Simple howtos

This collection of mini howtos is a dynamic copy of the Unix Toolbox. This page extracts the XML content directly from the original XHTML DOM and displays only the requested node. Source code here (use "save as").
Unix Toolbox revision 14.4
Copyright (c) 2007-2012 Colin Barschel. Some rights reserved under Creative Commons [Attribution - Share Alike]
File System
VPN with SSH
Encrypt Files
Encrypt Partitions
SSL Certificates
Useful Commands
Install Software
Convert Media
Disk Quota
Online Help


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


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


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 > 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 > 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 &


The program top displays running information of processes. See also the program htop from (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:


Terminate or send a signal with kill or killall.
# ping -i 60 > 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: