The test command is your best bet.

Look at the bottom -z is for zero test and -n is for non-zero test.


man test
test(1) test(1)
NAME
test - condition evaluation command
SYNOPSIS
test expr
[ expr ]
DESCRIPTION
The test command evaluates the expression expr and, if its value is
True, returns a zero (true) exit status; otherwise, a nonzero (false)
exit status is returned. test also returns a nonzero exit status if
there are no arguments. The following primitives are used to
construct expr:
-r file True if file exists and is readable.
-w file True if file exists and is writable.
-x file True if file exists and is executable.
-f file True if file exists and is a regular file.
-d file True if file exists and is a directory.
-c file True if file exists and is a character special
file.
-b file True if file exists and is a block special file.
-p file True if file exists and is a named pipe (fifo).
-u file True if file exists and its set-user-ID bit is
set.
-g file True if file exists and its set-group-ID bit is
set.

-k file True if file exists and its sticky bit is set.

-s file True if file exists and has a size greater than
zero.

-h file True if file exists and is a symbolic link.

-t [fildes] True if the open file whose file descriptor number is fildes (1 by default) is associated with a terminal device.

-z s1 True if the length of string s1 is zero.


-n s1 True if the length of the string s1 is non-zero.

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