exec su -c "external_com" - muser

PID USER      PRI  NI  VIRT   RES   SHR S CPU% MEM%   TIME+  Command
    1 root       20   0 37352  2500  2192 S  0.0  0.0  0:00.02 su -c external_com
    5 muser      20   0 85548  6292  5156 S  0.0  0.0  0:00.47 └─ external_com
   13 muser      20   0 85548  6292  5156 S  0.0  0.0  0:00.35    ├─ external_com
   12 muser      20   0 85548  6292  5156 S  0.0  0.0  0:00.02    ├─ external_com