This variable identifies whether or not the tcl interpreter
is operating in interactive mode or not.  If its value is 1 ,
then the interpreter acts as if it were an interactive interpreter. See [tclsh] for a list of the
differences between interactive and non-interactive mode.

[Ro] You can shorten commands in interactive mode, e.g.: "inf" for "info".

[RS] 2004-03-24 Yup - and this is especially delightful on a [PocketPC] where less stylus-tapping is better. If the abbreviation is unambiguous, like ''ll'' for [llength], it saves time and screen space; if it is ambiguous (e.g. 'l'), it is another [introspection] helper - the educational error message shows you the possible completions, just as [[info command l*]] would have done. 

 % l
 ambiguous command name "l": label labelframe ladd lappend ldiff lexpr lframe
 lindex linsert list list:add list:clear list:filter list:popup list:show 
 listbox ljtext llength lmap lmirrorx load loadvfs loop lower lpop lpush lradio 
 lrange lreplace lrevert ls lsearch lset lsort lspread lsuniq lswap luniq
 % ll
 wrong # args: should be "llength list"

Just make sure that the interactive command is tried at global level (0), so that [unknown] (which provides abbreviation mode) finds itself at level 1.
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[Tcl syntax help]