Variables

Question 1

Setting a variable

Answer 1

>myvar=’This is my variable!’

• Note no spaces before or after the = sign
• Note use of ‘ instead of “
• Note that this makes the variable exclusively available to shell script

Question 2

Calling a variable

Answer 2

>echo $myvar

• Note the use of $

>echo foo${myvar}crap

fooThis is my variable!crap #

• Note the use of { } to specify the variable when spaces are not available

Question 3

Making an environmental variable available for other scripts

Answer 3

• You need to export the variable after creating it so that it is available to all scripts. Remember that defining it only makes it available to shell scripts.

> export myvar

• Or you can define and export the variable in one line

>export myvar=’this is my variable!’

• Note that the export command does not work from inside a script

Question 4

Deleting/ erasing a variable

Answer 4

>unset myvar

Question 5

Manipulating a string variable

Answer 5

• To extract and display parts of a variable you need to use the ${ } notation to specify variable and manipulation details.

>myvar=’this is my variable’

>echo ${myvar:0:4}

this

• Variable expansion is the replacement of the variable by its value

Question 6

In a script what is referred to when stating ${1} or its sidekick ${1%.*}

Answer 6

• This refers to the environmental varaible that is fed to the script as its first argument in the command line

>./myscript.sh balls bucket

• here if the second argument needs to be referred to in the script it will appear as ${2} Or if manipulated ${2:0:4}

Question 7

Referring to command line arguments inside a script

Answer 7

!/usr/bin/env bash

echo name of script is $0
echo first argument is $1
echo second argument is $2
echo seventeenth argument is ${17}
echo number of arguments is $#
echo list of all my arguments is $@

• $0 by default yields the name of the script
• from the 10th argument onwards the argument number needs to be in brackets, hence ${17}
• the number of arguments fed to the script can be called by $#
• List of all arguments passed to script is called by $@

Question 8

What’s a loop control variable

&

How is it set

Answer 8

Answer is two in one.

The loop control variable is set inside the loop itself and its value is used to control the loop operation.

#!/usr/bin/env bash
 for **x** in one two three four

do
echo number $x
done

output:
number one
number two
number three
number four