09-21-2007
Well "ls" will do something while "xargs ls" will hang waiting for input. Suppose you wanted to do an ls on a file called data1 and another called data2. You could do "ls data1 data2". Or you could do "echo data1 data2 | xargs ls". xargs reads from stdin and runs the command using the input as arguments. "ls `echo data1 data2`" is almost the same thing. But xargs can read thousands of words and run as many separate ls commands as needed.
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seek(n) Tcl Built-In Commands seek(n)
__________________________________________________________________________________________________________________________________________________
NAME
seek - Change the access position for an open channel
SYNOPSIS
seek channelId offset ?origin?
_________________________________________________________________
DESCRIPTION
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 back-
wards 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 nonblocking 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.
EXAMPLES
Read a file twice:
set f [open file.txt]
set data1 [read $f]
seek $f 0
set data2 [read $f]
close $f
# $data1 == $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
SEE ALSO
file(n), open(n), close(n), gets(n), tell(n), Tcl_StandardChannels(3)
KEYWORDS
access position, file, seek
Tcl 8.1 seek(n)