This is a command in the [TclX] package.
    :   '''select''' ''readFileIds'' ?''writeFileIds''? ?''exceptFileIds''? ?''timeout''?
Discovers which channels are ready for reading, writing or have an exceptional condition pending. Each of ''readFileIds'', ''writeFileIds'' and ''exceptFileIds'' is a list of channel handles (as produced by [open] or [socket]) to check for the given condition.  The ''timeout'' is how long to wait (a floating-point number of seconds) for a response if none of the given channels currently are in any of the given states; if omitted, this command will wait indefinitely.

This mechanism is also the foundation of the [fileevent] command and Tcl's event loop on [Unix]. This command is constrained in what it can do on [Windows].
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[Richard Suchenwirth] 2007-12-13 - The verb ''select'' has many facets, a popular one being its use in [SQL]. Here I chose it for a slight abstraction over [tk_chooseDirectory] and [tk_getOpenFile], mostly to escape from  "quoting hell". It starts from the directory that the associated variable contains, and does not change the variable if canceled.

 proc select {what _var} {
    upvar #0 $_var var
    set res ""
    switch -- $what {
        dir  {set res [tk_chooseDirectory -initialdir $var]}
        file {set res [tk_getOpenFile -initialdir [file dir $var]]}
        default {error "usage: select dir|file"}
    }
    if {$res ne ""} {set var $res}
 }

Practical effect: while gaining the initial directory and cancel functionalities, calls like
     button $f.2 -text ... -command "set $varname \[tk_getOpenFile\]" 
could be simplified to
     button $f.2 -text ...  -command [list select file $varname]

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