console shows "unknown operand" from a if statement

Question

I'm trying to script an automatic md5sum check for my embedded system running uClinux. The script is generated on my computer as well as the tar file I want to check.

The script goes like this :

#!/bin/sh
filename='My_File'
md5='d4deeac6f655ee5d4b9ec150fc6957a5'

if test ! -e $filename.tar
then
    echo Update file does not exist
    exit 1
fi  

if [ -z `md5sum "$filename.tar" | grep $md5` ]
then
    echo 'md5sum is not correct'
    exit 1
else
    echo 'md5sum is correct'
fi  

tar -xvf "$filename.tar"
[...]

The md5sum check run as expected, i-e the script stops when the checksum is wrong and executes to the end otherwise. But when the checksum is correct, I get this message from the console :

[: My_File.tar: unknown operand

I don't understand why I get this, and I think this is not accurate to let my script like this. Can someone explain me what's the shell is doing and how to get rid of this message ?

Thanks

Answer 1

Quote the output of md5sum so it's not split into multiple words. -z only expects one operand.

if [ -z "`md5sum "$filename.tar" | grep $md5`" ]

While we're here, might as well switch to the nicer $(...) syntax.

if [ -z "$(md5sum "$filename.tar" | grep $md5)" ]