In the shell, the condition in the IF-Z to-D means "go"

[-A file] is true if file exists.

[-B file] True if file exists and is a block special file.

[-C file] True if file exists and is a word special.

[-D file] True if file exists and is a directory.

[-E File] True if file exists.

[-F file] True if file exists and is a normal file.

[-G file] True if file exists and Sgid has been set.

[-H file] True if file exists and is a symbolic connection.

[-K file] True if file exists and the sticky bit has been set.

[-P file] If file exists and is a name pipe (f if O) is true.

[-R File] True if file exists and is readable.

[-S file] True if file exists and the size is not 0.

[-T FD] true if the file descriptor FD is open and points to a terminal.

[-u file] True if file exists and suid (set user ID) is set.

[-W file] True if file exists and is writable.

[-X file] True if file exists and is executable.

[-o file] True if file exists and is a valid user ID.

[-G file] True if file exists and is a valid user group.

[-L file] True if file exists and is a symbolic connection.

[-N file] If file exists and has been mod if IED since it is last read is true.

[-S file] True if file exists and is a socket.

[File1-nt FILE2] If FILE1 have been changed more recently than FILE2, or if FILE1 exists and FILE2 does not is true.

[File1-ot FILE2] If FILE1 is older than FILE2, or FILE2 exists and FILE1 does not exist, it is true.

[File1-ef FILE2] True if FILE1 and FILE2 point to the same device and node number.

[-O Optionname] true if the shell option "Optionname" is turned on.

[-Z STRING] "STRING" is true if the length is zero.

[-N STRING] or [string] "string" is true if the length is not 0 non-zero.

[STRING1 = = STRING2] If 2 strings are the same. "=" May is used instead of "= =" for strict POSIX compliance is true.

[STRING1! = STRING2] True if the strings are not equal.

[STRING1 < STRING2] if "STRING1" sorts before "STRING2" lexicographically in the current locale is true.

[STRING1 > STRING2] If the ' STRING1 ' sorts after ' STRING2 ' lexicographically in the ' current locale ' is true.

[ARG1 OP ARG2] "OP" is one of-eq,-ne,-lt,-le,-gt Or-ge. These arithmetic binary operators return true if "ARG1" is equal to, not equal to, less than, less than or equal to, great Er than, or greater than or equal to "ARG2", respectively. "ARG1" and "ARG2" are integers.

In the shell, the condition in the IF-Z to-D means "go"