Getting started with Shell Scripting

Shell scripting is a simple yet powerful way of performing some task using shell. A shell script is just like any program file which contains sequence of statements. Shell scripts can be useful when you want to execute a same set of commands everytime say for installing dependencies on multiple systems. What makes a shell script so powerfull is that you can directly write your shell commands as statements which allows you to do a lot of things ranging from linkinking two files for e.g getting some value from say a python script and supplying it as an input to say a php file, to monitoring the entire system. All this is possible because you are working on the system level.
Now that we know what can we do using shell script let's try to make one of our own.

Let's try to create a directory and list all the files in a specific directory.

Creating bash script step by step

1. Open your terminal and create a new script file.

Let's call our script file list_file.sh, every bash script has extension .sh.

vi list_files.sh

Press I to insert.

2. Specifying executable binary

The first line of the script contains the absolute path of the program by which it is executed.

Note: Although it is not necessary to specify program it is followed as a good practice.

Since we are creating a bash script the path will be /usr/bin/bash
Enter the following line:

#!/usr/bin/bash

3. Adding comments

You can add comments by adding # followed by comment in a bash script

# Script to create a directory in home with given name and a file containing list of files present in /var/www/html

4. Declaring variable

Now let's create a variable which contains name of the folder which we are going to create:

NAME="Aastha"

You can create variables just by writing variable_name = value, no need to specify data type.

5. Creating folder

Now let's create a folder in home directory:

mkdir ~/$NAME

Notice that NAME is preceeded by $, here $NAME represents value stored in NAME variable.

6. Generating file with list of files present in /var/www/html

We can use echo followed by text, to print some text on stdout(Standard Outoput) but we can also use it to add some text in a specified file using > operator like this:

echo `ls /var/www/html` > ~/$NAME/file_list.txt

Here ~/$NAME/file_list.txt is the name of out put file which contains result of ls /var/www/html.
The ` denotes that echo will first evaluate the expression and then use its value.

The whole file will look like this:

#!/usr/bin/bash
# Script to create a directory in home with given name and a file containing list of files present in /var/www/html
NAME="Aastha"
mkdir ~/$NAME
echo `ls /var/www/html` > ~/$NAME/file_list.txt

7. Saving the file

We are done with all the statements, let's save and close the file. Press Esc then type :wq that is write and quit and press enter.

8. Executing the script

To execute the script first we will need to change its permissions by:

chmod +x list_files.sh 

Which will grant executing permissions to all.

Now let's execute the script:

./list_files.sh 

And that's it! You can see a file is created in the specified directory.

This was just a basic shell script in the next article we'll see how to take command line arguments using flags