ChannelId must be an identifier for an open channel such as the Tcl standard input channel (stdin), the return value from an invocation of open or socket, or the result of a channel creation command provided by a Tcl extension. The channel must have been opened for input.
If varName is omitted the line is returned as the result of the command. If varName is specified then the line is placed in the variable by that name and the return value is a count of the number of characters returned.
If end of file occurs while scanning for an end of line, the command returns whatever input is available up to the end of file. If channelId is in non-blocking mode and there is not a full line of input available, the command returns an empty string and does not consume any input. If varName is specified and an empty string is returned in varName because of end-of-file or because of insufficient data in non-blocking mode, then the return count is -1. Note that if varName is not specified then the end-of-file and no-full-line-available cases can produce the same results as if there were an input line consisting only of the end-of-line character(s). The eof and fblocked commands can be used to distinguish these three cases.
set chan [open "some.file.txt"]
set lineNumber 0
while {[gets $chan line] >= 0} {
puts "[incr lineNumber]: $line"
}
close $chan