POSIX Shell Conventions: Is it given that lowercase names are reserved for binaries and uppercase names are intended for variables?

Question

Background

I'm writing some scripts for standing up WSL instances with regularity, due to accidentally ruining a previous install, and the painful process of getting it back to a workable state. As part of this, I'm learning a lot about how Unix is managed, since it differs greatly from Windows.

Question

Is there a convention for avoiding name collisions between built-ins/binaries and variables?

This question came to mind because of certain common words for binaries, like groups and file, and I was having to be careful in situations like for file in ~/.ssh.

My Current Understanding

In some cases, it looks like all-lowercase names are the norm for binaries, sometimes kebab-case, and obviously you can't write kebab-case variables. I also don't normally see uppercase names used outside of things like $PATH or $HOME. That said, while those can be considered variables, they also follow the C convention of all-caps for runtime constants, since they are largely static. You can add things to $PATH, but it usually handled by a start-up process, like ~/.bashrc.

Example Code

# User Setup
## Perform setup for each user account provisioned (should only be one)
for USERHOME in /home/*; do
    USER="$(basename "$USERHOME")"
    
    # make directory for SSH keys
    mkdir -p "$USERHOME/.ssh"
    # make development directories
    mkdir -p "$USERHOME/bitbucket" "$USERHOME/github" "$USERHOME/gitlab"
    
    # Copy SSH keys from host machine into each user
    for fp in /mnt/c/Users/some_user/.ssh/*; do
        f="$USERHOME/.ssh/$(basename "$fp")"
        
        # If the SSH file isn't found in the current user's $HOME/.ssh
        if ! test -e "$f"; then
            # Copy from host machine
            cp "$fp" "$f"
        fi

        # test if no extension, which is common for private SSH keys
        # The test compares target file with a substitution that returns everything after a period (commonly the extension)
        # Note: Problem occurs due to directory (~/.ssh) containing period.
        fname = $(basename "$f")
        if test -f "$f" && test "$fname" = "${fname#*.}"; then
            # Private keys shouldn't be available to other users
            chmod 600 "$f"
        else
            # Other files (like public keys) can be read-only to all users
            chmod 644 "$f"
        fi
    done
    
    # Add ssh-agent start command, and add SSH keys
    if ! grep -q "ssh-agent" "$USERHOME/.bashrc"; then
        tee -a "$USERHOME/.bashrc" <<- EOF > /dev/null
            # Note: ssh-agent process count will include grep scan. Check for only one for not-started
            if test \$(ps -ef | grep -c "ssh-agent") -eq 1; then
                eval "\$(ssh-agent -s)"
            elif test -z $SSH_AGENT_PID; then
                export SSH_AUTH_SOCK=$(ls -1 /tmp/ssh-*/agent.* | head -n1)
                export SSH_AGENT_PID="${SSH_AUTH_SOCK#*.}"
            fi

            if test \$(ls ~/.ssh/*_ed25519 2>/dev/null); then
                ssh-add ~/.ssh/*_ed25519
            fi

            if test \$(ls ~/.ssh/*_rsa 2>/dev/null); then
                ssh-add ~/.ssh/*_rsa
            fi
        EOF
    fi
    
    # Add current user to Docker permissions group
    if test $(groups $USER | grep -c "docker") -eq 0; then
        usermod -aG docker "$USER"
    fi

  if test -d /root/.cargo; then
    cp /root/.cargo "$USERHOME"
    echo '. "$HOME/.cargo/env"' >> "$USERHOME/.bashrc"
  fi

    # Switch ownership of the user's HOME to current user (defaults to root)
    chown -fR "$USER" "$USERHOME"
done