$ grub-mkrescue -o kernel.iso isodir 2>&1 > /dev/null

xorriso 1.4.4 : RockRidge filesystem manipulator, libburnia project.

Drive current: -outdev 'stdio:kernel.iso'
Media current: stdio file, overwriteable
Media status : is blank
Media summary: 0 sessions, 0 data blocks, 0 data, 34.2g free
Added to ISO image: directory '/'='/tmp/grub.wW6wRX'
xorriso : UPDATE : 896 files added in 1 seconds
Added to ISO image: directory '/'='/home/bla/isodir'
xorriso : UPDATE : 900 files added in 1 seconds
xorriso : NOTE : Copying to System Area: 512 bytes from file '/usr/lib/grub/i386-pc/boot_hybrid.img'
ISO image produced: 9839 sectors
Written to medium : 9839 sectors at LBA 0
Writing to 'stdio:kernel.iso' completed successfully.