Linux BASH Shell Scripting [EN].pdf

Linux BASH Shell Scripting

Basics of Creating and Utilizing BASH Scripts

Pete Nesbitt

May 2006

The information presented here should act as a guide to creating quality scripts using the Linux builtin

BASH scripting language. BASH (Bourne Again Shell) is a scripting language as well as the default

command interpretor in most Linux distributions, including Red Hat Linux. The ability to create

quality scripts is arguably the most time and error saving aspect of Linux Systems Administration. The

fact that the default shell or user environment in Linux is BASH makes makes learning to create

scripts both very valuable as well as simple. In the following document we will look at fundamentals of

scripts, best practices creating and testing of scripts. Shell scripts can be very simple or extremely

complex involving dozens of support files, but regardless of the complexity, they should be created in a

consistent, selfexplanatory and easy to read format.

We won't cover regular expressions, or shell utilities like loops since that information is readily

available on the Internet and in books. If after looking at an example, you do not understand an aspect

like “ while read x in `ls` ”, be sure to research how these functions work, or ask a more experienced

staff member for help. Mimicking a process without understanding is of little or no value, and

misunderstanding the process can lead to dangerous scripts and frustration.

In order to achieve the best value from this document, you should be familiar with common commands,

basic regular expressions and GNU utilities. Anything used within the examples can be researched

using the man command or by searching Google. For example, you will want to be familiar with grep,

sed, awk, and cut .

IMPORTANT NOTE: If you try and cut and paste the examples in this document, you will probably

need to manually replace the special characters since the word processor modifies them for

presentation. Ones copied from the command line will be okay, but ones typed directly into this

document will need to be corrected. These include, but are not limited to ' ” ` / \ |

The Bash Shell mantra:

Every formal script should begin with command line interpreter definition.

In BASH, it is a virtual poem:

Hash Bang Slash

Bin Slash Bash

Literally:

#!/bin/bash

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Linux BASH Shell Scripting

Basics of Creating and Utilizing BASH Scripts

Pete Nesbitt

May 2006

In addition to what I call a formal script , you can achieve script like functionality by entering a series

of command at the prompt.

What can I do with a script?

Any task you can run at the command line can be incorporated into a script. This is because working at

the command line and writing a script are essentially the same thing. Unix and Linux utilities and

commands tend to be very simple and singular in task. However, stringing a series of such utilities

together can create very flexible, powerful and extensible command line or script. There is no

environment that can be manipulated to perform with such flexibility and adaptability than the

command line and it's related script files.

Any task that is complicated, repetitive or needs to be performed numerous times should be scripted.

This saves time, eliminates the chances of typos, and often can be reused later.

Creating a script:

1. start by laying out the flow with only comments of what each step will perform, don't worry

about the actual code until you define the steps you want to perform.

2. use the command line to test pieces of your script

3. take small bits of code that you have tested at the command line, then move them into the script

4. always test in a safe area, not working on important data or files until you have confirmed the

scripts works as expected.

Learning to write quality BASH Scripts:

1. Be consistent. Develop a layout for your scripts and stick to that style.

2. Take your time. Scripting is not overly complicated, but does rushing things will just result in

errors and frustration.

3. Practice makes perfect. Try and script as many things as possible, especially complicated tasks.

This will help develop your skills working with sets of commands as opposed to doing one

thing, saving to a file, working on that file, then saving those results and so on. Even if it is only

to be used once, unless time is of the essence, go script crazy. You will be a better, more

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Linux BASH Shell Scripting

Basics of Creating and Utilizing BASH Scripts

Pete Nesbitt

May 2006

valuable Systems Administrator in the long run.

4. Review the work of others. In scripting, like almost everything else Unix or Linux related, there

are many ways to do any given task. See how others do things, then decide what makes sense

for you.

Best Practices:

Always write your scripts in a way that others can easily follow the flow and functionality.

● give your scripts a meaningful name and include .sh as a suffix

● always define the command interpreter on the first line

● include your name and the date

● include the file name and a brief description of what the script will do

● define functions, variables and include files before the main code

● include comments (lines staritn with a # ) and blank lines for readability

● indent stanzas or code blocks for readability

● fully qualify your commands

 this adds security as well as making it cron safe

 although this may break portability, you can use another shell script to interigate the system

and ensure portability when defining commands as variables.

● add an 'End Of File' comment

What to do if your script doesn't work:

1. add some output to see how things progress

 add an echo line at each main step, that will tell you what does work

■ example:

after a line like “ let x=$x+1 ”

add “ echo “the value of x is now: $x” ” to see if the value is changing as expected

2. if you get an error stating “permission denied” the script is not executable

3. try snipits on the command line or in a smaller script to find and fix a failing line

4. use Google or other resources to confirm your code, such as a loop format

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Linux BASH Shell Scripting

Basics of Creating and Utilizing BASH Scripts

Pete Nesbitt

May 2006

Sample Script Template: (this is not a physical template, but a general format you may want to adopt)

#!/bin/bash

# file: myscript.sh

# created by Pete Nesbitt May 2006

# this script will perform some fancy task.

# set some variables to be used throughout the script

TARGET=”remote.host.com”

DATE_STAMP=”`date +%y%m%d`”

EMAIL_RECIPIENTS=”admin@yourdomain.com”

# here we will manipulate something

some code goes here

# now we need to do this other thing

some other code goes here

# eof

Functions:

A function is simple a code block that can be called several times without the need to rewrite the whole

thing every time.

In BASH a function is defined as:

my_function()

{

the code block

}

Then can be used anywhere in the script like so:

do some stuff...

my_function

some more code

See the Samples section at the end for real world examples.

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Linux BASH Shell Scripting

Basics of Creating and Utilizing BASH Scripts

Pete Nesbitt

May 2006

Building a Functions Library:

Whether you use the include statement or just have an inventory of code snipits, you can be consistent

and avoid recreating the wheel by accumulating a library of shell functions, tasks and variables.

A Simple Script to Try:

First lets make a quick script to create some files that we can later manipulate, then we will rename

them. Since these are simple examples, lets cheat a bit here, using the command line instead of a formal

script.

Create the original files:

count=0;while [ $count lt 10 ]; do touch file_$count;let count=${count}+1; done

We now have 10 files, named file_0 thru file_9

Lets rename them. We could do each separately using:

“mv file_0 new_name_0;mv file_1 new_name_1; ... mv file_9 new_name_9”

but that seems like a lot of work and a lot of opportunity for error.

A simpler and more efficient method would be to create a loop that would rename each file

automatically. This is very similar to the way we created the files. This time though, we will do each

step separately. Note the prompt changes when BASH expects more information before it can complete

the command string. This is called the second level prompt (the primary prompt is a $ for most users

and a # for the root user).

$ for x in `ls`

> do

> n="`echo $x|cut d_ f2`"

> mv $x new_file_$n

> done

We now have 10 files, named new_file_0 thru new_file_9

If you now do an uparrow, the previous command is displayed as:

for x in `ls`; do n="`echo $x|cut d_ f2`"; mv $x new_file_$n; done

Whenever doing mass changes, it is prudent to create a test directory to use when building the script or

command line, so you do not damage your real source files in the case of an error.

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