unpack image can not work

[boystray]

the command and result is here:

ThinkPad-T420:~/rom/mkbootimg_tools$ sh mkboot boot.img output
output exists, delete?(N/y)y
Unpack & decompress boot.img to output
mkboot: 214: mkboot: Bad substitution

[Cryolitia]

I have the same problem, you can jast solve it by using bash instead of sh.