linux man page for tell

tell(n)                                                        Tcl Built-In Commands                                                       tell(n)

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NAME
       tell - Return current access position for an open channel

SYNOPSIS
       tell channelId
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DESCRIPTION
       Returns an integer string giving the current access position in channelId.  This value returned is a byte offset that can be passed to seek
       in order to set the channel to a particular position.  Note that this value is in terms of bytes, not  characters  like  read.   The  value
       returned is -1 for channels that do not support seeking.

       ChannelId  must  be  an  identifier for an open channel such as a Tcl standard channel (stdin, stdout, or stderr), the return value from an
       invocation of open or socket, or the result of a channel creation command provided by a Tcl extension.

EXAMPLE
       Read a line from a file channel only if it starts with foobar:
              # Save the offset in case we need to undo the read...
              set offset [tell $chan]
              if {[read $chan 6] eq "foobar"} {
                  gets $chan line
              } else {
                  set line {}
                  # Undo the read...
                  seek $chan $offset
              }

SEE ALSO
       file(n), open(n), close(n), gets(n), seek(n), Tcl_StandardChannels(3)

KEYWORDS
       access position, channel, seeking

Tcl                                                                     8.1                                                                tell(n)