A shell is a program that reads commands and launches other programs. It provides a textual interface for working with files, processes and operating-system tools.

Commands and arguments

A command line contains a program name followed by arguments:

cp report.txt backup.txt

The shell parses the text and asks the operating system to start the requested program with those arguments.

Current directory and paths

The shell process has a current working directory. Relative paths are interpreted from that directory, while absolute paths begin at the filesystem root.

Commands such as cd, pwd and directory-listing tools make navigation explicit.

Redirection

Standard input and output can be connected to files:

program > result.txt
program < input.txt

Pipelines

A pipeline connects one process's standard output to another process's standard input:

producer | filter | consumer

This lets small independent tools be composed into larger workflows.

Environment and exit status

Shells pass environment variables to child processes and receive an exit status when a command terminates. Scripts use these mechanisms to automate repeated command-line procedures.

The shell is therefore both an interactive interface and a programming environment for composing operating-system processes.