gets − Read a line from a channel

gets channelId ?varName?

     This command reads the next line from channelId,
returns everything in the line up to (but not including) the
end‐of‐line character(s), and discards the end‐of‐line

     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.

This example reads a file one line at a time and prints it
out with the current line number attached to the start of
each line.

     set chan [open "some.file.txt"]
     set lineNumber 0
     while {[gets $chan line] >= 0} {
         puts "[incr lineNumber]: $line"
     close $chan


file(3tcl), eof(3tcl), fblocked(3tcl),

blocking, channel, end of file, end of line, line, non‐
blocking, read