Control Flow in Shell Scripts

Control flow (Decision construct) in shell scripting enable the program to execute some block of code based on some conditions evaluating to true or false. Control statements in shell is generally achieved by use of if statements and its derivatives. In this article, we are going to walk you throughout the process of writing your first control structure.

Creating a script file with correct exension

Shell scripting can be done in particular shell implementation like Bash, Zsh, Ksh and sh. Each shell has its specific file extension like .sh, .zsh or .ksh. Therefore, you have to know what shell is powering your terminal before you name your file. To do so execute the following command:
echo $0 # prints the special variable representing name of current program, in this case the shell
After ascertaining the current shell environment, create a script file with corresponding extension. For example, bash has the extension .sh, and zsh has the extension .zsh.

Including a shebang and the file path

Shebang (combination of octothrope/hash and exclamation, #!) is an interpreter directive which is followed by a path to the interpreter/shell program you want to execute the script. It is not strictly required but considered good practice because it ensures portability and predictability. Since we already know the shell program, we then use it to get its path:

which bash # replace bash with your shell echo $SHELL #prints the file path to the shell

After determining the path, open the file using your preferred text editor:

nano script.sh or vim script.sh

Include the shebang followed closely by the file path at the top of the file: #!/bin/bash

Execute to test

Add an echo command for the purpose of testing:
echo “We good so far”
Since systems file permissions prevents unauthorized read, writes and execution of files, you need to grant execute permissions to the shell program:
chmod o+x script.sh #grant every user execution permissions.
You can then execute the file either by specifying the shell program (repetition if you included shebang) or let the shebang direct.
sh script.sh ./script.sh
The test string will be printed on the terminal indicating successful creation of script file.

Control Flow Statements in Shell

There are three control flow statements in shell program and both are based on an a expression (mostly between square brackets unless implemented in C-style) evaluating to boolean value. An exit status of zero means true while all non-zero status means false. All if statements are terminated with fi, which is if spelled backwards.

Operators for Control Flow

Conditions for the if statements are implemented using operators evaluating to boolean.

Other shell operators like string comparison operators and file test operators can also be used within the square brackets.

a. if ... then

The condition between the square brackets executed and ifthe exit status is zero, the shell command inside the if block is executed.

        if [2 -gt 1]; then 
	        echo “Two is greater than  one”
        fi
                    

b. if … else

The condition between the square brackets executed and if the exit status is zero, the shell command inside the if block is executed, otherwise the statements inside the else block executed. It provides a way to ensure either of two block will be executed.

        if [2 -gt 3]; then 
            echo “Two is greater than three”
        else
            echo “Two is less than three”
                    

NOTE: This examples are bordering useless, we used them to reduce cognitive load by focusing on syntax.

c. if … elif

The condition between the square brackets executed and if the exit status is zero, the shell command inside the if block is executed, otherwise the expression inside the square brackets for elif is also checked. It provides a way to check multiple conditions and provide blocks for each.

        Age=18
        if [ $age -gt 18 ]; then 
            echo “You’re over 18, you can vote.”
        elif[ $age -eq 18 ];
            echo “You’re 18, you can vote.”
        else
            echo “You’re too young to vote.”
        fi
                    

d. Nested if

This control structure involves placing an if statement inside another if statement to enable expressing a more complex logic. In conclusion, in this article you have learned how to correctly create shell script file, operators for decision construct and the four control flow implementations.

        if [ $age -gt 20];
            if[ $age -gt 13 ]; then 
                echo “You’re a teenager”
            fi
        fi