# Bash Scripting

## Resources

### Tutorials/ Getting Started

* [shellscript.sh](https://www.shellscript.sh/) "shell scripting tutorial"
* [Linux Documentation Project - Bash Guide for Beginners](http://tldp.org/LDP/Bash-Beginners-Guide/html/index.html)
* [BashGuide](http://mywiki.wooledge.org/BashGuide) "aid people interested in learning to work with BASH"
* [Serious Shell Programming](https://legacy.gitbook.com/book/freebsdfrau/serious-shell-programming/details)

### Reference

* [GNU Bash Reference Manual](https://www.gnu.org/software/bash/manual/html_node/index.html)
* [Cheatsheet](https://devhints.io/bash) devhints.io
* [Bash Hackers Wiki](http://wiki.bash-hackers.org/)

### Tips & Tricks

* [Shell Scripts Matter](https://dev.to/thiht/shell-scripts-matter) "a summary of everything that can, and should be done when writing shell scripts."
* [ShellCheck](https://www.shellcheck.net/) "static analysis tool for shell scripts"
* [Unofficial Bash Strict Mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/) "causes bash to behave in a way that makes many classes of subtle bugs impossible."
* [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml)

### Talks

* [Introduction to Advanced Bash Usage](https://www.youtube.com/watch?v=uqHjc7hlqd0) by [James Pannacciulli](https://twitter.com/_jpnc)

## General Advice

### bash is not sh

While `bash` is "sh-compatible" some features of `bash` will break or cause unexpected behaviour in `sh`.

* Read [bash Is Not sh](https://rainbowhackerhorse.github.io/bash-Is-Not-sh/) for a more detailed explanation.

### Forget the .sh extension

Don't give scripts an `.sh` extension.

1. The [Google Shell Style Guide](https://google.github.io/styleguide/shell.xml) advises against it unless it's a library
2. It's a `bash` script, not an `sh` script

## Pipes

### grep

If you're piping to `grep` multiple times only the last `grep` in the sequence can be called with `-q`.

## Examples

### Shebang

First line of the script that indicates which interpreter is used to execute the file. The `#!` must be at the very start of the file, with no spaces or blank lines before it.

```bash
#!/usr/bin/env bash
```

1. [Rational](https://stackoverflow.com/questions/21612980/why-is-usr-bin-env-bash-superior-to-bin-bash/21613044#21613044)
   * "`#!/usr/bin/env` searches `PATH` for `bash`, and `bash` is not always in `/bin`, particularly on non-Linux systems."
2. [Rational](https://stackoverflow.com/questions/16365130/the-difference-between-usr-bin-env-bash-and-usr-bin-bash/16365367#16365367)
   * "This way, you don't have to look for it in a specific place on the system, as those paths may be in different locations on different systems. As long as it's in your path, it will find it."

### Execute Script

Execute `bash` and tell it to read the script `myscript`. When executing the script this way the shebang line (`#!`) is just a comment, `bash` does nothing with it

```bash
bash myscript
```

We can give the script executable permission. Instead of calling `bash` manually, we can execute `myscript` directly.

```bash
chmod +x myscript  # Mark myscript as executable
./myscript  # Directly execute myscript
```

When `myscript` is executed this way, the shebang line (`#!`) is used to determine which interpreter to use.

### Debug Mode

Print every command before its execution, replacing the variables with their real values.

```bash
set -x
```

[The Set Builtin](https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html)

### Unofficial Strict Mode

```bash
set -euo pipefail
# -e: Exit if any command returns non-zero status code
# -u: Prevent using undefined variables
# -o pipefail: Force pipelines to fail on first non-zero status code
```

[Unofficial Bash Strict Mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://wiki.hacksoc.co.uk/help-guides/programming-scripting/bash-scripting.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.