How to Master Conditional Logic in Bash

The use of conditional logic is a very essential part of any programming language. The conditional logic can be implemented in Bash in different ways to solve the programming problems. The methods of using conditional logic in Bash through the different types of “if” and “case” statements to compare the string and numeric values, checking the variable content, checking the existence of the file or directory, etc. are shown in this tutorial.

List of Content:

1. Use of the “If” Statement
2. Use of the “If-Else” Statement
3. Use of the “If-Elif-Else” Statement
4. Use of the “If” Statement to Check an Empty Variable
5. Use of the “If” Statement with Logical Operator
6. Use of the Nested “If” Statements
7. Use of the “If” Statement to Check the File Existence
8. Use of the “If” Statement to Check the Directory Existence
9. Use of the “If” Statement with Regex
10. Use of the “Case” Statement

Use of the “If” Statement

This example shows the simple use of the “if” statement in Bash. Six types of comparison operators can be used to compare the numeric values in Bash. These are “-eq” (equal), “-ne” (not equal), “-le” (less than equal), “-ge” (greater than equal), “-lt” (less than), and “-gt” (greater than). The uses of “-lt” and “-eq” are shown in the following script whether the number is less than 99 or has not been checked using the “-lt” operator. The number is even or odd and is checked by the “-eq” operator.

Output:

The following output appears after executing the previous script:

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Use of the “If-Else” Statement

The use of the “if-else” statement is shown in the following script. A string value is taken from the user and checks whether the value is “BLUE” or not using the “if-else” statement.

Output:

The following output appears after executing the previous script if “red” is taken as input:

The following output appears after executing the previous script if “blue” is taken as input:

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Use of the “If-Elif-Else” Statement

The use of the “if-elif-else” statement is shown in the following script. A number is taken from the user and checked with the different values until any match is found. If any match is found, the corresponding message is printed. If no match is found, the default message is printed.

Output:

The following output appears after executing the script with the value of 8723:

The following output appears after executing the script with the value of 9078:

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Use of the “If” Statement to Check an Empty Variable

The method of checking whether a variable is empty not using an “if” statement is shown in the following script. The “-z” option is used in the “if” statement to do this task.

Output:

The following output appears after executing the script with the value of 690:

The following output appears after executing the script if no input value is taken:

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Use of the “If” Statement with Logical Operators

Three types of logical operators can be used in the Bash conditional statement. These are logical OR (||), logical AND (&&), and logical NOT (!). A code value is taken from the user. If the input value is non-empty, the value is checked with two code values using logical OR. If the value matches with any code, the corresponding message is printed. If no matching code is found, the default message is printed.

Output:

The following output appears after executing the script with the input value of “CSE-108”:

The following output appears after executing the script with the input value of “BBA-56”:

 

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Use of the Nested “If” Statements

When an “if” condition is used inside another “if” condition, it is called a nested “if” statement. The method of using the nested “if” is shown in the following script. Two mark values are taken from the user. If the input values are non-empty, the first “if” condition checks whether the value of “$theory” is greater than or equal to 60 or not. If the first “if” condition returns “true”, the second “if” condition checks whether the value of the “$lab” is greater than or equal to 50 or not. If the second “if” condition also returns “true”, a success message is printed. Otherwise, a failure message is printed.

Output:

The following output appears if both or one of the input values are empty:

The following output appears if 78 is taken as theory marks and 45 is taken as lab marks. Here, the second “if” condition returns “false”:

The following output appears if 67 is taken as theory marks and 56 is taken as lab marks. Here, both “if” conditions return “true”:

The following output appears if 50 is taken as theory marks and 80 is taken as lab marks. Here, the first “if” condition returns “false”:

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Use of the “If” Statement to Check the File Existence

The existence of the file can be checked by the bash script in two ways. One is using the “-f” operator with “[]” brackets. Another is using the “test” command and the “-f” operator. A filename is taken and checks the existence of the file using the “if” condition with the “-f” operator. Then, another filename is taken and check the existence of the file using the “if” statement with the “test” command and the “-f” operator.

Output:

The following output appears after executing the script by taking the “test.txt” and “testing.txt” as the filenames. According to the output, both files exist in the current location and the “testing.txt” file is empty:

The following output appears after executing the script by taking the “f1.txt” and “test.txt” as the filenames. According to the output, the “f1.txt” file does not exist in the current location and the “test.txt” file is not empty:

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Use of the “If” Statement to Check the Directory Existence

The existence of the directory can be checked by the Bash script in two ways like the file. One is using the “-d” operator with “[]” brackets. Another is using the “test” command and the “-d” operator. A directory name is taken and checks the existence of the directory using the “if” condition with the “-d” operator. Then, another directory name is taken and checks the existence of the file using the “if” statement with the “test” command and the “-d” operator.

Output:

The following output appears after executing the script with the “temp” and “files” directory names. According to the output, both directories exist in the current location. Then, the “ls” command is executed to check the content of the directories:

The following output appears after executing the script with the “testing” and “new” directory names. According to the output, both directories do not exist in the current location. Then, the output of the “ls” command shows that both directories don’t exist:

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Use of the “If” Statement with Regex

The following script shows the method of validating the input data using an “if” statement with the regex. Here, two input values are taken from the user and are stored in the “$bookname” and “$bookprice” variables. The “if” condition is used in the script to check that the “$bookname” variable contains all alphabetic characters and the “$bookprice” contains a number.

Output:

The following output appears after executing the script with the input values of “Bash Programming” as the book name and 78 as the book price:

The following output appears after executing the script with the input values of 90 as the book name and “Bash” as the book price:

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Use of the “Case” Statement

The “case” statement is the alternative of the “if-elif-else” statement but all tasks of the “if-elif-else” statement can’t be done using the “case” statement. The simple use of the “case” statement is shown in the following script. A numeric value is taken from the user as the current month value. Then, the corresponding month is printed if any matching value is found in the “case” statement. Otherwise, the default message is printed.

Output:

The following output appears after executing the script with the value of 6:

The following output appears after executing the script with the value of 09:

The following output appears after executing the script with the value of 14:

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Conclusion

Different uses of conditional logic using the “if” and “case” statements are shown in the 10 examples of this tutorial. The concept of using the conditional logic in Bash will be cleared for the new Bash users after reading this tutorial.