There is one thing which we often get confuse in shell script, especially for those who are new to shell scripting
The difference between $var, ${var} and $(var)
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echo $var
echo ${var}
echo $(var)
I try to keep the concept as simple as that so that it should be helpful for new bees
Even before start, one thing we need to understand is in shell script to echo does not necessarily required quotes to print the message i.e. either double quote “” or single quote ‘’
Example:
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my_shell $ echo "Hello world"
Hello world
my_shell $ echo 'Hello world'
Hello world
my_shell $ echo Hello world
Hello world
In above example the output remain same for all the types of echo. Please try yourself w.r.t adding double quote in single quote and vice versa
$var - When we echo $var the value saved in var is printed
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my_shell $ var="World"
my_shell $ echo Hello $var
Hello World
${var} - There is no difference between $var and ${var} except the way it is used. i.e. if there is a value which need to be print just after variable var then variable var should be enclosed with curly brace i.e. ${var}
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my_shell $ var="Linkedin "
my_shell $ echo "$varis very famous"
very famous
In the above example $varis in echo is considered as one variable. Now in order to tackle this we need to use ${var}
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my_shell $ var="Linkedin "
my_shell $ echo "$varis very famous"
very famous
my_shell $ echo "${var}is very famous"
Linkedin is very famous
In the above example you can see now $var is differentiated between the word is just by enclosing in a brace
$(var): This is called command substitution and there is no way it is linked or related to $var or ${var}
This is used when we want to execute shell command and save the output to variable
Example:
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my_shell $ var=$(echo "Hello world")
my_shell $ echo $var
Hello world
my_shell $ echo $(pwd)
/home/bhavith/Bhavith
In above example observer how var saved the output of echo and pwd output
Another example:
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my_shell $ echo "List of files in $(pwd) is
>
> ---------------------
> $(ls)
> ---------------------"
List of files in /home/bhavith/Bhavith is
---------------------
c++
Development
Documents
doWhile
Junk
opensource
Youtube
---------------------
In the above example observe how pwd and ls is executed and echoed to console