Android : how to check if a file exist using adb shell

I have a bash script which goes as follows

if [ -f "/sdcard/testfile"]
then 
echo "exists" > /sdcard/outfile
else
echo "does not exist" > /sdcard/outfile
fi

I have sufficient permission to run this with /system/bin/sh I am calling this script from my application and running this with /system/bin/sh But after running I am getting false, even if the file '/sdcard/testfile' is there. When I am explicitly running in adb shell, I am getting this error

**[: not found**

Is there any other way to accomplish this task? I cannot just use java.io.File because of permission issue of application, therefore I am adhering to shell script (command).