Listing | Priority | Background/Foreground | Top | Kill
Listing and PIDs
Each process has a unique number, the PID. A list of all running process is retrieved with
ps.
# ps -auxefw # Extensive list of all running process
However more typical usage is with a pipe or with pgrep (for OS X install proctools from
MacPorts):
# ps axww | grep cron
586 ?? Is 0:01.48 /usr/sbin/cron -s
# ps axjf # All processes in a tree format (Linux)
# ps aux | grep 'ss[h]' # Find all ssh pids without the grep pid
# pgrep -l sshd # Find the PIDs of processes by (part of) name
# echo $$ # The PID of your shell
# fuser -va 22/tcp # List processes using port 22 (Linux)
# pmap PID # Memory map of process (hunt memory leaks) (Linux)
# fuser -va /home # List processes accessing the /home partition
# strace df # Trace system calls and signals
# truss df # same as above on FreeBSD/Solaris/Unixware
Priority
Change the priority of a running process with renice. Negative numbers have a higher
priority, the lowest is -20 and "nice" have a positive value.
# renice -5 586 # Stronger priority
586: old priority 0, new priority -5
Start the process with a defined priority with nice. Positive is "nice" or weak, negative
is strong scheduling priority. Make sure you know if /usr/bin/nice or the shell built-in is
used (check with # which nice).
# nice -n -5 top # Stronger priority (/usr/bin/nice)
# nice -n 5 top # Weaker priority (/usr/bin/nice)
# nice +5 top # tcsh builtin nice (same as above!)
While nice changes the CPU scheduler, an other useful command ionice will schedule the disk
IO. This is very useful for intensive IO application (e.g. compiling). You can select a
class (idle - best effort - real time), the man page is short and well explained.
# ionice c3 -p123 # set idle class for pid 123 (Linux only)
# ionice -c2 -n0 firefox # Run firefox with best effort and high priority
# ionice -c3 -p$$ # Set the actual shell to idle priority
The last command is very useful to compile (or debug) a large project. Every command
launched from this shell will have a lover priority. $$ is your shell pid (try echo $$).
FreeBSD uses idprio/rtprio (0 = max priority, 31 = most idle):
# idprio 31 make # compile in the lowest priority
# idprio 31 -1234 # set PID 1234 with lowest priority
# idprio -t -1234 # -t removes any real time/idle priority
Background/Foreground
When started from a shell, processes can be brought in the background and back to the
foreground with [Ctrl]-[Z] (^Z), bg and fg. List the processes with jobs. When needed
detach from the terminal with disown.
# ping cb.vu > ping.log
^Z # ping is suspended (stopped) with [Ctrl]-[Z]
# bg # put in background and continues running
# jobs -l # List processes in background
[1] - 36232 Running ping cb.vu > ping.log
[2] + 36233 Suspended (tty output) top
# fg %2 # Bring process 2 back in foreground
# make # start a long compile job but need to leave the terminal
^Z # suspended (stopped) with [Ctrl]-[Z]
# bg # put in background and continues running
# disown -h %1 # detatch process from terminal, won't be killed at logout
No straight forward way to re-attach the process to a new terminal, try reptyr (Linux).
Use nohup to start a process which has to keep running when the shell is closed (immune to
hangups).
# nohup ping -i 60 > ping.log &
Top
The program top displays running information of processes. See also the program htop from
htop.sourceforge.net (a more powerful version of top) which runs on Linux and FreeBSD
(ports/sysutils/htop/). While top is running press the key h for a help overview. Useful
keys are:
* u [user name] To display only the processes belonging to the user. Use + or blank to
see all users
* k [pid] Kill the process with pid.
* 1 To display all processors statistics (Linux only)
* R Toggle normal/reverse sort.
Signals/Kill
Terminate or send a signal with kill or killall.
# ping -i 60 cb.vu > ping.log &
[1] 4712
# kill -s TERM 4712 # same as kill -15 4712
# killall -1 httpd # Kill HUP processes by exact name
# pkill -9 http # Kill TERM processes by (part of) name
# pkill -TERM -u www # Kill TERM processes owned by www
# fuser -k -TERM -m /home # Kill every process accessing /home (to umount)
Important signals are:
* 1 HUP (hang up)
* 2 INT (interrupt)
* 3 QUIT (quit)
* 9 KILL (non-catchable, non-ignorable kill)
* 15 TERM (software termination signal)
