Workbook 6 – Part 2 The Bash Shell RH030 Linux Computing Essentials

Workbook 6 – Part 2The Bash Shell

RH030 Linux Computing Essentials

Linux+ Guide to Linux Certification, 2e

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Workbook 6 Objectives This workbook is all about the shell.

Last week we discussed What is a Shell

What a shell is and what it does How subshells is and how it works Overviewed shell features

Shell variables Should understand difference of Local verse Environmental variables

Creating new shell variables Exporting variables

Identifying common shell environment variables $LOGNAME, $SHELL,$HOME, $PATH

Manipulating common shell environment variables ~/.bashrc


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Workbook 6 Objectives

This week Shell feature = Command Line expansion

Startup & Login environment files. global & local environment files. Sequence of the startup environmental files Use of each startup environmental file Manipulating common shell environment variables permanently Creating permanent aliases/variables


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Key Concepts The bash shell expands certain command line metacharacters before

interpreting the command. Tilde expansion expands tokens that begin with a tilde (~) to users home directories. Brace expansion expands tokens with braces ({}) into multiple words, each of which

contains a single word from the specified list. Command substitution expands text enclosed within backticks (``) or "dollar

parenthesis" ($()) into the output produced by the enclosed command. Double quotes ("..." ), single quotes ('...'), and the backslash character

can be used to protect characters from being expanded by the shell.

Chapter 4 – Command line expansion


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Using Command Line Expansion

Expansion Area Syntax Expands To

History ! Refer to a previous command line

Brace {} Create this specified text

Tilde ~ Refer’s to a user's home directory

Variable $, ${...} Shell and environment variables

Arithmetic $((...)) Numeric calculation

Command Substitution ` ...`, Allows command to run in subshell

Pathname *, ?, [^...] Matchs to filenames in filesystem


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Brace Quick way to create items Create this specified text

touch file{01,02,03,04}

[student@station student]$ ls

file01 file02 file03 file04

Creating a hierarchy structure

[student@station student]$ mkdir chap{01,02,03,04}/{html,text}


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It can get quite complicated $touch soap/episode{1,2,3,4}.{draft,final}.{txt,pdf}

Will produce the following directory listing.

[student@station student]$ ls soap/episode1

episode2.draft.pdf episode3.draft.pdf episode4.draft.pdfepisode1.draft.txt episode2.draft.txt episode3.draft.txt episode4.draft.txt episode1.final.pdf episode2.final.pdf episode3.final.pdf episode4.final.pdf episode1.final.txt episode2.final.txt episode3.final.txt episode4.final.txt


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Pathname Expansion

Character Matching* 0 or more characters

? exactly one characters

[...] exactly one of the included characters

[^...] exactly one of the excluded characters


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Different Quotes do different things


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Using quota’s Sometime the entire line needs to be enclosed in quota’s1. Double quotes tell the shell to: ( “ )

Display the value of this variable used.

LOGIN=John

echo “Welcome $LOGIN”

Welcome John

2. Single quotes tell the shell to: ( ‘ ) Display the name of this variable used

LOGIN=John

echo ‘Welcome $LOGIN’

Welcome $LOGIN

3. Back quotes ( ` ) tell the shell to run text this as a commandecho “ Welcome $LOGIN today is `date` ”


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Command Substitution

The "old school" syntax for command substitution is to encapsulate the

command in `back ticks` The more modern syntax supported by the bash shell is similar to arithmetic

expansion, but with only one set of parentheses: $(subcommand)

[prince@station prince]$ mkdir reports.$(date +%d%b%Y)


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Quoting

Put example of quotes from 1st ppt here


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Using quota’s Here with no quotes, bash interprets the > and < characters as requests

to redirect the command's output (and input).

$ echo <pre> little red $CAR </pre>

$ -bash: syntax error near unexpected token `newline'

Here the double quotes protected the < and > characters. The dollar sign, however, is interpreted as a marker for a variable.

$ echo “<pre> little red $CAR <pre>“

$ <pre>little red corvette </pre>

Here the single quotes stop the variable expansion

$ echo '<pre>little red $CAR</pre>' $ <pre>little red $CAR</pre>


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Escape Characters Shell expansions occur before the command is run.

Occasionally, some commands use arguments that contain characters in them that are special to the bash shell.

Meaning that for bash they have some other meaning. Bash needs to be told NOT to read them as special characters.

but to use them in the current syntax.

\<special character>

A common example is the find command.

find –name “*.chap” –exec rm {} \;


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Use of escape character

echo \<pre\>little red $CAR\</pre\>

<pre>little red corvette</pre>

echo \<pre\>little red \$CAR\</pre\>

<pre>little red $CAR</pre>


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Chapter 5 – Shell Customizations Key Concepts The bash shell internally implements certain simple commands

(internal Commands) Which affect the shell's behavior. These are referred to as builtin commands. Shell aliases are established and examined with the alias command. Shell aliases are removed with the unalias command. The bash shell prompt can be customized using the PS1 variable. Shell flags can can be set with the set -f command, and cleared with set +f. Shell options are examined, set, and unset using the shopt command.


