shell 中相关判断参数意义

* -b file = True if the file exists and is block special file.

* -c file = True if the file exists and is character special file.

* -d file = True if the file exists and is a directory.

* -e file = True if the file exists.

* -f file = True if the file exists and is a regular file

* -g file = True if the file exists and the set-group-id bit is set.

* -k file = True if the files’ “sticky” bit is set.

* -L file = True if the file exists and is a symbolic link.

* -p file = True if the file exists and is a named pipe.

* -r file = True if the file exists and is readable.

* -s file = True if the file exists and its size is greater than zero.

* -s file = True if the file exists and is a socket.

* -t fd = True if the file descriptor is opened on a terminal.

* -u file = True if the file exists and its set-user-id bit is set.

* -w file = True if the file exists and is writable.

* -x file = True if the file exists and is executable.

* -O file = True if the file exists and is owned by the effective user id.

* -G file = True if the file exists and is owned by the effective group id.

* file1 -nt file2 = True if file1 is newer, by modification date, than file2.

* file1 ot file2 = True if file1 is older than file2.

* file1 ef file2 = True if file1 and file2 have the same device and inode numbers.

* -z string = True if the length of the string is 0.

* -n string = True if the length of the string is non-zero.

* string1 = string2 = True if the strings are equal.

* string1 != string2 = True if the strings are not equal.

* !expr = True if the expr evaluates to false.

* expr1 -a expr2 = True if both expr1 and expr2 are true.

* expr1 -o expr2 = True is either expr1 or expr2 is true.