работа со строками в bash - 4: Bad substitution

root ~ # outf=`fdisk -l|grep 'Disk /dev/sd'`
Disk /dev/md127 doesn't contain a valid partition table
Disk /dev/md126 doesn't contain a valid partition table
Disk /dev/dm-0 doesn't contain a valid partition table
Disk /dev/dm-1 doesn't contain a valid partition table
Disk /dev/dm-2 doesn't contain a valid partition table
Disk /dev/dm-3 doesn't contain a valid partition table
Disk /dev/dm-4 doesn't contain a valid partition table
Disk /dev/dm-5 doesn't contain a valid partition table
Disk /dev/dm-6 doesn't contain a valid partition table
root ~ # echo $outf 
Disk /dev/sda: 500.1 GB, 500107862016 bytes Disk /dev/sdb: 500.1 GB, 500107862016 bytes
root ~ # t=${outf:1:13}
root ~ # echo $t
isk /dev/sda:
root ~ # echo $BASH_VERSION 
4.1.5(1)-release