Spin up an Ubuntu box, ls /bin, ls /usr/bin and half an hour of manually trying different things. Good times!
Final list is:
shuf,tail,more,uniq,sort,tac,head,ls,find,expr,expand,echo,dir,erb,fold
Contents of "food" was the string "food".
https://advancedpersistentjest.com/2018/05/21/writeups-sign-cpushop-cats-rctf-part-1/