Quoting

Double Quotes

Let's say we want we assign a string with spaces to a variable. If we try:

var=This is a test

We get an error:

$ var1=This is a test.

-bash: is: command not found

The Shell assigns "This" to the variable and thinks the next word is a command.

To accomplish this we can surround the text with quotes.

ajay.mittal@AMITTAL-NB03 ~/temp

$ var1="This is a test."

ajay.mittal@AMITTAL-NB03 ~/temp

$ echo $var1
This is a test.

Double quotes will not ignore dollar signs , back quotes  or backslashes.

ajay.mittal@AMITTAL-NB03 ~/temp

$ var2="This is var1:$var1"

ajay.mittal@AMITTAL-NB03 ~/temp

$ echo $var2

This is var1:This is a test.

Using the back quote:

Back Quotes

ajay.mittal@AMITTAL-NB03 ~/temp

$ var1=" `date` is todays date"

ajay.mittal@AMITTAL-NB03 ~/temp

$ echo $var1

Sun, Oct 28, 2018 6:15:10 PM is todays date

The backquote indicates execution of a command and is interpreted by
the shell.

Single Quotes

Single quotes are more restrictive. They treat the string as it is
without any interpretation.

[amittal@hills sed]$ var1="Some value"

[amittal@hills sed]$ echo '$var1'

$var1

The value of the variable "var1" is not printed out.

Backslash

We can use the backslash to assign and remove special meaning to characters .

[amittal@hills sed]$ echo "a a" | grep "\s"

a a

[amittal@hills sed]$ echo "aa" | grep "\s"

The above states that "grep" should look for the "space" character .

[amittal@hills sed]$ echo "aa^" | grep "\^"

aa^

The above looks for the carat character . With the backslash we are
actually looking at the character caret instead of looking for the
pattern at the beginning of the string.

Command Substitution

[amittal@hills ~]$ var1=`date`

[amittal@hills ~]$ echo $var1

Tue Oct 30 17:42:18 PDT 2018

Using the back quote character we can execute the command inside and
the use the output. In the above the "date" command is executed and
the output is placed in the variable "var1" . We can confirm that by
echoing the contents of the variable "var1" .