categories of variables:1. System Variable env2. Custom variable set> env
Environment Variables
System Environment Variables/etc/profile/ETC/BASHRC
Personal Environment Variables. Bash_profile. BASHRC. Bash_history. Bash_logout
Positional variables$$ The PID of the shell itself (ProcessID)$! PID of the Shell's last running background process$? End code of the last Run command (return value)$- Flag at a glance using the SET command$* All parameter lists. such as "$*" in the Case of "" ", in the form of" $ $ ... $n "output all parameters.[email protected]All parameter lists. such as "[email protected]" with "" "in the case, with" $ "" $ "... All parameters are output in the form "$n".$# Number of arguments added to the shell $The name of the shell itself$1~ $nEach parameter value added to the shell. $ $ is the 1th parameter, and the $ = is the 2nd parameter .... By displaying the results you can see:$# is the number of arguments passed to the scriptthe name of the script itself$ $ is the first parameter passed to the shell script$ $ is the second parameter passed to the shell script[email protected] is a list of all parameters passed to the script
Shell constants and variables
