Learning shell interpretation of commands

When we log in, the $ sign will be visible in the shell terminal (# prompt if logged in as root or administrator). The Bash shell runs scripts as interpreter. Whenever we type a command, the BASH shell will read them as series of words (tokens). Each word is separated by a space ( ), semi colon (;), or any other command delimiter. We terminate the command by pressing the Enter key. This will insert a new line character at the end of the command. The first word is taken as a command, then consecutive words are treated as options or parameters.

The shell processes the command line as follows:

• If applicable, substitution of history commands

• Converting command line into tokens and words

• Updating history

• Processing of quotes

• Defining functions and substitution of alias

• Setting up of pipes, redirection, and background

• Substitution of variables (such as $name and $user) is performed

• Command substitution (echo `cal` and echo `date`) is performed

• Globing is performed...