# uname -a                           # Get the kernel version (and BSD version)
# lsb_release -a                     # Full release info of any LSB distribution
# cat /etc/SuSE-release              # Get SuSE version
# cat /etc/debian_version            # Get Debian version

Use /etc/DISTR-release with DISTR= lsb (Ubuntu), redhat, gentoo, mandrake, sun (Solaris), and so on. See also /etc/issue.

# uptime                             # Show how long the system has been running + load
# hostname                           # system's host name
# hostname -i                        # Display the IP address of the host. (Linux only)
# man hier                           # Description of the file system hierarchy
# last reboot                        # Show system reboot history

Hardware Informations

Kernel detected hardware

# dmesg                              # Detected hardware and boot messages
# lsdev                              # information about installed hardware
# dd if=/dev/mem bs=1k skip=768 count=256 2>/dev/null | strings -n 8 # Read BIOS

Linux

# cat /proc/cpuinfo                  # CPU model
# cat /proc/meminfo                  # Hardware memory
# grep MemTotal /proc/meminfo        # Display the physical memory
# watch -n1 'cat /proc/interrupts'   # Watch changeable interrupts continuously
# free -m                            # Used and free memory (-m for MB)
# cat /proc/devices                  # Configured devices
# lspci -tv                          # Show PCI devices
# lsusb -tv                          # Show USB devices
# lshal                              # Show a list of all devices with their properties
# dmidecode                          # Show DMI/SMBIOS: hw info from the BIOS

FreeBSD

# sysctl hw.model                    # CPU model
# sysctl hw                          # Gives a lot of hardware information
# sysctl hw.ncpu                     # number of active CPUs installed
# sysctl vm                          # Memory usage
# sysctl hw.realmem                  # Hardware memory
# sysctl -a | grep mem               # Kernel memory settings and info
# sysctl dev                         # Configured devices
# pciconf -l -cv                     # Show PCI devices
# usbdevs -v                         # Show USB devices
# atacontrol list                    # Show ATA devices
# camcontrol devlist -v              # Show SCSI devices

Load, statistics and messages

The following commands are useful to find out what is going on on the system.

# top                                # display and update the top cpu processes
# mpstat 1                           # display processors related statistics
# vmstat 2                           # display virtual memory statistics
# iostat 2                           # display I/O statistics (2 s intervals)
# systat -vmstat 1                   # BSD summary of system statistics (1 s intervals)
# systat -tcp 1                      # BSD tcp connections (try also -ip)
# systat -netstat 1                  # BSD active network connections
# systat -ifstat 1                   # BSD network traffic through active interfaces
# systat -iostat 1                   # BSD CPU and and disk throughput
# tail -n 500 /var/log/messages      # Last 500 kernel/syslog messages
# tail /var/log/warn                 # System warnings messages see syslog.conf

Users

# id                                 # Show the active user id with login and group
# last                               # Show last logins on the system
# who                                # Show who is logged on the system
# groupadd admin                     # Add group "admin" and user colin (Linux/Solaris)
# useradd -c "Colin Barschel" -g admin -m colin
# usermod -a -G <group> <user>       # Add existing user to group (Debian)
# groupmod -A <user> <group>         # Add existing user to group (SuSE)
# userdel colin                      # Delete user colin (Linux/Solaris)
# adduser joe                        # FreeBSD add user joe (interactive)
# rmuser joe                         # FreeBSD delete user joe (interactive)
# pw groupadd admin                  # Use pw on FreeBSD
# pw groupmod admin -m newmember     # Add a new member to a group
# pw useradd colin -c "Colin Barschel" -g admin -m -s /bin/tcsh 
# pw userdel colin; pw groupdel admin

Encrypted passwords are stored in /etc/shadow for Linux and Solaris and /etc/master.passwd on FreeBSD. If the master.passwd is modified manually (say to delete a password), run # pwd_mkdb -p master.passwd to rebuild the database.

To temporarily prevent logins system wide (for all users but root) use nologin. The message in nologin will be displayed (might not work with ssh pre-shared keys).

# echo "Sorry no login now" > /etc/nologin       # (Linux)
# echo "Sorry no login now" > /var/run/nologin   # (FreeBSD)

Limits

Some application require higher limits on open files and sockets (like a proxy web server, database). The default limits are usually too low.

