Verilog testbench code using gEDA and iVerilog

My assignment is to code a simple 2 to 4 decoder and then display the possible outcomes and waveform.

I am using the gEDA suite along with Icarus Verilog (iVerilog) as a compiler and GTKWave for the waveform.

This is my first time coding with Verilog or working with the gEDA suite. From googling it appears I need to follow this designflow:

think about a design you want to implement. In my case a Decoder
implement the design in VHDL/Verilog.
implement a testbench in VHDL/Verilog.
Compile design file and testbench file with iVerilog
Use testbench and .vcd dump file to display waveform using GTKWave

The testbench file wont compile and I am not sure why, I have tried several variations and I keep getting errors. Any help is much appreciated. Thank you.

Here is my design file code:

// 2 to 4 Decoder
// File Name: decoder.v

module decoder(X,Y,E,Z);
    input X,Y,E;
    output [0:3]Z;
    wire [0:3]Z;
    wire X1, Y1;

    not
        inv1(X1,X),
        inv2(Y1,Y);
    and
        and1(Z[0],X1,Y1,E),
        and2(Z[1],Y1,X,E),
        and3(Z[2],Y,X1,E),
        and4(Z[3],X,Y,E);
endmodule

Here is my testbench code:

module decoder_tb;
    input X,Y,E;
    output [0:3]Z;
    //wire [0:3]Z;
    //wire X1, Y1;  

    // should create .vcd dump file for GTKWave
    initial
        begin
            $dumpfile("decoder.vcd");
            $dumpvars();    
        end

    decoder decode(X,Y,E,Z);
    initial 
        begin
            $display($time,"<< Z[0]=%d   Z[1]=%d   Z[2]=%d   Z[3]=%d >>", Z[0] , Z[1] , Z[2] , Z[3] );  
        end 

    initial 
        begin 
         #0
         X = 0; Y = 0; E = 1; 
         #5 
         X = 0; Y = 1; E = 1;
         #10 
         X = 1; Y = 0; E = 1;
         #15 
         X = 1; Y = 1; E = 1;
        end 
endmodule

The commands in terminal I am using are:

iverilog -o decoder.vvp decoder.v decoder_tb.v
gtkwave decoder.vcd

EDIT: Here is the exact error message

aj@aj:~/verilogCode$ iverilog -o decoder.vvp decoder.v decoder_tb.v
decoder_tb.v:26: error: X is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : X is declared here as wire.
decoder_tb.v:26: error: Y is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : Y is declared here as wire.
decoder_tb.v:26: error: E is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : E is declared here as wire.
decoder_tb.v:28: error: X is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : X is declared here as wire.
decoder_tb.v:28: error: Y is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : Y is declared here as wire.
decoder_tb.v:28: error: E is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : E is declared here as wire.
decoder_tb.v:30: error: X is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : X is declared here as wire.
decoder_tb.v:30: error: Y is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : Y is declared here as wire.
decoder_tb.v:30: error: E is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : E is declared here as wire.
decoder_tb.v:32: error: X is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : X is declared here as wire.
decoder_tb.v:32: error: Y is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : Y is declared here as wire.
decoder_tb.v:32: error: E is not a valid l-value in decoder_tb.
decoder_tb.v:6:      : E is declared here as wire.
12 error(s) during elaboration.

Solution

In your testbench, change input to reg and output to wire. This fixes the compile errors for me (although, I am not using gEDA or iVerilog):

module decoder_tb;
    reg X,Y,E;
    wire [0:3]Z;

My simulators gave much more meaningful error messages than yours in this case:

Identifier 'X' does not appear in port list.