SCRIPT : Conditional Expressions.

 Conditional Expressions.
      A conditional expression is used with the [[ compound command to test
      attributes of files and to compare strings.  Word splitting and file
      name generation are not performed on the words between [[ and ]].
      Each expression can be constructed from one or more of the following
      unary or binary expressions:

           -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.
           -f file             True if file exists and is an ordinary file.
           -g file             True if file exists and is has its setgid bit
                               set.
           -h file             True if file exists and is a a symbolic link.
           -k file             True if file exists and is has its sticky bit
                               set.
           -n string           True if length of string is nonzero.
           -o option           True if option named option is on.
           -p file             True if file exists and is a fifo special
                               file or a pipe.
           -r file             True if file exists and is readable by
                               current process.
           -s file             True if file exists and has size greater than
                               zero.
           -t fildes           True if file descriptor number fildes is open
                               and associated with a terminal device.
           -u file             True if file exists and is has its setuid bit
                               set.
           -w file             True if file exists and is writable by
                               current process.
           -x file             True if file exists and is executable by
                               current process.  If file exists and is a
                               directory, the current process has permission
                               to search in the directory.
           -z string           True if length of string is zero.
           -L file             True if file exists and is a symbolic link.
           -O file             True if file exists and is owned by the
                               effective user ID of this process.
           -G file             True if file exists and its group matches the
                               effective group ID of this process.
           -S file             True if file exists and is a socket.
           file1 -nt file2     True if file1 exists and is newer than file2.
           file1 -ot file2     True if file1 exists and is older than file2.
           file1 -ef file2     True if file1 and file2 exist and refer to
                               the same file.
           string = pattern    True if string matches pattern.
           string != pattern   True if string does not match pattern.
           string1 < string2   True if string1 comes before string2 based on
                               the ASCII value of their characters.
           string1 > string2   True if string1 comes after string2 based on
                               the ASCII value of their characters.
           exp1 -eq exp2       True if exp1 is equal to exp2.
           exp1 -ne exp2       True if exp1 is not equal to exp2.
           exp1 -lt exp2       True if exp1 is less than exp2.
           exp1 -gt exp2       True if exp1 is greater than exp2.
           exp1 -le exp2       True if exp1 is less than or equal to exp2.
           exp1 -ge exp2       True if exp1 is greater than or equal to
                               exp2.

      A compound expression can be constructed from these primitives by
      using any of the following, listed in decreasing order of precedence.

           (expression)                       True, if expression is true.
                                              Used to group expressions.
           ! expression                       True if expression is false.
           expression1 && expression2         True, if expression1 and
                                              expression2 are both true.
           expression1 || expression2         True, if either expression1 or
                                              expression2 is true.