bsearch is one of the many useful commands available in the Tclx extension.
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 criteria 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).
See also the lsearch command's -sorted option, though that works on Tcl lists in memory.