arch 	show architecture of machine(1)
uname -m 	show architecture of machine(2)
uname -r 	show used kernel version
dmidecode -q 	show hardware system components - (SMBIOS / DMI)
hdparm -i /dev/hda 	displays the characteristics of a hard-disk
hdparm -tT /dev/sda 	perform test reading on a hard-disk
cat /proc/cpuinfo 	show information CPU info
cat /proc/interrupts 	show interrupts
cat /proc/meminfo 	verify memory use
cat /proc/swaps 	show file(s) swap
cat /proc/version 	show version of the kernel
cat /proc/net/dev 	show network adpters and statistics
cat /proc/mounts 	show mounted file system(s)
lspci -tv 	display PCI devices
lsusb -tv 	show USB devices
date 	show system date
cal 2007 	show the timetable of 2007
date 041217002007.00  	set date and time - MonthDayhoursMinutesYear.Seconds
clock -w	save date changes on BIOS
shutdown -h now 	shutdown system(1)
init 0 	shutdown system(2)
telinit 0 	shutdown system(3)
shutdown -h hours:minutes & 	planned shutdown of the system
shutdown -c 	cancel a planned shutdown of the system
shutdown -r now 	reboot(1)
reboot 	reboot(2)
logout 	leaving session
cd /home  	enter to directory '/ home'
cd .. 	go back one level
cd ../.. 	go back two levels
cd 	go to home directory
cd ~user1 	go to home directory
cd - 	go to previous directory
pwd 	show the path of work directory
ls 	view files of directory
ls -F 	view files of directory
ls -l 	show details of files and directory
ls -a 	show hidden files
ls *[0-9]* 	show files and directory containing numbers
tree 	show files and directories in a tree starting from root(1)
lstree 	show files and directories in a tree starting from root(2)
mkdir dir1 	create a directory called 'dir1'
mkdir dir1 dir2 	create two directories simultaneously
mkdir -p /tmp/dir1/dir2 	create a directory tree
rm -f file1 	delete file called 'file1'
rmdir dir1 	delete directory called 'dir1'
rm -rf dir1 	remove a directory called 'dir1' and contents recursively
rm -rf dir1 dir2 	remove two directories and their contents recursively
mv dir1 new_dir 	rename / move a file or directory
cp file1 file2 	copying a file
cp dir/* . 	copy all files of a directory within the current work directory
cp -a /tmp/dir1 . 	copy a directory within the current work directory
cp -a dir1 dir2 	copy a directory
ln -s file1 lnk1  	create a symbolic link to file or directory
ln file1 lnk1 	create a physical link to file or directory
touch -t 0712250000 file1 	modify timestamp of a file or directory - (YYMMDDhhmm)
file file1 	outputs the mime type of the file as text
iconv -l 	lists known encodings
iconv -f fromEncoding -t toEncoding inputFile > outputFile 	creates a new from the given input file by assuming it is encoded in fromEncoding and converting it to toEncoding.
find . -maxdepth 1 -name \*.jpg -print -exec convert "{}" -resize 80x60 "thumbs/{}" \; 	batch resize files in the current directory and send them to a thumbnails directory (requires convert from Imagemagick)
find / -name file1  	search file and directory into root filesystem from '/'
find / -user user1 	search files and directories belonging to 'user1'
find /home/user1 -name \*.bin 	search files with '. bin' extension within directory '/ home/user1' 
find /usr/bin -type f -atime +100 	search binary files are not used in the last 100 days
find /usr/bin -type f -mtime -10 	search files created or changed within 10 days
find / -name \*.rpm -exec chmod 755 '{}' \; 	search files with '.rpm' extension and modify permits
find / -xdev -name \*.rpm 	search files with  '.rpm' extension ignoring removable partitions as cdrom, pen-drive, etc.…
locate \*.ps 	find files with the '.ps' extension - first run 'updatedb' command
whereis halt 	show location of a binary file, source or man
which halt 	show full path to a binary / executable
mount /dev/hda2 /mnt/hda2  	mount disk called hda2 - verify existence of the directory '/ mnt/hda2'
umount /dev/hda2 	unmount disk called hda2 - exit from mount point '/ mnt/hda2' first
fuser -km /mnt/hda2 	force umount when the device is busy
umount -n /mnt/hda2 	run umount without writing the file /etc/mtab - useful when the file is read-only or the hard disk is full
mount /dev/fd0 /mnt/floppy 	mount a floppy disk
mount /dev/cdrom /mnt/cdrom 	mount a cdrom / dvdrom
mount /dev/hdc /mnt/cdrecorder 	mount a cdrw / dvdrom
mount /dev/hdb /mnt/cdrecorder 	mount a cdrw / dvdrom
mount -o loop file.iso /mnt/cdrom 	mount a file or iso image
mount -t vfat /dev/hda5 /mnt/hda5 	mount a Windows FAT32 file system
mount /dev/sda1 /mnt/usbdisk 	mount a usb pen-drive or flash-drive
mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share 	mount a windows network share
df -h  	show list of partitions mounted
ls -lSr |more 	show size of the files and directories ordered by size
du -sh dir1 	estimate space used by directory 'dir1'
du -sk * | sort -rn  	show size of the files and directories sorted by size
rpm -q -a --qf '%10{SIZE}t%{NAME}n' | sort -k1,1n 	show space used by rpm packages installed sorted by size (fedora, redhat and like)
dpkg-query -W -f='${Installed-Size;10}t${Package}n' | sort -k1,1n 	show space used by deb packages installed sorted by size (ubuntu, debian and like)
groupadd group_name  	create a new group
groupdel group_name 	delete a group
groupmod -n new_group_name old_group_name 	rename a group
useradd -c "Name Surname " -g admin -d /home/user1 -s /bin/bash user1 	create a new user belongs "admin" group
useradd user1 	create a new user
userdel -r user1 	delete a user ( '-r' eliminates home directory)
usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1 	change user attributes
passwd 	change password
passwd user1 	change a user password (only by root)
chage -E 2005-12-31 user1 	set deadline for user password
pwck 	check correct syntax and file format of '/etc/passwd'  and users existence
grpck 	check correct syntax and file format of '/etc/group'  and groups existence
newgrp group_name 	log in to a new group to change default group of newly created files
ls -lh  	show permits
ls /tmp | pr -T5 -W$COLUMNS 	divide terminal into 5 columns
chmod ugo+rwx directory1 	set permissions reading (r), write (w) and (x) access to users owner (u) group (g) and others (o)
chmod go-rwx directory1 	remove permits reading (r), write (w) and (x) access to users group (g) and others (or
chown user1 file1 	change owner of a file
chown -R user1 directory1 	change user owner of a directory and all the files and directories contained inside
chgrp group1 file1 	change group of files
chown user1:group1 file1 	change user and group ownership of a file
find / -perm -u+s 	view all files on the system with SUID configured
chmod u+s /bin/file1 	set SUID bit on a binary file - the user that running that file gets same privileges as owner
chmod u-s /bin/file1 	disable SUID bit on a binary file
chmod g+s /home/public 	set SGID bit on a directory - similar to SUID but for directory
chmod g-s /home/public 	disable SGID bit on a directory
chmod o+t /home/public 	set STIKY bit on a directory - allows files deletion only to legitimate owners
chmod o-t /home/public 	disable STIKY bit on a directory
chattr +a file1  	allows write opening of a file only append mode
chattr +c file1 	allows that a file is compressed / decompressed automatically by the kernel
chattr +d file1 	makes sure that the program ignores Dump the files during backup
chattr +i file1 	makes it an immutable file, which can not be removed, altered, renamed or linked
chattr +s file1 	allows a file to be deleted safely
chattr +S  file1 	makes sure that if a file is modified changes are written in synchronous mode as with sync
chattr +u file1 	allows you to recover the contents of a file even if it is canceled
lsattr 	show specials attributes
bunzip2 file1.bz2  	decompress a file called 'file1.bz2'
bzip2 file1 	compress a file called 'file1'
gunzip file1.gz 	decompress a file called 'file1.gz'
gzip file1 	compress a file called 'file1'
gzip -9 file1 	compress with maximum compression
rar a file1.rar test_file 	create an archive rar called 'file1.rar'
rar a file1.rar file1 file2 dir1 	compress 'file1', 'file2' and 'dir1' simultaneously
rar x file1.rar 	decompress rar archive
unrar x file1.rar 	decompress rar archive
tar -cvf archive.tar file1 	create a uncompressed tarball
tar -cvf archive.tar file1 file2 dir1 	create an archive containing 'file1', 'file2' and 'dir1'
tar -tf archive.tar 	show contents of an archive
tar -xvf archive.tar 	extract a tarball
tar -xvf archive.tar -C /tmp 	extract a tarball into / tmp
tar -cvfj archive.tar.bz2 dir1 	create a tarball compressed into bzip2
tar -xvfj archive.tar.bz2 	decompress a compressed tar archive in bzip2
tar -cvfz archive.tar.gz dir1 	create a tarball compressed into gzip
tar -xvfz archive.tar.gz 	decompress a compressed tar archive in gzip
zip file1.zip file1 	create an archive compressed in zip
zip -r file1.zip file1 file2 dir1 	compress in zip several files and directories simultaneously
unzip file1.zip 	decompress a zip archive
rpm -ivh package.rpm  	install a rpm package
rpm -ivh --nodeps package.rpm 	install a rpm package ignoring dependencies requests
