defining functions in TWRP
When in recovery mount your /system using
adb shell mount /system

though twrp can be twitchy otherwise use in-twrp mount function to mount system and you should have full room to do what you need.
In recovery you have a functioning kernel, so after mounting your system you should have every thing at your disposal

adb shell mount /system
adb shell
~ # mount | grep system
/dev/block/platform/msm_sdcc.1/by-name/system on /system type ext4 (rw,seclabel,relatime,data=ordered)
~ # export PATH=/system/bin:/system/xbin:$PATH
~ # which busybox
/system/xbin/busybox
~ # which sh
/system/bin/sh