bsearch is one of the many useful commands available in the Tclx extension.
bsearch fileId key ?retvar? ?compare_proc?
Search an opened file fileId containing lines of text sorted
into ascending order for a match. Key contains the string
to match. If retvar is specified, then the line from the
file is returned in retvar, and the command returns 1 if key
was found, and 0 if it wasn't. If retvar is not specified
or is a null name, then the command returns the line that
was found, or an empty string if key wasn't found.
By default, the key is matched against the first white-space
separated field in each line. The field is treated as an
ASCII string. If compare_proc is specified, then it defines
the name of a Tcl procedure to evaluate against each line
read from the sorted file during the execution of the
bsearch command. Compare_proc takes two arguments, the key
and a line extracted from the file. The compare routine
should return a number less than zero if the key is less
than the line, zero if the key matches the line, or greater
than zero if the key is greater than the line. The file
must be sorted in ascending order according to the same cri-
teria compare_proc uses to compare the key with the line, or
erroneous results will occur.
This command at one time did not work on files containing binary data (bytes of zero).