Gain insights into the intricacies of the Bash shell, ideal for beginners. Delve into essential skills with interactive learning designed to ensure lasting knowledge without requiring prior experience.

dockerbash.tar.gz

tset

A course for anyone interested in understanding all aspects of the bash shell. Designed for use by beginners, it assumes as little knowledge as possible but teaches you all the intricacies of bash it took me decades to learn by trial and error. 

The interactive nature of the course instills the knowledge in a way you will not forget!

Master the Bash Shell

# How Important is this Lesson? 

Most bash scripts won't be completely comprehensible or safely written unless
**exit codes** are understood.

# What Is An Exit Code?

After you run a command, function or builtin, a special variable is set that
tells you what the result of that command was. If you’re familiar with HTTP
codes like 200 or 404, this is a similar concept to that.

To take a simple example, type this in:


When that special variable `$?` is set to `0` it means that the previous command completed
successfully (**line 2**). When the special variable `$?` is set to non-zero (in this case `127` it means that it did not complete successfully.


# Standard Exit Codes

There are guidelines for exit codes for those that want to follow standards.
Be aware that not all programs follow these standards (`grep` is the most common
example of a non-standard program, as you will learn)!

Some key ones are:

- `0` - 'command successfully run' (OK)
- `1` - Used when there is an error but no specific number reserved to indicate what it was (general error)
- `2` - Misuse of shell `builtin` command
- `126` - Permission problem or command is not executable
- `127` - No file found matching this command
- `128` - Invalid exit argument given (eg exit 1.76)
- `128+n` - Process killed with signal `n`, eg 130 = terminated with signal 2

> Note: Signals will be covered in [part 4](https://www.educative.io/courses/master-the-bash-shell/traps-and-signals)

Since codes `3`-`125` are not generally reserved, you might use them for your own
purposes in your applications.

# Exit Codes and `if` Statements 

So far so simple, but unfortunately (and because they are useful) exit codes can
be used for many different reasons, not just to tell you whether the command
completed successfully or not. Just as with exit codes in HTTP, the application
can use exit codes to indicate something went wrong, or it can return a ‘200 OK’
and give you a message directly.

Try to predict the output of this:



When that special variable `＄?` is set to `0` it means that the previous command completed
successfully (**line 2**). When the special variable `＄?` is set to non-zero (in this case `127` it means that it did not complete successfully.

In this lesson, you will find out what an exit code is, how you can set one in a script and a function, some conventions around exit codes, and other 'special' parameters in bash.

Core Bash

Scripting Bash

Bash in Practice

Advanced Bash

Exit Codes

How Important is this Lesson?

What Is An Exit Code?

Standard Exit Codes