When to wrap quotes around a shell variable in Linux?

Most of the programmers who are new to shell scripting and dealing with string variables, have a doubt regarding when they have to wrap
quotes around a shell variable. In this article, we are going to perform some tests and try to understand when and where quotes around a
shell variable are required.

Table of Contents

List of scenarios when “quotes” are required

Sometimes when we are forgot to wrap a quotes around a shell variable it gives error or undesired output. most crucial condition is when
we are working with string having a space in it like “Good morning!”.

When we working with a string variable it’s a good habit to wrap it around quotes.

Scenario 1: Using length of string in an expression

Here we will perform a practical that find length of sting usind expr.

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#declare a sting with space
strValue="Hello world!"

# expr command with two argument first is LENGTH 
# and second is string variable wrapped in quotes.
length=`expr length "$strValue"`

#print the length of string
echo "Length of the string is : $length "

Output :

Length of the string is : 12

Now we qill try this above program without wrapping quotes around “$string” variable.

#declare a sting with space
string="Hello world!"

# expr command with two argument first is LENGTH 
# and second is string variable wraped in quotes.
length=`expr length $string`

#print the length of string
echo "Length of the string is : $length "

Output :

expr: syntax error: unexpected argument ‘world!’
Length of the string is :  

When we provided the second argument to the expr without quotes, then it is considered as multiple arguments. Like,

length=`expr length Hello world!`

And when we provide second argument to the expr with quotes, the it considers the argument as one whole argument like,

length=`expr length "Hello world!"`

Scenario 2: Comparing string in bash script

Here, we will perform a practical example to compare two string, which have a space.

#read the first string from user
echo "Enter first string:"
read string1

#read the second string from user
echo "Enter second string:"
read string2

#compare two string
if [ "$string1" = "$string2" ]
then
    echo "Strings are equal."   
else
    echo "Strings are not equal."
fi

Output :

Enter first string:
red hat
Enter second string:
red hat
Strings are equal.

If we compare two string varible without quote then it give undesired output. like,

#read the first string from user
echo "Enter first string:"
read string1

#read the second string from user
echo "Enter second string:"
read string2

#compare two string
if [ $string1 = $string2 ]
then
    echo "Strings are equal."   
else
    echo "Strings are not equal."
fi

Output :

Enter first string:
red hat
Enter second string:
red hat
file1.sh: line 51: [: too many arguments
Strings are not equal.

Here we got same type of error or warning as exapmple:1.

Scenario 3: Printing string array

In this practical, we will declare a string array and print it using for loop.

# declare the array
arr=("tata nexon" "Toyota Fortuner" "swift" "Toyota Fortuner" "celerio")

# print the array using for loop
for list in "${arr[@]}"; do
    echo $list
done

Output :

tata nexon
Toyota Fortuner
swift
Toyota Fortuner
celerio

Now we perform same above program with quote variable

# declare the array
arr=("tata nexon" "Toyota Fortuner" "swift" "Toyota Fortuner" "celerio")

# print the array using for loop
for list in ${arr[@]}; do
    echo $list
done

Output :

tata
nexon
Toyota
Fortuner
swift
Toyota
Fortuner
celerio

You can see the difference between these two programs. It doesn’t give any error but gives the undesired output. As it takes “Toyota Fortuner” as a two seperate arguments.

Summary

In this article, we learned that when we are working with string with spaces, then we always wrap quotes around a shell variable because without quotes it takes the argument as multiple arguments and gives error or sometimes gives undesired output. Thanks.

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Thanks for reading.

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