Truncate the file behind the given channel at the given location (or the current by default). Operation is (probably) not supported by non-file channels.

    :   '''chan truncate''' ''channelId'' ?''length''?

Like the ''ftruncate()'' call in C's stdio.

Introduced in Tcl 8.5.

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[AMG]: For one possible use of this command, see [How do I remove one line from a file?].

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