Shell environment variables & parameter variables

Environment variables
$HOME	Home directory of the current user
$PATH	A colon-delimited list of directories to search for commands
$PS 1	A command prompt, usually a $ character, but in bash, you can use some more complex values. For example, the string [\[email protected]\h \w]$ is a popular default value that gives the user name, machine name, and current directory name, as well as a $ prompt.
$SP 2	A second-level prompt that prompts for subsequent input, usually the > character
$IFS	Enter a domain prompt. When the shell reads the input, it gives a set of characters used to separate the words, usually space-time grids, tabs, and line breaks
$	The name of the shell script
$#	The number of arguments passed to the script
$$	The process number of the shell script, which is typically used by the script to generate a unique temporary file, such as/tmp/tmpfileshell

Parameter variables
$1,$2,...	Parameters of the script program
$*	All parameters are listed in a variable, separated by the first character in the environment variable ifs, and the delimited character changes if the IFS is modified
[Email protected]	It is an ingenious variant of $* that does not use IFS environment variables, so even if IFS is empty, the parameters will not be squeezed together

Shell environment variables & parameter variables