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Tuesday, September 21, 2021

How do you pass a Named Argument in a Shell Script?

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The shell scripts in Linux allow you to write programs with hard-coded values and programs that can take user inputs at runtime. These user inputs are known as parameters or arguments. All of us are generally familiar with passing normal arguments to the shell scripts. However, you might sometimes feel the need to pass “Named Arguments” to your shell scripts. This article will guide you more about what exactly are named arguments and their need in shell scripts. After that, we will share an extensive example of passing the named arguments to a shell script in Ubuntu 20.04.

Named Arguments in Shell Scripts in Ubuntu 20.04

By now, we clearly understand that we can easily write such shell scripts in Ubuntu 20.04 that are capable of taking arguments as input from the user while executing these scripts from the terminal. However, a different type of arguments known as “Named Arguments” is also commonly used within the shell scripts. A named argument is the one that is symbolized by a “name-value” pair. This “name-value” pair is defined within the shell script, and the corresponding argument is passed in the same manner while executing the script as you pass the normal arguments.

Then what is the significance of using the named arguments in shell scripts? Well, at times, you define the arguments in your shell scripts, but while running those scripts, you might not necessarily need to provide all of those arguments. The named arguments allow you to skip the values of as many of these arguments as you want while running your script. In that case, if you had used the normal arguments, your system would have rendered an error message and would not have allowed you to proceed with the execution of your script if you would have skipped any pre-defined arguments within your script.

Moreover, at times, you might also change the order of passing the arguments while running a shell script, i.e., you pass the arguments in random order instead of the one that is followed while declaring the variables within your shell script. The named arguments easily let you do this, whereas on the other hand, if you would have used the normal arguments in this situation, then again, an error message would have been generated because of not following the correct order of arguments. Therefore, you can say that the named arguments provide more flexibility to the users while providing inputs instead of restricting them.

Now, when you realize the importance of using the named arguments in shell scripts, let us see how we can use these arguments within a shell script on a Ubuntu 20.04 system.

Example of Passing Named Arguments to a Shell Script in Ubuntu 20.04

For passing named arguments to a shell script in Ubuntu 20.04, we have designed a shell script that is shown in the image below. We will be executing this shell script with different arguments or input values in this section.

In this shell script, we have used the built-in “getopts” function in the shell inside a while loop. This function basically takes named parameters as input from the user. Then, we have defined three different cases, i.e., “n, a, and g” corresponding to our variables “Name, Age, and Gender” respectively. These cases are defined against the “flag” variable upon which our case-esac statement will execute. The case-esac statement in the shell is basically an equivalent of the Switch statement in C. Then, inside the case-esac block, we have listed all the three cases that were declared earlier, i.e., n, a, and g. Against each case, we have declared a variable that equals the argument provided by the user as an input while executing the script. Each argument provided by the user will be assigned to the relevant variable every time this script will be executed. Then, we finally have three “echo” commands that are there to print the values of the variables name, age, and gender, respectively.

After designing this script, we will first execute it with normally named arguments in the correct order as follows:

$ bash Named.sh –n Aqsa –a 27 –g Female

Here, Named.sh represents the name of our shell script. Moreover, you can see from this command that we have listed the flags first, followed by their respective arguments to be passed. Also, you can clearly notice that we have passed these arguments in the exact order as they were defined in our shell script.

Now when this shell script is executed, you will be able to see the values assigned to all of your named parameters on the terminal, as shown in the image below:

Once we have executed this script in the normal flow, we can try out a little experiment by executing this shell script with the same input values but in a slightly different order, as shown in the following command:

$ bash Named.sh –n Aqsa –g Female –a 27

You can see in this command that we have changed the order of the gender and age arguments from the one that was defined initially in the shell script. Now, we will try to find out if these values are correctly assigned to our named arguments or not.

When this command is executed, you will be able to see from its output that regardless of the order in which the arguments are passed while executing the shell script, they will still be assigned to the correct variables as shown in the image below:

In some situations, a user might not want to reveal his/her age. In that situation, he/she will execute this shell script in the following manner:

$ bash Named.sh –n Aqsa –g Female

In the output of this shell script, when it is executed with the arguments shown above, you can clearly see that our system has not generated any error messages; rather, it has smoothly executed our script with the provided parameters while leaving the Age variable blank.

In the same manner, you can also try skipping the Gender variable while only providing the value for the Name variable in the manner shown below:

$ bash Named.sh –n Aqsa

The corresponding output for this command is shown in the following image:

Finally, we will try to execute this script without providing any arguments as follows:

Again, you can see from the following output that no error message is generated; rather, our script has executed successfully even without any provided arguments.

Conclusion

From the detailed example discussed in this tutorial, we can conclude that it will still be executed successfully regardless of the order of the named arguments provided to a shell script. Moreover, even if you will not provide any arguments to your script, it can still be executed without any errors. However, the only thing you need to be careful about while passing named arguments is using the correct flag followed by its corresponding value while executing your shell script.

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