3. Parts of a command

After you log in, you will get a prompt, that is, a request to type a command. The part of Unix that reads and executes your command is called a shell, and there are several different shells. Most users use the bash shell, which prompts with a dollar sign “$”. Some people prefer tcsh, which prompts with a percent sign “%”. Type this command to find out which shell you have:

echo $0

Here are the parts of a Unix command:

commandname [argument]… [<inputfile] [> outputfile]

commandname: The name of the command comes first.
argument: If there are any arguments to the command, they follow the command name.
inputfile: If there is an input redirection symbol “<”, input comes from inputfile, otherwise it comes from the keyboard.
outputfile: If there is an output redirection symbol “>”, output is sent to outputfile, otherwise it is sent to the screen.

It is also possible to link up several commands so that the output of each command is used as the input of the next command:

command₁ | command₂ | … | command_n

In this example, the output of command₁ is used as the input for command₂; the output of command₂ is used as the input for command₃; and so on.

Summary of common Unix commands

3. Parts of a command