rpm -U package.rpm 	upgrade a rpm package without changing configuration files
rpm -F package.rpm 	upgrade a rpm package only if it is already installed
rpm -e package_name.rpm 	remove a rpm package
rpm -qa 	show all rpm packages installed on the system
rpm -qa | grep httpd 	show all rpm packages with the name "httpd"
rpm -qi package_name 	obtain information on a specific package installed
rpm -qg "System Environment/Daemons" 	show rpm packages of a group software
rpm -ql package_name 	show list of files provided by a rpm package installed
rpm -qc package_name 	show list of configuration files provided by a rpm package installed
rpm -q package_name --whatrequires 	show list of dependencies required for a rpm packet
rpm -q package_name --whatprovides 	show capability provided by a rpm package
rpm -q package_name --scripts 	show scripts started during installation / removal
rpm -q package_name --changelog 	show history of revisions of a rpm package
rpm -qf /etc/httpd/conf/httpd.conf 	verify which rpm package belongs to a given file
rpm -qp package.rpm  -l 	show list of files provided by a rpm package not yet installed
rpm --import /media/cdrom/RPM-GPG-KEY 	import public-key digital signature
rpm --checksig package.rpm 	verify the integrity of a rpm package
rpm -qa gpg-pubkey 	verify integrity of all rpm packages installed
rpm -V package_name 	check file size, permissions, type, owner, group, MD5 checksum and last modification
rpm -Va 	check all rpm packages installed on the system - use with caution
rpm -Vp package.rpm 	verify a rpm package not yet installed
rpm2cpio package.rpm | cpio --extract --make-directories  *bin* 	extract executable file from a rpm package
rpm -ivh /usr/src/redhat/RPMS/`arch`/package.rpm 	install a package built from a rpm source
rpmbuild --rebuild package_name.src.rpm 	build a rpm package from a rpm source
yum install package_name  	download and install a rpm package
yum localinstall package_name.rpm 	That will install an RPM, and try to resolve all the dependencies for you using your repositories.
yum update 	update all rpm packages installed on the system
yum update package_name 	upgrade a rpm package
yum remove package_name 	remove a rpm package
yum list 	list all packages installed on the system
yum search package_name 	find a package on rpm repository
yum clean packages 	clean up rpm cache erasing downloaded packages
yum clean headers 	remove all files headers that the system uses to resolve dependency
yum clean all 	remove from the cache packages and headers files
dpkg -i package.deb  	install / upgrade a deb package
dpkg -r package_name 	remove a deb package from the system
dpkg -l 	show all deb packages installed on the system
dpkg -l | grep httpd 	show all deb packages with the name "httpd"
dpkg -s package_name 	obtain information on a specific package installed on system
dpkg -L package_name 	show list of files provided by a package installed on system
dpkg --contents package.deb 	show list of files provided by a package not yet installed
dpkg -S /bin/ping 	verify which package belongs to a given file
apt-get install package_name  	install / upgrade a deb package
apt-cdrom install package_name 	install / upgrade a deb package from cdrom
apt-get update 	update the package list
apt-get upgrade 	upgrade all of the installed packages
apt-get remove package_name 	remove a deb package from system
apt-get check 	verify correct resolution of dependencies
apt-get clean 	clean up cache from packages downloaded
apt-cache search searched-package 	returns list of packages which corresponds string "searched-packages"
cat file1  	view the contents of a file starting from the first row
tac file1 	view the contents of a file starting from the last line
more file1 	view content of a file along
less file1 	similar to 'more' command but which allows backward movement in the file as well as forward movement
head -2 file1 	view first two lines of a file
tail -2 file1 	view last two lines of a file
tail -f /var/log/messages 	view in real time what is added to a file
cat file1 file2 ... | command <> file1_in.txt_or_file1_out.txt  	general syntax for text manipulation using PIPE, STDIN and STDOUT
cat file1 | command( sed, grep, awk, grep, etc...) > result.txt 	general syntax to manipulate a text of a file, and write result to a new file
cat file1 | command( sed, grep, awk, grep, etc...) >> result.txt 	general syntax to manipulate a text of a file and append result in existing file
grep Aug /var/log/messages 	look up words "Aug" on file '/var/log/messages'
