dontdiewhentherearenoprocessesmatchingregexp

Reference: /n/sources/patch/applied/dontkill
Date: Sun Jun 15 19:30:16 CES 2008
Signed-off-by: rsc@swtch.com

--- /rc/bin/dontkill	Sun Jun 15 19:30:04 2008
+++ /rc/bin/dontkill	Sun Jun 15 19:30:03 2008
@@ -6,4 +6,5 @@
 	exit usage
 }
 # see /sys/src/9/port/proc.c:/^killbig
-exec chmod -w /proc/^`{psu | awk '$NF ~ /'$1'/ {print $2}'}^/ctl
+pids=`{psu | awk '$NF ~ /'$1'/ {print $2}'}
+~ $#pids 0 || chmod -w /proc/^$pids^/ctl