How to Trace Unix System Calls for a Process

The following platforms support a trace utility that can be used to identify what a process is doing:
O/S Version Trace Utility
Sun Solaris 2.x, Unixware 7.0 truss, e.g.:

Unixware 7.0
$ truss -aefo

Solaris
$ truss -rall -wall -p

HP/UX 11 tusc, e.g.:
$ tusc -afpo

IBM AIX 4.x sctrace, e.g.:
$ sctrace -Amo

Linux strace, e.g.:
$ strace -fo

$ strace -p

SGI IRIX 6.x par, e.g.:
$ par -siSSo

Compaq Tru64 Unix trace, e.g.:
$ trace -fo

Sequent Dynix/PTX truss, e.g.:
$ truss -aefo