# What Bash Is — Bash / Shell Scripting

Source: https://www.skillbyai.com/en/bash/f-shells

> Explain what a shell does and how Bash relates to sh, zsh and POSIX.

## A command interpreter and a language

A **shell** reads commands, either typed interactively or from a file, and runs programs for you. It also expands variables and wildcards, connects programs with pipes and redirects input and output. **Bash** (the Bourne Again SHell, from the GNU project) is the most widely used shell for scripting: it is the default login shell on most Linux distributions and is installed almost everywhere, including in CI runners and containers. Bash implements the **POSIX shell** standard plus many extensions: `[[ ]]` tests, arrays, `$(( ))` arithmetic, `local`, process substitution and more. Other shells: **`sh`** is the POSIX shell, which on Debian and Ubuntu is **dash** (small and fast, without Bash extensions); **zsh** is the default interactive shell on macOS and is popular interactively; **fish** focuses on usability but is not POSIX-compatible. Current Bash is version 5.x, but **macOS ships the old Bash 3.2**, which lacks associative arrays and other features, something to remember when scripts must run on Macs.

## Where the shell sits

The shell sits between you (or a script) and the operating system, launching programs and wiring them together.

![Concentric layers: a person icon and a document icon on the outside, a shell ring, then a kernel core, with small program boxes launched from the shell ring.](assets/figures/bash/section-1-map.svg) — Figure 1.1 — The shell between users, scripts and the operating system.

## Which shell am I using?

Interactive shell, script interpreter and versions can all differ.

```bash
echo "$SHELL"            # your login shell (not necessarily the one running now)
echo "$BASH_VERSION"     # set only when running inside bash
ps -p $$ -o comm=         # the shell process running this command

ls -l /bin/sh             # on Debian/Ubuntu: a link to dash
bash --version | head -1

# run a script explicitly with bash, regardless of your interactive shell
bash ./deploy.sh
```

## Bash scripts need bash, not sh

Running a Bash script with `sh script.sh` on Ubuntu uses dash, which fails on `[[`, arrays and other Bash features with confusing errors. Use a `#!/usr/bin/env bash` line and run the script directly.

**Quiz:** On Ubuntu, what does /bin/sh usually point to?

- [x] dash
- [ ] bash
- [ ] zsh
- [ ] fish

*Answer:* dash. Debian and Ubuntu use dash as /bin/sh for speed and strict POSIX behaviour.