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Using alias’s alias command:

Used to list all the available aliases.

Using aliases Short alias runs a long command-line Can create a single alias for multiple commands But are only temporarily available unless you make them permanent. Locally this is done using the .bashrc environment file.

Creating temporary alias’s alias c=clear alias c=“ clear;date;ls –l ” alias ts="touch $(date +timestamp.%H:%M:%S)" alias tsc="find . -newer"


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~/.bashrc = local permanent aliases

Making aliases permanent echo alias c=clear >> .bashrc . ./.bashrc


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Remember - Last week we looked at variablesThere are 2 types of variables:

1. User-defined variables: Custom variables Created and defined by the local user

2. Environment variables: Contain system information or properties that the system and programs both

use in the running of the overall of the system and access regularly. Such as your unique login id.


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Examples of Some Frequently Used Environmental Variables Name Description

LOGNAME 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.


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Working with Variables & their values Displaying all currently available variables & their current values

set

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. echo $HOME

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”


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Modifying existing variables Customizing Your Prompt

echo $PS1“[\u@\h \W]$” Default setting

PS1=“Hello World”echo $PS1Hello World

/home/sheila $pwd/home/sheila $/home/sheila $ PS1= “This month is `date +%h ` @”This month is May @

The back quotes (`) are used to run it as a command instead of interpreting it literally as a string of text.


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set verse shopt These two builtin commands are used to configure the shell's

behavior using shell options. In other words they turn on functions.

One is the set command, which can also be used to modify the shell's behavior using (usually) single letter flags

set –f set –o noclobber (c shell function)

The other is shopt, which is used to configure shell options. shopt –s cdspell shopt –u cdspell


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Chapter 6 – Sourcing Shell Scripts and Shell Initialization Key Concepts Shell scripts are sourced with the source or . commands. Shell scripts are all executed in the same shell that sources the script. Bash shells can be either login or non-login shells. Bash shells can be either interactive or non-interactive shells. /etc/profile, files within the /etc/profile.d directory, and ~/.bash_profile are sourced on

startup of every login shell. /etc/bashrc and ~/.bashrc are sourced on startup of every shell. ~/.bash_logout is sourced by every exiting login shell.


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source = run this script [blondie@station blondie]$ cat prompts.script

PS1="whadda you want? "

#PS1="\a\u@\H \$(date --iso-8601) \t [\!] \$ "

#PS1="[\u@\h \W]\$ "

In order to try out the first prompt, she sources the file prompts.script.

[blondie@station blondie]$ source prompts.script

whadda you want?

Can also use . ./prompts.script Can also use bash ./prompts.script


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Environment Files Environment files are used as part of the logon process

During login all users are assigned many default values. Such as a default shell, path, home directory etc

All the different shells each use their own environment files. When their shell is started it reads it’s own environment files.

There are 2 types of environment files.

1. Global – used when you login and given to everyone

2. Local – Also read when you login PLUS when you start a new subshell

– used only to the local specific user.


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Login Process Environment files are just :

Script configuration files

Some are run everytime you login.

There are 2 types: Global environment files

/etc/profile /etc/bashrc

local Bash environment Files

~/.bash_profile ~/.bashrc ~/.bash_logout


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Last week we discussed exporting variables Exporting variables = Making the entire system aware of them.

It declares the existence of defined variables within the system. A variable needs to be exported or declared to the entire system so that

they are available for use. Making them available to all subshells.

export command = 2 uses Used on it’s own It will lists all the currently exported / declared available variables

export teacher Used with an existing variable it will export/declare that variable. This then ensures that all sub-shells have access to this variable


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Items - you would export in Global Environment

Exports the login id you used to connect to the system with. $LOGNAME for login name etc

Exports PATH for Default Command Path $PATH sets default path of directories where the shell will look when a

command is executed Exports MAIL for Default Mailbox location

$MAIL sets default path of directories containing each specific users mailbox location.

Sets TERM Variable Default Terminal Type screen and keyboard

Displays Contents of /etc/motd File Holds any motd is ‘message of the day’

Sets Default File Creation Permissions sets umask values which determines the default permissions

Checks for Mail Checks mail and will print a mail message upon login


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Order of the environment files /etc/profile is read first

It contains the global variables. It exports the global variables.

~/.bash_profile is read next It contains the local variables. It exports the local variables.

~/.bashrc is read next It contains the local aliases. Code within it causes it to read

the /etc/bashrc if it exists.

/etc/bashrc is read last It contains the global aliases.


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Note During login different shells use different Local Environment Files


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Summary echo $? && || set env export $ PATH $HOME $SHELL $PS1 alias set shopt source . ./script1 bash script1 /etc/profile /etc/bashrc ~/.bash_profile ~/.bashrc ~/.bash_logout

Documents

Workbook 6 – Part 2 The Bash Shell RH030 Linux Computing Essentials