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Pro Widely available
You can expect a reasonably recent version of Tcl to either installed or available in the repositories of any popular open source *nix.
Pro Sane quoting rules
Unlike in *sh you don't need to quote your variable substitutions ('"$1"').
Pro Standard library
Unlike *sh Tcl has a rich standard library.
Pro Everything is a string
Tcl can operate at the same level of abstraction as the POSIX shell, which makes it easier to manipulate the output of other programs.
With a little care you can have the same script work on Linux, *BSD, OS X and Windows.
Pro Standalone packages
Tcl enables easy deployment through self-contained binaries known as starpacks.