Meaning of -z to -d in if condition judgment in shell#
[ -a FILE ]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 character special file.[ -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 regular file.[ -g FILE ]True if FILE exists and has the SGID set.[ -h FILE ]True if FILE exists and is a symbolic link.[ -k FILE ]True if FILE exists and has the sticky bit set.[ -p FILE ]True if FILE exists and is a named pipe (FIFO).[ -r FILE ]True if FILE exists and is readable.[ -s FILE ]True if FILE exists and is not empty.[ -t FD ]True if file descriptor FD is open and points to a terminal.[ -u FILE ]True if FILE exists and has the SUID (set user ID) 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 owned by the effective user ID.[ -G FILE ]True if FILE exists and is owned by the effective group ID.[ -L FILE ]True if FILE exists and is a symbolic link.[ -N FILE ]True if FILE exists and has been modified since it was last read.[ -S FILE ]True if FILE exists and is a socket.[ FILE1 -nt FILE2 ]True if FILE1 exists but FILE2 does not.[ FILE1 -ot FILE2 ]True if FILE1 is older than FILE2, or FILE2 exists and FILE1 does not.[ FILE1 -ef FILE2 ]True if FILE1 and FILE2 point to the same device and inode number.[ -o OPTIONNAME ]True if the shell option "OPTIONNAME" is enabled.[ -z STRING ]True if the length of "STRING" is zero.[ -n STRING ] or [ STRING ]True if the length of "STRING" is non-zero.