Linux

Per shell/script

The shell limits are governed by ulimit. The status is checked with ulimit -a. For example to change the open files limit from 1024 to 10240 do:

# ulimit -n 10240                    # This is only valid within the shell

The ulimit command can be used in a script to change the limits for the script only.

Per user/process

Login users and applications can be configured in /etc/security/limits.conf. For example:

# cat /etc/security/limits.conf
*   hard    nproc   250              # Limit user processes
asterisk hard nofile 409600          # Limit application open files

System wide

Kernel limits are set with sysctl. Permanent limits are set in /etc/sysctl.conf.

# sysctl -a                          # View all system limits
# sysctl fs.file-max                 # View max open files limit
# sysctl fs.file-max=102400          # Change max open files limit
# echo "1024 50000" > /proc/sys/net/ipv4/ip_local_port_range  # port range
# cat /etc/sysctl.conf
fs.file-max=102400                   # Permanent entry in sysctl.conf
# cat /proc/sys/fs/file-nr           # How many file descriptors are in use

FreeBSD

Per shell/script

Use the command limits in csh or tcsh or as in Linux, use ulimit in an sh or bash shell.

Per user/process

The default limits on login are set in /etc/login.conf. An unlimited value is still limited by the system maximal value.

System wide

Kernel limits are also set with sysctl. Permanent limits are set in /etc/sysctl.conf or /boot/loader.conf. The syntax is the same as Linux but the keys are different.

# sysctl -a                          # View all system limits
# sysctl kern.maxfiles=XXXX          # maximum number of file descriptors
kern.ipc.nmbclusters=32768           # Permanent entry in /etc/sysctl.conf
kern.maxfiles=65536                  # Typical values for Squid
kern.maxfilesperproc=32768
kern.ipc.somaxconn=8192              # TCP queue. Better for apache/sendmail
# sysctl kern.openfiles              # How many file descriptors are in use
# sysctl kern.ipc.numopensockets     # How many open sockets are in use
# sysctl net.inet.ip.portrange.last=50000 # Default is 1024-5000
# netstat -m                         # network memory buffers statistics

See The FreeBSD handbook Chapter 11http://www.freebsd.org/handbook/configtuning-kernel-limits.html for details.

Solaris

The following values in /etc/system will increase the maximum file descriptors per proc:

set rlim_fd_max = 4096               # Hard limit on file descriptors for a single proc
set rlim_fd_cur = 1024               # Soft limit on file descriptors for a single proc

Runlevels

Linux

Once booted, the kernel starts init which then starts rc which starts all scripts belonging to a runlevel. The scripts are stored in /etc/init.d and are linked into /etc/rc.d/rcN.d with N the runlevel number.
The default runlevel is configured in /etc/inittab. It is usually 3 or 5:

# grep default: /etc/inittab                                         
id:3:initdefault:

The actual runlevel can be changed with init. For example to go from 3 to 5:

# init 5                             # Enters runlevel 5

• 0       Shutdown and halt
• 1       Single-User mode (also S)
• 2       Multi-user without network
• 3       Multi-user with network
• 5       Multi-user with X
• 6       Reboot

Use chkconfig to configure the programs that will be started at boot in a runlevel.

# chkconfig --list                   # List all init scripts
# chkconfig --list sshd              # Report the status of sshd
# chkconfig sshd --level 35 on       # Configure sshd for levels 3 and 5
# chkconfig sshd off                 # Disable sshd for all runlevels

Debian and Debian based distributions like Ubuntu or Knoppix use the command update-rc.d to manage the runlevels scripts. Default is to start in 2,3,4 and 5 and shutdown in 0,1 and 6.

# update-rc.d sshd defaults          # Activate sshd with the default runlevels
# update-rc.d sshd start 20 2 3 4 5 . stop 20 0 1 6 .  # With explicit arguments
# update-rc.d -f sshd remove         # Disable sshd for all runlevels
# shutdown -h now (or # poweroff)    # Shutdown and halt the system

FreeBSD

The BSD boot approach is different from the SysV, there are no runlevels. The final boot state (single user, with or without X) is configured in /etc/ttys. All OS scripts are located in /etc/rc.d/ and in /usr/local/etc/rc.d/ for third-party applications. The activation of the service is configured in /etc/rc.conf and /etc/rc.conf.local. The default behavior is configured in /etc/defaults/rc.conf. The scripts responds at least to start|stop|status.

