The ceph osd ls command simply returns a list of the OSD numbers currently deployed within the cluster.

    
    # ceph osd ls | head -3
    0
    1
    2
    # ceph osd ls |wc
        280     280    1010
    #
  

This information could be derived by processing the output of ceph osd tree, but this is a convenient way to drive all in a single command