I sorted my RNA-Seq bam files after mapping by star, and here is my key command line in a for cycle bash file: 

    samtools sort $file -o ${filename}.sorted.bam

At last, the one error file shows like:  

    [bam_sort_core] merging from 6 files and 1 in-memory blocks...
    [bam_sort_core] merging from 4 files and 1 in-memory blocks...
    [bam_sort_core] merging from 8 files and 1 in-memory blocks...
    [bam_sort_core] merging from 9 files and 1 in-memory blocks...
    [bam_sort_core] merging from 7 files and 1 in-memory blocks...
    [bam_sort_core] merging from 4 files and 1 in-memory blocks...
    [bam_sort_core] merging from 5 files and 1 in-memory blocks...
    [bam_sort_core] merging from 3 files and 1 in-memory blocks...
    [bam_sort_core] merging from 6 files and 1 in-memory blocks...
    [bam_sort_core] merging from 13 files and 1 in-memory blocks...
    [bam_sort_core] merging from 8 files and 1 in-memory blocks...
    [bam_sort_core] merging from 2 files and 1 in-memory blocks...
    ...

Something wrong here? I searched but still don't know why, so I'm not sure these sorted files be used for next step?  
What's more, I have M(like 63) files, and there M-N(like 6, yes, few) lines, it means not all file will meet this problem? It makes me more confused. 

Any ideas will be appreciated!!!
These are not error messages, just debugging notes. 

Large files cannot be sorted in memory thus get saved into temporary files. Once the sort completes the temporary files are removed.

There is nothing to be concerned about