# /etc/rc.d/sshd status
sshd is running as pid 552.
# shutdown now                       # Go into single-user mode
# exit                               # Go back to multi-user mode
# shutdown -p now                    # Shutdown and halt the system
# shutdown -r now                    # Reboot

The process init can also be used to reach one of the following states level. For example # init 6 for reboot.

• 0       Halt and turn the power off (signal USR2)
• 1       Go to single-user mode (signal TERM)
• 6       Reboot the machine (signal INT)
• c       Block further logins (signal TSTP)
• q       Rescan the ttys(5) file (signal HUP)

Windows

Start and stop a service with either the service name or "service description" (shown in the Services Control Panel) as follows:

net stop WSearch
net start WSearch                    # start search service
net stop "Windows Search"
net start "Windows Search"           # same as above using descr.

Reset root password

Linux method 1

At the boot loader (lilo or grub), enter the following boot option:

init=/bin/sh

The kernel will mount the root partition and init will start the bourne shell instead of rc and then a runlevel. Use the command passwd at the prompt to change the password and then reboot. Forget the single user mode as you need the password for that.
If, after booting, the root partition is mounted read only, remount it rw:

# mount -o remount,rw /
# passwd                             # or delete the root password (/etc/shadow)
# sync; mount -o remount,ro /        # sync before to remount read only
# reboot

FreeBSD method 1

On FreeBSD, boot in single user mode, remount / rw and use passwd. You can select the single user mode on the boot menu (option 4) which is displayed for 10 seconds at startup. The single user mode will give you a root shell on the / partition.

# mount -u /; mount -a               # will mount / rw
# passwd
# reboot

Unixes and FreeBSD and Linux method 2

Other Unixes might not let you go away with the simple init trick. The solution is to mount the root partition from an other OS (like a rescue CD) and change the password on the disk.

• Boot a live CD or installation CD into a rescue mode which will give you a shell.
• Find the root partition with fdisk e.g. fdisk /dev/sda
• Mount it and use chroot:

# mount -o rw /dev/ad4s3a /mnt
# chroot /mnt                        # chroot into /mnt
# passwd
# reboot

Kernel modules

Linux

# lsmod                              # List all modules loaded in the kernel
# modprobe isdn                      # To load a module (here isdn)

FreeBSD

# kldstat                            # List all modules loaded in the kernel
# kldload crypto                     # To load a module (here crypto)

Compile Kernel

Linux

# cd /usr/src/linux
# make mrproper                      # Clean everything, including config files
# make oldconfig                     # Reuse the old .config if existent
# make menuconfig                    # or xconfig (Qt) or gconfig (GTK)
# make                               # Create a compressed kernel image
# make modules                       # Compile the modules
# make modules_install               # Install the modules
# make install                       # Install the kernel
# reboot

FreeBSD

Optionally update the source tree (in /usr/src) with csup (as of FreeBSD 6.2 or later):

# csup <supfile>

I use the following supfile:

*default host=cvsup5.FreeBSD.org  # www.freebsd.org/handbook/cvsup.html#CVSUP-MIRRORS
*default prefix=/usr 
*default base=/var/db
*default release=cvs delete tag=RELENG_7
src-all

To modify and rebuild the kernel, copy the generic configuration file to a new name and edit it as needed (you can also edit the file GENERIC directly). To restart the build after an interruption, add the option NO_CLEAN=YES to the make command to avoid cleaning the objects already build.

# cd /usr/src/sys/i386/conf/
# cp GENERIC MYKERNEL
# cd /usr/src
# make buildkernel KERNCONF=MYKERNEL
# make installkernel KERNCONF=MYKERNEL

To rebuild the full OS:

# make buildworld                    # Build the full OS but not the kernel
# make buildkernel                   # Use KERNCONF as above if appropriate
# make installkernel
# reboot
# mergemaster -p                     # Compares only files known to be essential
# make installworld
# mergemaster -i -U                  # Update all configurations and other files
# reboot

For small changes in the source you can use NO_CLEAN=yes to avoid rebuilding the whole tree.

# make buildworld NO_CLEAN=yes       # Don't delete the old objects
# make buildkernel KERNCONF=MYKERNEL NO_CLEAN=yes

Repair grub

So you broke grub? Boot from a live cd, [find your linux partition under /dev and use fdisk to find the linux partion] mount the linux partition, add /proc and /dev and use grub-install /dev/xyz. Suppose linux lies on /dev/sda6:

# mount /dev/sda6 /mnt               # mount the linux partition on /mnt
# mount --bind /proc /mnt/proc       # mount the proc subsystem into /mnt
# mount --bind /dev /mnt/dev         # mount the devices into /mnt
# chroot /mnt                        # change root to the linux partition
# grub-install /dev/sda              # reinstall grub with your old settings

Fans are one of the few components in a PC which have unsealed moving parts. By their nature, they have to be open to the air to suck heat out of the machine and push cool air in. As a result, they're extremely susceptible to all the airborne particles that make up household dust.

