It's not the Terminal which creates the zero byte file but a bunch of Python scripts which either don't like your ttf font or the dummy DSIG file. Or the original ttf file already contains a digital signature!
The best you can try is the following:
Dump the copied ttf file (in my example I copied the TrueType font file Andale Mono.ttf to a folder named FontMod on my desktop):
ttx /Users/user/Desktop/FontMod/Andale\ Mono.ttf
Remove or rename the original ttf file in the working folder. Open the resulting ttx file with a decent text editor like TextWranglerTextWrangler, BBEdit or even nano.
If the ttx file doesn't contain a <DSIG>...</DSIG> section simply add the below snippet at the end just in front of the closing </ttFont> tag.
<DSIG>
<!-- note that the Digital Signature will be invalid after recompilation! -->
<tableHeader flag="0x1" numSigs="1" version="1"/>
<SignatureRecord format="1">
-----BEGIN PKCS7-----
0000000100000000
-----END PKCS7-----
</SignatureRecord>
</DSIG>
and save the result.
If the ttx file already contains a DSIG section you may try to replace the PKCS7 part:
<DSIG>
<!-- note that the Digital Signature will be invalid after recompilation! -->
<tableHeader flag="0x1" numSigs="1" version="1"/>
<SignatureRecord format="1">
-----BEGIN PKCS7-----
MIIllQYJKoZIhvcNAQcCoIIlhjCCJYICAQExCzAJBgUrDgMCGgUAMGEGCisGAQQBgjcCAQSgUzBR
...
tCPnxeRQVRtMEWQXYd27Fkhu/dLhBloN2ti3CGPKPTneEW7TrcwcpUH4yvX9gyzjXQ==
-----END PKCS7-----
</SignatureRecord>
</DSIG>
by
<DSIG>
<!-- note that the Digital Signature will be invalid after recompilation! -->
<tableHeader flag="0x1" numSigs="1" version="1"/>
<SignatureRecord format="1">
-----BEGIN PKCS7-----
0000000100000000
-----END PKCS7-----
</SignatureRecord>
</DSIG>
and save the result.
In either case rebuild the font file:
ttx /Users/user/Desktop/FontMod/Andale\ Mono.ttx
If you still need a snippet to create a disg.ttx file use something like that:
...
cat > dsig.ttx<<EOF
<?xml version="1.0" encoding="UTF-8"?>
<ttFont sfntVersion="\x00\x01\x00\x00" ttLibVersion="3.0">
<DSIG>
<!-- note that the Digital Signature will be invalid after recompilation! -->
<tableHeader flag="0x1" numSigs="1" version="1"/>
<SignatureRecord format="1">
-----BEGIN PKCS7-----
0000000100000000
-----END PKCS7-----
</SignatureRecord>
</DSIG>
</ttFont>
EOF
Please note that the xml encoding and the ttLibVersion probably have to be adapted to the respective versions in the ttf file.