GNU tar User Guide - Magnetic Tape Control

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5.4.2 Magnetic Tape Control

Backup scripts access tape device using special hook functions. These functions take a single argument – the name of the tape device. Their names are kept in the following variables:

— Backup variable: MT_BEGIN

The name of begin function. This function is called before accessing the drive. By default it retensions the tape:
          MT_BEGIN=mt_begin
          
          mt_begin() {
              mt -f "$1" retension
          }
     

— Backup variable: MT_REWIND

The name of rewind function. The default definition is as follows:
          MT_REWIND=mt_rewind
          
          mt_rewind() {
              mt -f "$1" rewind
          }
     

— Backup variable: MT_OFFLINE

The name of the function switching the tape off line. By default it is defined as follows:
          MT_OFFLINE=mt_offline
          
          mt_offline() {
              mt -f "$1" offl
          }
     

— Backup variable: MT_STATUS

The name of the function used to obtain the status of the archive device, including error count. Default definition:
          MT_STATUS=mt_status
          
          mt_status() {
              mt -f "$1" status
          }
     