On unix systems the mtx utility is useful for media movement within a tape library. This article describes some of the most common commands for tape movement within a library. The mtx utility takes commands of the form
mtx -f <sg device> <command>

Library inventory

mtx -f <sg device> status
For example:
# mtx -f /dev/sg6 status
  Storage Changer /dev/sg6:4 Drives, 121 Slots ( 1 Import/Export )
Data Transfer Element 0:Empty
Data Transfer Element 1:Empty
Data Transfer Element 2:Empty
Data Transfer Element 3:Empty
      Storage Element 1:Full :VolumeTag=SDE121S                         
      Storage Element 2:Full :VolumeTag=SDE122S                         
      Storage Element 3:Full :VolumeTag=SDE123S                         
      Storage Element 4:Full :VolumeTag=SDE124S                         
      Storage Element 5:Full :VolumeTag=SDE125S                         
      Storage Element 6:Full :VolumeTag=SDE126S                         
      Storage Element 7:Empty
      Storage Element 8:Empty
      Storage Element 120:Empty
      Storage Element 121 IMPORT/EXPORT:Full :VolumeTag=SDE120S

Drive numbers are specified starting from zero. i.e Drive 0, Drive 1 etc. While storage element addresses are specified starting from 1. i.e Slot 1, Slot 2 etc

Load a tape drive

mtx -f <sg device> first <drive number>
The above command loads the tape drive specified with a tape cartridge from the first storage slot. If driver number is not specified, then the first drive (drive 0) is used
# mtx -f /dev/sg6 first
Loading media from Storage Element 1 into drive 0...done
#mtx -f /dev/sg6 first 3
Loading media from Storage Element 1 into drive 3...done

Unload a tape drive

mtx -f <sg device> unload
The above command unloads the first tape drive
For example:
# mtx -f /dev/sg6 unload
Unloading drive 0 into Storage Element 1...done
When unloading a tape drive, the tape drive number can also be specified using the command below
mtx -f <sg device> unload <slot number> <drive number>
For example:
mtx -f /dev/sg6 unload 1 3
Unloading drive 3 into Storage Element 1...done

Moving from one tape to next

It is possible to load the cartridges in a sequential manner, for example load the first cartridge, unload the first cartridge, load the second cartridge etc. This is possible using the 'next' command

As an example:
# mtx -f /dev/sg6 first
Loading media from Storage Element 1 into drive 0...done
# mtx -f /dev/sg6 next
Unloading drive 0 into Storage Element 1...done
Loading media from Storage Element 2 into drive 0...done

Similar to the first command the next command also takes a drive number. 
Also the next slot that would be use will depend on the slot from which the current tape was loaded. 
For example consider the following

Data Transfer Element 2:Full (Storage Element 4 Loaded):VolumeTag = SDE123S
Drive 2 was loaded with a tape from storage slot 4
The following command will load drive 2 with tape from storage slot 5
# mtx -f /dev/sg6 next 2
Unloading drive 2 into Storage Element 4...done
Loading media from Storage Element 5 into drive 2...done

Specifying the tape drive and slots for load and unload

It is possible to specify the tape drive to use in a multi-tape drive library system and also the specific storage slot to use.

For example:
# mtx -f /dev/sg6 load 4 3
Loading media from Storage Element 4 into drive 3...done
# mtx -f /dev/sg6 unload 10 3
Unloading drive 3 into Storage Element 10...done

Moving tapes between slots and changer ports

mtx -f <sg device> transfer <source slot> <destination slot>
# mtx -f /dev/sg6 transfer 2 121
# mtx -f /dev/sg6 status
      Storage Element 2:Empty
      Storage Element 121 IMPORT/EXPORT:Full :VolumeTag=SDE121S

The above command also exports a tape from the library, while the opposite would have imported a tape into the library

Further reading

Read the man page of mtx for all commands and options supported by mtx. Some take libraries will require that you eject a tape from the drive before media movement is possible, also some tape libraries require that you position the picker to an element first before media movement is possible etc.