tell manual page - Tcl Built-In Commands

## Tcl8.6.4 Documentation > Tcl Commands, version 8.6.4 > tell

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### NAME

tell — Return current access position for an open channel

### SYNOPSIS

tell channelId

### 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, open, close, gets, seek, Tcl_StandardChannels

### KEYWORDS

access position, channel, seeking
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