External Dependencies on Unix Commands

hackdaddy has asked for the wisdom of the Perl Monks concerning the following question:

Because my Perl utility script has external dependencies on Unix commands, how can I be sure that the Unix commands will be available in the location I expect them to be? How can I be sure that they will be the versions I expect? How can I be sure the Unix command is not an alias?

/bin/echo
/bin/which
/bin/df
/bin/uname
/bin/awk
/bin/ls
sqlplus
grep
find
gtar
[download]

On the Windows platform, there is a standard Windows API for querying the machine's hardware and configuration. When scripting in a Unix environment, how does the programmer ensure standard results when using external Unix commands?

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