Master Shell Basics: From What a Shell Is to Advanced Variable Tricks

What is a Shell?

Shell is a command interpreter that wraps the OS kernel, allowing users to interact with the system by typing commands which the shell parses and executes.

Common Shell Types

Bash (Bourne Again Shell) – default on most Linux and macOS.

Sh (Bourne Shell) – original Unix shell.

Tcsh (C Shell) – C‑like syntax.

Zsh – advanced features, default on macOS Catalina and later.

Fish – user‑friendly with autosuggestions.

What is a Shell Script?

A shell script is a text file containing a sequence of shell commands and control structures that can be executed non‑interactively.

Shebang

The first line #!/bin/bash tells the system which interpreter to use.

Executing Scripts

Run with bash script.sh or sh script.sh (no execute permission needed).

Make executable ( chmod +x script.sh) and run ./script.sh or by absolute path.

Variables in Shell

Variables are defined without spaces ( name="value") and accessed with $name or ${name}. Variable names may contain letters, numbers, and underscores but cannot start with a digit.

System Variables

$HOME

, $PWD, $SHELL, $USER and similar built‑in variables.

Special Variables

$?

– exit status of the last command (0 for success, 1‑255 for error). $# – number of positional parameters passed to a script or function. $* – all positional parameters as a single word (when quoted). $@ – all positional parameters as separate words (when quoted).

Environment Variables

Exported with export VAR=value and stored in configuration files such as ~/.bashrc, ~/.bash_profile, or global files like /etc/profile and /etc/profile.d/*.sh.

String Substitution and Substring Extraction

Examples of powerful parameter expansion: ${var:2:5} – extract a substring starting at offset 2 with length 5. ${var#pattern} – remove the shortest matching prefix. ${var##pattern} – remove the longest matching prefix. ${var%pattern} – remove the shortest matching suffix. ${var%%pattern} – remove the longest matching suffix. ${var/pattern/repl} – replace the first occurrence of pattern with repl. ${var//pattern/repl} – replace all occurrences.

Practical Example: Batch Rename Files

for f in *finished.jpg; do
    mv "$f" "${f//_finished/}"
done

Conclusion

Shell scripting is a lightweight, powerful tool for system automation, file manipulation, and quick prototyping, making it essential for developers and operations engineers.