This page addresses the problem how to tell if an executable is available.
Lars H: Isn't that what auto_execok is for?
Let's say we wish to check availability of 'cdrecord'.
The simplest method is to check with 'which':
if {[catch {exec which cdrecord} cerr]} {
puts stderr "'cdrecord' not found in $::env(PATH)"
} else {
set ::cdrecord_exec $cerr
puts stderr "using 'cdrecord' via $cdrecord_exec"
}However this method requires a POSIX host system to offer the command "which". In fact even a POSIX system might fail. For example, Cygwin have a hybrid mix of win32 tcl/tk and their POSIX environment. Command "which" would tell cdrecord is available as a symlink, but tcl/tk script running on win32 engine cannot invoke symlink, thus positive answer from 'which' doesn't grantee availability of the executable.
A more complicated method is to actually run the target executable to do a simple thing, like printing its version number. The code is a bit complicated because you need to worry if version number is written on stderr, resulting failed exec. Thus a second check at $::errorCode is necessary:
if {[catch {exec cdrecord -version}] == 0 || [lindex $::errorCode 0] == "NONE" } {
puts stderr "'cdrecord' is available"
}