Is there a way to lock a branch in GIT

I have an idea of locking a repository from users pushing files into it by having a lock script in the GIT update hook since the push can only recognize the userid as arguments and not the branches. So i can lock the entire repo which is just locking a directory.

Is there a way to lock a specific branch in GIT?

Or is there a way an Update Hook can identify from which branch the user is pushing and to which branch the code is pushed?

The branch being pushed to is the first parameter to the update hook. If you want to lock the branch myfeature for pushing, this code (placed in hooks/update) will do it:

#!/bin/sh
# lock the myfeature branch for pushing
refname="$1"

if [[ $refname == "refs/heads/myfeature" ]]
then
    echo "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
    echo "You cannot push to myfeature! It's locked"
    echo "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
    exit 1
fi
exit 0

The update hook, from the docs:

The hook executes once for each ref to be updated, and takes three parameters:

• the name of the ref being updated,
• the old object name stored in the ref,
• and the new objectname to be stored in the ref.

So... yes, it knows exactly what branch is being pushed, and can simply check that parameter and exit failure if it doesn't want the branch pushed to.

And if you want to (intelligently) do this before the user has uploaded the objects, you can use the pre-receive hook:

This hook executes once for the receive operation. It takes no arguments, but for each ref to be updated it receives on standard input a line of the format:

<old-value> SP <new-value> SP <ref-name> LF

where <old-value> is the old object name stored in the ref, <new-value> is the new object name to be stored in the ref and <ref-name> is the full name of the ref.

(those are spaces and line-feed)