Module Linux essentials - Module 9: Basic scripting

Module Linux essentials - Module 9 introduce basic scripting. This chapter will guide how to turn commands into a script. After studying this chapter you should be able to: Basic text editing, basic shell scripting. Inviting you to refer.

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This slide deck is for LPI Academy instructors to use for lectures for LPI Academy courses

©Copyright Network Development Group 2013

Module 9 Basic Scripting

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Exam Objective 3.3 Turning Commands into a

Script

Objective Summary

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Text Editors

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A script is a sequence of

commands

• If you can type it on the command line, it can be scripted

• You can branch based on tests and loop over collections

• This is all available from the command

line, too

• Scripts are repeatable and consistent and free you to do more interesting work

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Editors

• Scripts are text files

• LibreOffice will add formatting, making it unusable

• nano and vi / vim are popular editors

• Of the two, nano is far easier to use

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• Type as normal, use arrow keys to move around

• Most commands are Control + another

character

• Context sensitive commands are at the

bottom of the screen, e.g ^X to exit, ^W to find

• ^G gets you help anywhere

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shebang

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• #! is the shebang (hash + bang)

• Tells the kernel what shell interpreter to use to run the script

• Must be on the first line

• #!/bin/sh

• #!/usr/bin/ruby

• Lets you run the script with /script no matter which shell the user has

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Working with Variables

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• Temporary storage of data in memory

• Assigning (no $):

• Using (need a $):

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Special Variables

• $1 $9 are the arguments to the script

• /test.sh hello there # $1=“hello” $2=“there”

• $? is the exit code of the last command to

be run

• use “exit 1” to exit your own script with

error code 1

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Working with Conditionals

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Conditionals

if something; then

do this

fi

if something; then

do this

elif something else;

then

do that

else

try this

fi

# something returns

an exit code If it is 0, then “do this” will be executed up until fi

# alternative syntax, allows for multiple tests and a default option if you want

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tests

test –f /tmp/foo # test if file exists

test ! –f /tmp/foo # test if file doesn’t exist test $A –eq 1 # is $A = 1 (numeric)

test “$B” = “Hello” # string comparison

test $A –lt 10 # $A < 10?

alternatively, [ is the same as test:

if test –f /tmp/foo; then # it works

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Case

case ”$GREETING" in

hello|hi)

echo "hello yourself"

;;

goodbye)

echo "nice to have met you"

echo "I hope to see you again"

;;

*)

echo "I didn't understand that"

esac

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Loops

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The for loop

• Operates over a fixed set of items or a glob

NAMES=“Alice Bob Charlie”

for N in $NAMES; do

echo “Hello $N”

done

for FILE in *; do

ls –l $FILE

done

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While loops

• Tests the loop condition each time

while [ ! –f /tmp/foo ]; do

try_to_make_foo # makes /tmp/foo on success

sleep 1 # wait 1 sec

done

• Used when the number of loops is

unknown or changes each loop

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