bastion@host:~$ cat pids.txt
1111
2222
3333
4444
bastion@host:~$ echo $(cat pids.txt)
1111 2222 3333 4444
for i in `echo $(cat killpids.txt)`
UPD: a note with alternative commands from Liviu Chircu, OpenSIPS core developer:
1. instead of `echo $(cat pids.txt)`, a better alternative would be: `cat pids.txt | xargs` 2. instead of "for i in `echo $(cat pids.txt)`; do kill -9 $i; done", a better option would be: `cat pids.txt | xargs kill -9`