What is Shell?

The shell is the program that interprets the commands you type and translates them into calls to the operating system. The name comes from the image of a shell wrapped around the system’s core (the kernel) — you talk to the shell, and it talks to the kernel.

It is not just a command runner. It is also a complete programming language, with variables, conditionals, loops and functions, which makes it the default tool for automation on Unix-like systems.

The main shells

• Bash — the most widespread; the historical Linux default and what you find on most servers.
• Zsh — the macOS default since 2019; Bash-compatible and richer in autocomplete and customization.
• Fish — prioritizes usability with native suggestions and syntax highlighting, at the cost of not being compatible with Bash scripts.
• PowerShell — the Windows default; unlike the others, it passes structured objects between commands instead of text.
• sh — the minimal POSIX shell; writing scripts for it guarantees maximum portability.

Shell scripting in practice

#!/usr/bin/env bash
set -euo pipefail   # aborts on error, undefined variable or pipe failure

SOURCE="/var/www/app"
DEST="/backup"
DATE=$(date +%Y-%m-%d)
FILE="$DEST/app-$DATE.tar.gz"

if [[ ! -d "$SOURCE" ]]; then
  echo "Source not found: $SOURCE" >&2
  exit 1
fi

tar -czf "$FILE" "$SOURCE"
echo "Backup created: $FILE"

# removes backups older than 30 days
find "$DEST" -name "app-*.tar.gz" -mtime +30 -delete

That first line after the shebang deserves attention. set -euo pipefail turns the shell’s default behavior — pressing on after errors — into something predictable. Without it, a backup script can fail silently and delete old files believing it saved the new ones.

Redirection and pipes

command > output.txt        # redirects output (overwrites)
command >> output.txt       # appends to the end
command 2> errors.txt       # redirects only the errors
command > all.txt 2>&1      # merges output and errors into one file
command < input.txt         # reads from standard input
command1 | command2         # connects one's output to the other's input

Where the shell shows up professionally

Every CI/CD pipeline runs shell commands. Every Dockerfile has RUN instructions interpreted by it. Server maintenance, automation of repetitive tasks and incident investigation happen mostly in the terminal. The articles on essential terminal commands and SSH in practice cover the necessary foundation.