBSOLETE xserverlbxproxy starthere xorg.conf xinit xsm[elvis@station elvis]$


17

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@station elvis]$ ls -l /etc/passwd -rw-r--r-- 1 root root 3694 Aug 15 16:26 /etc/passwd [elvis@station elvis]$ echo $? 0

In contrast, the following example shows how the ls command failed

[elvis@station elvis]$ ls -l /etc/password ls: /etc/password: No such file or directory [elvis@station elvis]$ echo $? 1 echo


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@station elvis]$ mkdir /tmp/boring && mv numbers.txt /tmp/boring

[elvis@station elvis]$ ls


19

Wth || bash will execute the second command only if the first command "fails" (i.e., had a return value of 1).

[elvis@station elvis]$ mkdir /tmp/boring || mv numbers.txt /tmp/boring

[elvis@station elvis]$ ls

In the above example, if the mkdir command succeeded, Will the file be moved?

Difference of && or ||&


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.


21

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


22

The shell manages the variablesThere are 2 types of variables:

1. 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.

2. 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.


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


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


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


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.

PS1 Sets how your prompt will be displayed.


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


28

Examples - Environment Variables

Table 8-3: Common BASH environment variables


29

Examples - Environment Variables

Table 8-3 (continued): Common BASH environment variables


30

Local VariablesUser-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.


31

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.


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


33

Summary bash echo $? && || set env export $ PATH $HOME $SHELL /etc/profile /etc/bashrc ~/.bash_profile ~/.bashrc

Documents

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