A simple shell command such as echo a b c consists of the command itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping. • Simple Commands: The most common type of command. • Pipelines: Connecting the input and output of several commands. • Lists: How to execute commands sequentially. • Compound Commands: Shell commands for control flow. • Coprocesses: Two-way communication between commands. • GNU Parallel: Running commands in parallel.