Test switches and special parameters in shell scripts

1. Test Switch

Some test instructions are included in the shell, and the following table lists the meanings of these test instructions and whether they are available for the test command, bash, Ksh.

Switch	Test	Bash	Ksh	Defined
-A FILE		Support	Support	Just the file exists.
-B FILE	Support	Support	Support	File exists and is a block file like a disk device such as/dev
-C FILE	Support	Support	Support	File exists and is a character file like a TTY device such as/dev
-D FILE	Support	Support	Support	File exists and is a standard directory
-E FILE	Support	Support	Support	Just the file exists.
-F FILE	Support	Support	Support	The file exists and is a standard file similar to a flat file
-G FILE	Support	Support	Support	The file exists and is a set-group-id. This refers to a file license that changes the user's effective group for file enforceability
-G FILE	Support	Support	Support	The file exists, and its group ownership is the user's valid group ID
-H FILE	Support	Support	Support	The file exists, and it is a symbolic link. Same as-l
-K FILE	Support	Support	Support	The file exists, and it is glued to a bit set. This means that only the owner of the file or the owner of the directory can delete the file
-L STRING	Support			and a numeric value similar to/usr/bin/test-l STRING-GT 5 && Echo, comparing string lengths
-L FILE	Support	Support	Support	The file exists, and it is a symbolic link. As with-h
-N STRING	Support	Support	Support	The length of the STRING is nonzero
-N FILE		Support	Support	File exists, but has been modified since the most recent read-in
-O OPTION	Support	Support	Support	If the OPTION of the shell is opened, returns True, such as Set-x
-O FILE	Support	Support	Support	The file exists and its owner is determined by a valid user ID
-P FILE	Support	Support	Support	File exists, and it is a named pipe (or FIFO)
-R FILE	Support	Support	Support	File exists and is readable
-S FILE	Support	Support	Support	File exists, and its size is greater than 0 bytes
-S FILE	Support	Support	Support	File exists, and it is a socket
-T [FD]	Support	Support	Support	Open FD (file descriptor) in a terminal, default is stdout
-U FILE	Support	Support	Support	The file exists and Set-user-id is set
-W FILE	Support	Support	Support	File exists, and can be written
-X FILE	Support	Support	Support	File exists and is executable
-Z STRING	Support	Support	Support	The length of the STRING is 0

2. Special Parameters

The special parameters in the shell are the internal variables of the shell, which are to be used with the prefix $.

Like echo $? is to get the value of the shell internal variable "?".

List of content variables:

Parameters	Defined
*	From 1 onwards, complete the list of all positional variables. If a double quotation mark is quoted, the word that becomes the first character of the IFS (internal file delimiter) value is divided
@	From 1 onwards, complete the list of all positional variables. If double quotes are quoted, each positional parameter becomes a single word
#	Number of positional parameters in decimal
?	The return code of the last or most recent foreground task. If the task is killed by a signal, the return value is 128 plus the value of the signal. For example: The standard kill signal value is 15, then the return value is 143
-	All flags sent to the shell, or flags provided by the SET command
$	Shell process ID. If it is in a child shell, it is extended to the value of the current shell, not the child shell.
!	Process ID of the command that was recently put into the background execution
_	Expands to the last parameter of the previous command
0	Extended to the name of a shell or shell script
1 ... 9	The positional parameters provided to the shell, function, or script. A value larger than 9 can be accessed using ${number}

Test switches and special parameters in shell scripts