Common conditions in the Shell to judge

-B file if it exists and is a block special file, True
-C file If it exists and is a character special file, True
-D file if it exists and is a directory, true
-e File is true if it exists
-F file is true if it exists and is a rule file
-G file is true if it exists and the value of the Sgid bit is set
-H file is true if it exists and is a compliant link
-K file if it exists and the value of "sticky" bit is set
-P file If it exists and is a named pipe, true
-R file if it exists and is readable, true
-S file is true if it exists and its size is greater than 0
-U file is true if it exists and the SUID bit is set
-W file is true if it exists and is writable
-X file is true if the file exists and is executable
-O file is true if it exists and is owned by a valid user ID

-Z String True if string length is 0
-N String True if string length is not 0
string1 = string2 true if two strings are equal
String1! = string2 true if two strings are not equal

Int1-eq int2 if Int1 equals Int2, then true
Int1-ne Int2 If int1 is not equal to Int2, then true
Int1-lt Int2 If Int1 is less than Int2, it is true
Int1-le Int2 If int1 is less than or equal to Int2, then true
INT1-GT Int2 If Int1 is greater than Int2, then true
Int1-ge Int2 If int1 is greater than or equal to Int2, then true

!expr If expr is false, the compound expression is true. Expr can be any valid test expression
Expr1-a expr2 if Expr1 and Expr2 are true, the whole is true.
Expr1-o expr2 if Expr1 and Expr2 have a true-to-real

Special variable The name of the command that is being executed. For shell scripts, this is the path to the activated command
$n the variable corresponds to the parameter that was taken when the script was activated. n is a positive integer that corresponds to the position of the parameter ($1,$2 ...)
$# provides the parameter number of the script
$* all these parameters are enclosed in double quotation marks. If a script receives two parameters, $* equals $1$2
$@ all these parameters are enclosed in double quotation marks, respectively. If a script receives two parameters, $@ is equivalent to $1$2
$? Exit status after the previous command was executed
$$ the process number of the current shell. For shell scripts, this is the process ID when it is executing
$! The process number of the previous background command