Book Image

Mastering Bash

By : Giorgio Zarrelli
Book Image

Mastering Bash

By: Giorgio Zarrelli

Overview of this book

System administration is an everyday effort that involves a lot of tedious tasks, and devious pits. Knowing your environment is the key to unleashing the most powerful solution that will make your life easy as an administrator, and show you the path to new heights. Bash is your Swiss army knife to set up your working or home environment as you want, when you want. This book will enable you to customize your system step by step, making your own real, virtual, home out of it. The journey will take you swiftly through the basis of the shell programming in Bash to more interesting and challenging tasks. You will be introduced to one of the most famous open source monitoring systems—Nagios, and write complex programs with it in any languages. You’ll see how to perform checks on your sites and applications. Moving on, you’ll discover how to write your own daemons so you can create your services and take advantage of inter-process communication to let your scripts talk to each other. So, despite these being everyday tasks, you’ll have a lot of fun on the way. By the end of the book, you will have gained advanced knowledge of Bash that will help you automate routine tasks and manage your systems.

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Table of Contents (15 chapters)
Preface
Preface
What this book covers
What you need for this book
Who this book is for
Conventions
Reader feedback
Customer support
Free Chapter
1
Let's Start Programming
Let's Start Programming
I/O redirection
Messing around with stdin, stdout, and stderr
Time for the interpreter: the sha-bang
Variables
Summary
2
Operators
Operators
Arithmetic operators
Assignment operators
Bitwise operators
Logical operators
Comma operator (,)
Operators evaluation order and precedence in decreasing relevance
Exit codes
Summary
3
Testing
Testing
What if...else
Test command recap
Summary
4
Quoting and Escaping
Quoting and Escaping
Special characters
Quoting and escaping
Summary
5
Menus, Arrays, and Functions
Menus, Arrays, and Functions
The case statement
Arrays
Functions
Summary
6
Iterations
Iterations
The for loop
Let's do something while, until…
Exiting the loop with break and continue
Time to give our client a menu
CLI, passing the arguments to the command line
Summary
7
Plug into the Real World
Plug into the Real World
What is Nagios?
Active and passive checks
Returning code and thresholds
Command and service definitions
Our first Nagios plugin
Summary
8
We Want to Chat
We Want to Chat
The Slack messaging service
Slack WebHooks
What is a JSON?
Do you like cURLing?
Formatting our messages
Message attachments
Our wee chatty script for Slack
Summary
9
Subshells, Signals, and Job Controls
Subshells, Signals, and Job Controls
What is a subshell?
Background processes
Signals
Job controls
Subshells and parallel processing
Summary
10
Lets Make a Process Chat
Lets Make a Process Chat
Pipes
Redirection to a file
The command substitution
The process substitution
Environment variables
Coprocesses
/dev/tcp and /dev/udp
Netcat
Summary
11
Living as a Daemon
Living as a Daemon
What is a daemon?
DEMO
disown
Double fork and setsid
Becoming a daemon
Trapping a daemon
Going dark with the daemon
Summary
12
Remote Connections over SSH
Remote Connections over SSH
What is SSH?
Configuration files
The sshd_config file
ssh_config
Passwordless connections
Proxies and tunnels
Summary
13
Its Time for a Timer
Its Time for a Timer
One shot at it
The cron scheduler
Summary
14
Time for Safety
Time for Safety
The restricted shell
Restricted sftp sessions with OpenSSH
Summary

Environment variables

Let's call it a proof of concept more than a real means to have processes communicate with each other. Who would really want to mess with the environment? Anyway, we are exploring some viable means to IPC, so we can take this in account even though we will not use it in the first instance. Let's have a look at env:

env
LS_COLORS=REDACTED
XDG_MENU_PREFIX=xfce-
LANG=en_GB.utf8
DISPLAY=:0.0
XDG_VTNR=7
SSH_AUTH_SOCK=/tmp/ssh-MgHTC62oCYDp/agent.1121
GLADE_CATALOG_PATH=:
XDG_SESSION_ID=2
XDG_GREETER_DATA_DIR=/var/lib/lightdm/data/zarrelli
USER=zarrelli
GLADE_MODULE_PATH=:
DESKTOP_SESSION=xfce
PWD=/home/zarrelli
HOME=/home/zarrelli
GUAKE_TAB_UUID=b07321dd-a221-41bd-8ecc-0ae94b9082b9
SSH_AGENT_PID=1159
QT_ACCESSIBILITY=1
XDG_SESSION_TYPE=x11
XDG_DATA_DIRS=/usr/share/xfce4:/usr/local/share/:/usr/share/:/usr/share
XDG_SESSION_DESKTOP=xfce
GLADE_PIXMAP_PATH=:
GTK_MODULES=gail:atk...
Previous Section
End of Section 6
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