Conditionals Execution (Decision Making)

Bash structured language constructs

You can use the if command to test a condition. For example, shell script may need to execute tar command only if a certain condition exists (such as backup only on Friday night).

If today is Friday
   execute tar command
otherwise
   print an error message on screen.

.

More About Logic

  • So far, the script you've used followed sequential flow:
#!/bin/bash
echo "Today is $(date)"
echo "Current directory : $PWD"
echo "What Users Are Doing:"
w
  • Each command and/or statement is executed once, in order in above script.
  • With sequential flow scripts, you cannot write complex applications (intelligent Linux scripts).
  • However, with if command you will be able to selectively run certain commands (or part) of your script.
  • You can create a warning message and run script more interactively using if command to execute code based on a condition.

But What Is A Condition?

  • A condition is nothing but an expression that evaluates to a boolean value (true or false).
  • In other words condition can be either true or false.
  • A condition is used in shell script loops and if statements.

So, How Do I Make One?

A condition is mainly a comparison between two values. Open a shell prompt (console) and type the following command:

echo $(( 5 + 2 ))

Sample Output:

7

Addition is 7. But,

echo $(( 5 < 2 ))

Sample Output:

0

Answer is zero (0). Shell simple compared two number and returned result as true or false. Is 5 is less than 2? No. So 0 is returned. The Boolean (logical data) type is a primitive data type having one of two values

  • True
  • False

In shell:

  • 0 value indicates false.
  • 1 or non-zero value indicate true.

Examples

Operator Example Description True / False Evaluates To
5 > 12 echo $(( 5 > 12 )) Is 5 greater than 12? No (false) 0
5 == 10 echo $(( 5 == 10 )) Is 5 equal to 10? No (false) 0
5 != 2 echo $(( 5 != 2 )) 5 is not equal to 2? Yes (true) 1
1 < 2 echo $(( 1 < 2 )) Is 1 less than 2? Yes (true) 1
5 == 5 echo $(( 5 == 5 )) Is 5 equal to 5? Yes (true) 1

Now, it makes no sense to use  echo command for comparisons. But, when you compare it with some value it becomes very useful. For example:

if [ file exists /etc/resolv.conf ]
then
    make a copy
else
    print an error on screen
fi

The test command is used to check file types and compare values. Test is used in conditional execution. It is used for:

    File attributes comparisons
    Perform string comparisons.
    Basic arithmetic comparisons. 

test command syntax

test condition

OR

test condition && true-command

OR

test condition || false-command

OR

test condition && true-command || false-command

Type the following command at a shell prompt (is 5 greater than 2? ):

test 5 -gt 2 && echo "Yes"
test 1 -lt 2 && echo "Yes"

Sample Output:

Yes
Yes

You need to use the test command while making decisions. Try the following examples and note down its output:

test 5 -eq 5 && echo Yes || echo No
test 5 -eq 15 && echo Yes || echo No
test 5 -ne 10 && echo Yes || echo No
test -f /etc/resolv.conf && echo "File /etc/resolv.conf found." || echo "File /etc/resolv.conf not found."
test -f /etc/resolv1.conf && echo "File /etc/resolv1.conf found." || echo "File /etc/resolv1.conf not found."

Now, you can use the if statement to test a condition. if command The general syntax is as follows:

if condition
then
     command1 
     command2
     ...
     commandN 
fi

OR

if test var == value
then
     command1 
     command2
     ...
     commandN 
fi

OR

if test -f /file/exists
then
     command1 
     command2
     ...
     commandN 
fi

OR

 if [ condition ]
then
      command1
      command2
      ....
      ..
fi

If given condition is true than the command1, command2..commandN are executed. Otherwise script continues directly to the next statement following the if structure. Open a text editor and create the script called verify.sh:

#!/bin/bash
read -p "Enter a password" pass
if test "$pass" == "jerry"
then
     echo "Password verified."
fi

Save and close the file. Run it as follows:

chmod +x verify.sh
./verify.sh

Sample Outputs:

Enter a password : jerry
Password verified.

Run it again:

./verify.sh

Sample Output:

Enter a password : tom

The if structure is pretty straightforward. The read command will read the password and store it to variable called pass. If $pass (i.e. password) is equal to "jerry", then "Password verified." is displayed. However, if it is not equal to "jerry", the script does not print any message and script will go to the next statement. Here is another example (number.sh):

#!/bin/bash
read -p "Enter # 5 : " number
if test $number == 5 
then
    echo "Thanks for entering # 5"
fi
if test $number != 5 
then
    echo "I told you to enter # 5. Please try again."
fi

Enter # 5 : 5 Thanks for entering # 5 Save and close the file. Run it as follows:

chmod +x number.sh
./number.sh

Sample Outputs:

Enter # 5 : 5
Thanks for entering # 5

Try it again:

./number.sh

Sample Outputs:

En

ter # 5 : 11
Source:https://bash.cyberciti.biz/guide/If_structures_to_execute_code_based_on_a_condition