gistfile1.sh

# Parse a provisioning profile
# Extract the first DeveloperCertificates <data> entry
# Remove any leading whitespace
# Remove any blank lines
# Base64 decode the blob
# Parse the .cer with OpenSSL
# Extract the first line, which is the certificate subject (the rest is the cert blob)
# End up with a string like: subject= /UID=AABBCCDDEE/CN=iPhone Developer: First Last (FFGGHHIIJJ)/C=US

# Note: Uses xmlstarlet to parse the plist, but you could probably use PlistBuddy or grep, too

security cms -D -i "/path/to/some.mobileprovision" | \
xml sel -t -v "/plist/dict/key[. = 'DeveloperCertificates']/following-sibling::array[1]/data[1]" | \
awk '{print $1}' | sed '/^$/d' | base64 -D | openssl x509 -subject -inform der | head -n 1