Bash Scripting-6- Using Logical operators in shell scripts

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Bash Scripting-6- Using Logical operators in
shell scripts
Posted by Prince DAN January 13, 2020 in DevOps
We have already discussed about the conditional
statements i.e. if-else, if-then-else elif
statements have also discussed some basic shell
scripts examples for those as well. In this
tutorial, we will discuss some advanced uses of
conditional statements by using logical
operators in shell scripts. Recommended Read
Bash Scripting-7- Using FOR loop in shell script
Also Read Recommended guide to install
POSTGRESQL from Source Logical operators or
boolean operators are used for performing logical
operations. Following 3 are types of logical
operators, 1- Logical AND ()- will compare
two inputs if both are true, it will return
true or else false.
2

1. Logical OR ()- Will check two conditions will
   return true, if any of them is true return
   false when both are false.
2. Not equal to (!)- will return true when the
   condition is false return false if the
   condition is true.

Using logical operators in shell scripts
Lets see an example to get a better
understanding of using logical operators in
shell scripts, !/bin/bash dir/home/ec2-user if
-d dir -w dir then echo dir
directory exists its writable fi So in this
script, we are checking if a directory exists
if its readable. So when both of these
conditions are satisfied, it will print the
message that the directory exists is readable.
Here we can also repace AND logical operator
with OR logical operator, the script will check
if either directory is readable or is writable
or both and will then print the message but will
print nothing, if both conditions are false.
Relational Operators
Relational operators are used to check relation
between two operands depending upon the
conditions, it will return a negative or positive
outcome. Following is the list of relational
operators, Relational Operators
lt
less than or equal to
gt
greater than or equal to
3
lt
less than
gt
greater than

Equal to
! not equal to These relational operators are
used inside double parenthesis i.e. (( )).
Double Parenthesis (( ))
These are used to performing/handling advanced
arithmetic operations following paramters can
be used with this,
val
post-increment
val
post decrement
val
pre-increment
val
pre decrement
!
logical NOT

Logical AND

Logical OR

exponential

bitwise NOT operator
4

bitwise AND operator

bitwise OR operator
ltlt
left bitwise shift
gtgt right bitwise shift Now lets discuss an
example of these concepts. !/bin/bash
num115 if (( num1 2 gt 100 )) then ((
num2number 2 )) echo Square of num1 is
num2 fi This simple script checks if the
square of a number is greater than 100 if it is
then it will calculate the square of the number
will print with a message.
Double Brackets
Double parenthesis is an advanced version
provides us with advanced features for
string comparisons. !/bin/bash If USER
ec2 then echo Welcome USER else
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echo Who are you USER fi With this script,
we are checking if there is any user that starts
with ec2 if there is any user starting with
ec2, it will print a welcome message or else will
ask for who the user is. With this, we end our
tutorial on using logical operators in shell
scripts also performing advanced operations
for arithmetic string comparisons. Please feel
free to send in any questions, queries
suggestions using the comment box below.
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