Bash Programming

Bash, a powerful programming language, is perfectly designed for command line and shell scripts. In this module, we’ll learn how to use Bash. Its features will help us solve our daily computer tasks and even automate some of them.

This module covers the general principles of computing operations and basic programming concepts in detail. It will also help us consolidate our new knowledge through exercises for each topic. Overall, this experience should be a great start to our Bash programming journey.

Basic Programming with Bash

Now that we have got everything about the current directory, we can change it.

Let’s suppose that we want to go to a directory named `usr`. We will call the `cd` command to go to the `/usr` path. We can do it this way:
```
cd /usr
```

Let’s connect to the following terminal and run the above command.

Autocompletion works for both command and its parameters. Let’s try it in the terminal above this way:
1. Type `cd /us` text.
2. Press the “Tab” key twice.
3. Bash displays the directories that start with `/us`.
4. Complete our command by typing `r` or `e`, and then press the “Tab” key once.

The following figure shows the result of these commands:


The `cd` command does not output anything if it succeeds. It changes the current directory, and that’s it. We can read the new path right before the `$` symbol of the command prompt in the figure above. This path equals `/usr`. The `cd` command accepts both absolute and relative paths. Relative paths are shorter. Therefore, we type them faster. We should prefer them when we navigate the file system using a command shell.

There is a simple rule to distinguish the type of path. An absolute path starts with a slash `/`. An example is `/usr/include`. A relative path starts with a directory name. An example is `usr/include`.

Now, we are in the `/usr` directory. We can get a list of its subdirectories and go to one of them. Let’s suppose that we want to go one level higher and reach the root directory.

There are two ways to do that: go to the absolute path `/` or the special relative path `..`. The `..` path always points to the parent directory of the current one. Use it in the `cd` call this way:

```txt
cd ..
```
We can try running this in the terminal above.

> In addition to the special relative path `..`, there is another special path `.`, which points to the current directory. If we execute the command `cd .`, nothing happens. We stay in the same place. We need the `.` path to run programs from the current directory.

Come back to the `/usr` directory. Then, run the `ls` utility there. It will show us the `share` subdirectory. Come to this directory and call `ls` again. We will find the `doc` directory there. It contains Bash documentation. Call the `cd` command in the following way to reach the documentation:

```txt
cd doc/bash
```

We are in the `/usr/share/doc/bash` directory now. Call the `ls` utility there. It will show us several files. One of them is `README`. It contains a brief description of the Bash interpreter. We found the documentation file. The next step is to print its contents. The `cat` utility does that. Here is an example of how to run it:
```txt
cat README
```

The following figure shows the output after the `cat` call.

The `cd` command does not output anything if it succeeds. It changes the current directory, and that’s it. We can read the new path right before the `＄` symbol of the command prompt in the figure above. This path equals `/usr`. The `cd` command accepts both absolute and relative paths. Relative paths are shorter. Therefore, we type them faster. We should prefer them when we navigate the file system using a command shell.

There is a simple rule to distinguish the type of path. An absolute path starts with a slash `/`. An example is `/usr/include`. A relative path starts with a directory name. An example is `usr/include`.

Now, we are in the `/usr` directory. We can get a list of its subdirectories and go to one of them. Let’s suppose that we want to go one level higher and reach the root directory.

There are two ways to do that: go to the absolute path `/` or the special relative path `..`. The `..` path always points to the parent directory of the current one. Use it in the `cd` call this way:

```txt
cd ..
```
We can try running this in the terminal above.

> In addition to the special relative path `..`, there is another special path `.`, which points to the current directory. If we execute the command `cd .`, nothing happens. We stay in the same place. We need the `.` path to run programs from the current directory.

Come back to the `/usr` directory. Then, run the `ls` utility there. It will show us the `share` subdirectory. Come to this directory and call `ls` again. We will find the `doc` directory there. It contains Bash documentation. Call the `cd` command in the following way to reach the documentation:

```txt
cd doc/bash
```

We are in the `/usr/share/doc/bash` directory now. Call the `ls` utility there. It will show us several files. One of them is `README`. It contains a brief description of the Bash interpreter. We found the documentation file. The next step is to print its contents. The `cat` utility does that. Here is an example of how to run it:
```txt
cat README
```

The following figure shows the output after the `cat` call.

Before We Begin

File System

Command Information

Bash Extra Features

Scripts Features

Variables and Parameters

Conditional Statements

Arithmetic Expressions and Operations

Bitwise Operations

Logical Operations and Increments

Loop Operators

Functions

Package Manager

Bash Programming from Scratch

Bash Scripts Tools

APPENDIX

Conclusion

Command cd

The `cd` command