IF-Operator

#!/bin/bash
X=""
if [ -n "$X" ]; then    # -n tests to see if the argument is non empty
        echo "the variable X is not the empty string"
else
	echo "empty!"
fi

http://pegasus.rutgers.edu/~elflord/unix/bash-tute.html: A brief summary of test operators

Here's a quick list of test operators. It's by no means comprehensive, but its likely to be all you'll need to remember (if you need anything else, you can always check the bash manpage … )

operator produces true if… number of operands

-n operand non zero length 1

-z operand has zero length 1

-d there exists a directory whose name is operand 1

-f there exists a file whose name is operand 1

-eq the operands are integers and they are equal 2

-neq the opposite of -eq 2

= the operands are equal (as strings) 2

!= opposite of = 2

-lt operand1 is strictly less than operand2 (both operands should be integers) 2

-gt operand1 is strictly greater than operand2 (both operands should be integers) 2

-ge operand1 is greater than or equal to operand2 (both operands should be integers) 2

-le operand1 is less than or equal to operand2 (both operands should be integers) 2

Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the file argument to one of the primaries is of the form `/dev/fd/N', then file descriptor N is checked. If the file argument to one of the primaries is one of `/dev/stdin', `/dev/stdout', or `/dev/stderr', file descriptor 0, 1, or 2, respectively, is checked.

-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 its set-group-id bit is set.

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

-k file True if file exists and its „sticky“ bit is 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 has a size greater than zero.

-t fd True if file descriptor fd is open and refers to a terminal.

-u file True if file exists and its set-user-id bit 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 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.

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

-N file True if file exists and has been modified since it was last read.

file1 -nt file2 True if file1 is newer (according to 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.

-o optname True if shell option optname is enabled. The list of options appears in the description of the `-o' option to the set builtin (see section 4.3 The Set Builtin).

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

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

string1 == string2 True if the strings are equal. `=' may be used in place of `=='.

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

string1 < string2 True if string1 sorts before string2 lexicographically in the current locale.

string1 > string2 True if string1 sorts after string2 lexicographically in the current locale.

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, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.