cc -Wall -Werror -Wmissing-prototypes -I../posix_spawn -g -O2
    -fsanitize=undefined -o cush -L../posix_spawn cush.o shell-grammar.o
    list.o shell-ast.o termstate_management.o utils.o signal_support.o
    -lspawn -ll -lreadline
/usr/bin/ld:
    termstate_management.o:/.../cs3214-cush/src/termstate_management.h:6:
    multiple definition of `shell_pgrp';
    cush.o:/.../cs3214-cush/src/termstate_management.h:6: first defined here
collect2: error: ld returned 1 exit status
make: *** [Makefile:27: cush] Error 1