There are times when a Tcl command can fail because of the data being passed in to it ... and when it fails, it can do so with a spectacular crash! For example, the glob command which matches files to a pattern (Tcl's ls or dir if you like to think of it that way) can go 'belly up' if there are no files at all that match the pattern:

no files matched glob pattern "*.lua"
   while executing
"glob *.lua"
   invoked from within
"set file [glob *.lua]"
   (file "tcl/falls" line 15)

OOooops!

If you want your Tcl program to continue, even on an error, you should use the catch command and run the command that may fail within a deferred block that's passed in to it ... something like:

catch {glob *.lua} yikes.

It works like this:
• If the command WORKS, the result is put back into the yikes variable, and catch returns a true value, but
• if the command FAILS, an error message is put into the yikes variable, and catch returns a false value.

There's a code example here that illustrates catching (on glob) with testihng the conditionals. And further examples here (failing) and here (caught). There's also a further explanation on the blog here, and a demonstration of how catch can handle a failure to open a file here.

When you are first writing a piece of code - writing what's known as a 'proof of concept' or a 'spike solution', you may not be thinking about error handling too much. But it reallity such code testing / defensive coding against errors is VITAL. So we make certain that we cover catch and loads of associated subjects on our Tcl Programming courses.
(written 2009-10-22)

 T214 - Tcl/Tk - Other Facilities in Tcl
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3287] Exceptions - Tcl style - (2011-05-12)
  [1338] Handling Binary data in Tcl (with a note on C) - (2007-09-09)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1277] AgtInvoke - a command to drive Agilent Tcl software extensions - (2007-07-26)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [461] Shortened interactive commands - (2005-10-11)
  [366] Error handling in Tcl through catch - (2005-07-02)
  [364] pu daily and p hourly - (2005-06-30)
  [239] What and why for the epoch - (2005-03-08)

T209 - Tcl/Tk - File and Directory Handling
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [3320] Reading the nth line from a file (Perl and Tcl examples) - (2011-06-09)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1407] Reading from another process in Tcl (pipes and sockets) - (2007-10-26)
  [785] Running external processes in Tcl and Tcl/Tk - (2006-06-29)
  [779] The fragility of pancakes - and better structures - (2006-06-26)

Back to Tcl - passing arrays and strings in and back out of procs

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A short form of if ... then ... else
Windows 7 and Open Source Programming
Beyond the Pale
What are Tcl lists?
Tcl - catching an error before your program crashes
Tcl - passing arrays and strings in and back out of procs
Melksham Town - asleep or awake?
Beauty in pictures
Tcl - a true interpretive, command based language
Python - how it saves on compile time