Check if a user is in a group

Question

I have a server running where I use php to run a bash script to verify certain information of a user. For example, I have a webhosting server set up, and in order to be able to

Answer 1

A slightly more error-proof method to check for group membership using zero char delimited fixed string grep.

if id -nGz "$USER" | grep -qzxF "$GROUP"
then
    echo User \`$USER\' belongs to group \`$GROUP\'
else
    echo User \`$USER\' does not belong to group \`$GROUP\'
fi

or using long opts

if id --name --groups --zero "$USER" | 
   grep --quiet --null-data --line-regexp --fixed-strings "$GROUP"
then
    echo User \`$USER\' belongs to group \`$GROUP\'
else
    echo User \`$USER\' does not belong to group \`$GROUP\'
fi