seek - Man Page

seek channelId offset ?origin?

Changes the current access position for channelId.

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.

The offset and origin arguments specify the position at which the next read or write will occur for channelId. Offset must be an integer (which may be negative) and origin must be one of the following:

start

The new access position will be offset bytes from the start of the underlying file or device.

current

The new access position will be offset bytes from the current access position; a negative offset moves the access position backwards in the underlying file or device.

end

The new access position will be offset bytes from the end of the file or device.  A negative offset places the access position before the end of file, and a positive offset places the access position after the end of file.

The origin argument defaults to start.

The command flushes all buffered output for the channel before the command returns, even if the channel is in non-blocking mode. It also discards any buffered and unread input. This command returns an empty string. An error occurs if this command is applied to channels whose underlying file or device does not support seeking.

Note that offset values are byte offsets, not character offsets.  Both seek and tell operate in terms of bytes, not characters, unlike read.

Read a file twice:

set f [open file.txt]
set data1 [read $f]
seek $f 0
set data2 [read $f]
close $f
# $data1 eq $data2 if the file wasn't updated

Read the last 10 bytes from a file:

set f [open file.data]
# This is guaranteed to work with binary data but
# may fail with other encodings...
fconfigure $f -translation binary
seek $f -10 end
set data [read $f 10]
close $f

file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)

access position, file, seek

chan(n), file(n), tell(n).