Contrary to popular myth, household dust is not primarily composed of human skin. In fact, although there are some skin cells present in virtually all house dust, its composition varies quite dramatically from area to area, season to season and room to room.

As you might expect, it includes pet hairs, pollen, plant seeds and broken-down insect remains, as well as lots and lots of particles of dirt. Both the organic and inorganic components of house dust can be troublesome to a machine with moving parts. Minute fragments of minerals can become trapped between bearings and shafts, while a build-up of softer material can act as a brake and slow a fan's rate of rotation.

Thankfully, with a little DIY work on a typical desktop PC case, you can increase the reliability of the fans, which in turn ensures proper cooling of all your system's important components: processor, video card, memory and power supply.

The simple answer to fan failure is to filter the air that enters your PC. Doing this isn't difficult at all, and here's a really simple way to cut out the muck.

1. Find a filter

The obvious filter material for a PC intake is that used for screening dust in a vacuum cleaner filter pad, since this material is designed expressly to perform this function.

These filter pads are cheap and readily available from all good electrical shops, and since you'll mark out and cut the filter material to fit your particular PC, it doesn't matter which vacuum cleaner model filters you buy.

You're after the largest area of filter you can get for your money, though, so go through the different models. You may be able to buy filter material in sheets, which could be even cheaper. It won't be a major outlay: you're typically looking at spending just a few pounds for a bag of filter pads.

2. Hold it in place

To get started, you'll need to find something to hold the filter material in place over the intake on your PC's case. You could stick it in place, but that would make it hard to replace periodically. Fortunately there's a ready-made holder for the filter material in the form of a simple J-shaped plastic extrusion.

This kind of extrusion is normally used to hide the rough edges of hardboard or acrylic sheet fittings, but it's just as good for holding vacuum filter material, and the gap is ideal for use as a slide fit. It's available in strips from DIY chains like B&Q and Homebase for a couple of pounds a metre.

You should be able to get all the lengths you need to add filters to a typical PC from one extrusion pack. Measure the size you need to hold the filter material in your PC and cut it to length with a craft knife.

In most cases, two strips placed at the top and bottom of the filter will work best, but for small areas a single strip may suffice. You can attach the strips of extrusion to the case with double-sided tape, applied first to the extrusion and then to the PC's case. This tape is readily available from most DIY stores.

3. Cut the filter to size

Measure the ventilation holes in your PC's case and cut out a cardboard template for each. Lay each template on the filter material and draw round it with a marker to make the inserts. Keep the templates, so you can renew the filters every so often.

You can fit filter pieces to the fan ports on power supplies and hold them in place by undoing the mounting screws and screwing them back through corresponding holes in the filter material.

Cut the filter material with scissors – a craft knife tends to snag. Though it's generally an unwoven, felt-like material, filter pads can still be cut into quite intricate shapes if needed.

4. Fit it to your PC

Fitting the filters, once you have the extrusion and filter material cut to size, is very simple. Stick the extrusion strips to the top and bottom of the case vent and slide the prepared filter strip between them so they're gripped in the channels.

Filters can be added inside or outside the case, whichever is more convenient (the result will look neater if they're added on the inside, of course). Try not to stick the extrusion over holes making up part of the case vent.

Adding the filter will reduce the airflow through the vents in comparison with unfiltered vents, but not significantly – a fan running slowly because of dust in its bearing will suffer far more.

5. Change it regularly

If all this Blue Peter-style DIY seems like overkill, it's worth mentioning that your author, who runs three working PCs pretty much continuously, was replacing a fan roughly every four months between these machines. This is in a domestic rural environment with few vehicle fumes or other typical urban contaminants to clog the workings up.

Since installing filters and nano-ceramic fans, not a single fan has had to be replaced and all are continuing to run quietly and provide specified levels of airflow. Changing the filters on each PC takes around 10 minutes to cut the material to size and another 10 minutes to fit. That's just 20 minutes every six months, probably less time than it typically takes to download and install a Service Pack.