The meaning of shell variable $#,$@,$0,$1,$2 in Linux/shell ${}, # #和 percent use example/export__linux

The meaning of shell variable $#,$@,$0,$1,$2 in Linux is explained:
Variable Description:
$$
The shell itself PID (ProcessID)
$!
The PID of the Shell's last running background process
$?
End code of the last command to run (return value)
$-
Flag list using the SET command
$*
A list of all parameters. Output all parameters in the form of "$ $n", as in the case of "$*" with "".
$@
A list of all parameters. such as "$@" with "" in the case, to "$ $" ... Output all parameters in the form of "$n".
$#
Number of arguments added to the shell
$
File name of the shell itself
$1~ $n

The value of each parameter added to the shell. $ is the 1th parameter, $ is the 2nd parameter ....

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In this paper, we introduce the ${}, # #和 percent usage example in the shell, and give the results in different situations.
Suppose a variable is defined as:
The code is as follows:
File=/dir1/dir2/dir3/my.file.txt
You can replace each of the different values with ${}:
${file#*/}: Delete the first/and left string: dir1/dir2/dir3/my.file.txt
${file##*/}: Delete the last/and left string: my.file.txt
${file#*.}  : Delete the first one. And the string to the left: file.txt
${file##*.}  : Delete the last one. And the string to the left: txt
${file%/*}: Delete the last/And right string:/dir1/dir2/dir3
${file%%/*}: Delete the first/and the right string: (null value)
${file%.*}: Delete the last one. And the string to the right:/dir1/dir2/dir3/my.file
${file%%.*}: Delete the first one. And the string to the right:/dir1/dir2/dir3/my
The method of memory is:
# is to remove the left (on the keyboard # on the left of the $)
% is to remove the right (on the keyboard% on the right side of the $)
A single symbol is a minimum match; two symbols is the maximum match
${file:0:5}: Extract leftmost 5 bytes:/dir1
${file:5:5}: Fetches 5 consecutive bytes to the right of the 5th byte:/dir2
You can also replace the string in the value of the variable:
${file/dir/path}: Replaces the first dir with the Path:/path1/dir2/dir3/my.file.txt
${file//dir/path}: Replace all dir with Path:/path1/path2/path3/my.file.txt

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1, executes the script is in a child shell environment to run, after the script executes the child shell to exit automatically;

2. How the system environment variables in a shell are copied into the sub shell (variables defined with export);

3. The system environment variable in a shell is only valid for the shell or its child shell, and the variable disappears at the end of the shell (and cannot be returned to the parent shell).

4. A variable that is not defined by export is valid only for the shell, and the child shell is not valid.

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-D: Determine whether the set is a directory
-Z: Determine if a variable exists with a value
-F: Judge whether a file is made
-L: Determine if a symbolic link is made
-R: Determining whether the made is readable
-S: Determine if the existing object length is 0
-W: Judge whether the formulated can be written
-X: Determines whether an existing object can be executed
。 : Negative sign of test condition
These file operations are often convenient for scripting, especially if they are used in an IF condition statement