grep ^Aug /var/log/messages 	look up words that begin with "Aug" on file '/var/log/messages'
grep [0-9] /var/log/messages 	select from file '/var/log/messages' all lines that contain numbers
grep Aug -R /var/log/* 	search string "Aug" at directory '/var/log' and below
sed 's/stringa1/stringa2/g' example.txt 	replace "string1" with "string2" in example.txt
sed '/^$/d' example.txt 	remove all blank lines from example.txt
sed '/ *#/d; /^$/d' example.txt 	remove comments and blank lines from example.txt
echo 'esempio' | tr '[:lower:]' '[:upper:]' 	convert from lower case in upper case
sed -e '1d' result.txt 	eliminates the first line from file example.txt
sed -n '/stringa1/p' 	view only lines that contain the word "string1"
sed -e 's/ *$//' example.txt 	remove empty characters at the end of each row
sed -e 's/stringa1//g' example.txt 	remove only the word "string1" from text and leave intact all
sed -n '1,5p;5q' example.txt 	view from 1th to 5th row
sed -n '5p;5q' example.txt 	view row number 5
sed -e 's/00*/0/g' example.txt 	replace more zeros with a single zero
cat -n file1 	number row of a file
cat example.txt | awk 'NR%2==1' 	remove all even lines from example.txt
echo a b c | awk '{print $1}' 	view the first column of a line
echo a b c | awk '{print $1,$3}' 	view the first and third column of a line
paste file1 file2 	merging contents of two files for columns
paste  -d  '+' file1 file2 	merging contents of two files for columns with '+' delimiter on the center
sort file1 file2 	sort contents of two files
sort file1 file2 | uniq 	sort contents of two files omitting lines repeated
sort file1 file2 | uniq -u 	sort contents of two files by viewing only unique line
sort file1 file2 | uniq -d 	sort contents of two files by viewing only duplicate line
comm -1 file1 file2 	compare contents of two files by deleting only unique lines from 'file1'
comm -2 file1 file2 	compare contents of two files by deleting only unique lines from 'file2'
comm -3 file1 file2 	compare contents of two files by deleting only the lines that appear on both files
dos2unix filedos.txt fileunix.txt  	convert a text file format from MSDOS to UNIX
unix2dos fileunix.txt filedos.txt 	convert a text file format from UNIX to MSDOS
recode ..HTML < page.txt > page.html 	convert a text file to html
recode -l | more 	show all available formats conversion
badblocks  -v  /dev/hda1  	check bad blocks in disk hda1
fsck  /dev/hda1 	repair / check integrity of linux filesystem on disk hda1
fsck.ext2  /dev/hda1 	repair / check integrity of ext2 filesystem on disk hda1
e2fsck  /dev/hda1 	repair / check integrity of ext2 filesystem on disk hda1
e2fsck -j /dev/hda1 	repair / check integrity of ext3 filesystem on disk hda1
fsck.ext3  /dev/hda1 	repair / check integrity of ext3 filesystem on disk hda1
fsck.vfat  /dev/hda1 	repair / check integrity of fat filesystem on disk hda1
fsck.msdos  /dev/hda1 	repair / check integrity of dos filesystem on disk hda1
dosfsck  /dev/hda1 	repair / check integrity of dos filesystems on disk hda1
mkfs /dev/hda1  	create a filesystem type linux on hda1 partition
mke2fs /dev/hda1 	create a filesystem type linux ext2 on hda1 partition
mke2fs -j /dev/hda1 	create a filesystem type linux ext3 (journal) on hda1 partition
mkfs -t vfat 32 -F /dev/hda1 	create a FAT32 filesystem
fdformat  -n /dev/fd0 	format a floppy disk
mkswap /dev/hda3 	create a swap filesystem
swapon /dev/hda3 	activating a new swap partition
swapon /dev/hda2 /dev/hdb3  	activate two swap partitions
dump -0aj -f /tmp/home0.bak /home   	make a full backup of directory '/home'
dump -1aj -f /tmp/home0.bak /home  	make a incremental backup of directory '/home'
restore -if /tmp/home0.bak 	restoring a backup interactively
rsync -rogpav --delete /home /tmp 	synchronization between directories
rsync -rogpav -e ssh --delete /home ip_address:/tmp  	rsync via SSH tunnel
rsync -az -e ssh --delete ip_addr:/home/public /home/local 	synchronize a local directory with a remote directory via ssh and compression
rsync -az -e ssh --delete /home/local ip_addr:/home/public 	synchronize a remote directory with a local directory via ssh and compression
dd bs=1M if=/dev/hda | gzip | ssh user@ip_addr 'dd of=hda.gz' 	make a backup of a local hard disk on remote host via ssh
dd if=/dev/sda of=/tmp/file1 	backup content of the harddrive to a file
tar -Puf backup.tar /home/user 	make a incremental backup of directory '/home/user'
( cd /tmp/local/ && tar c . ) | ssh -C user@ip_addr 'cd /home/share/ && tar x -p' 	copy content of a directory on remote directory via ssh
( tar c /home ) | ssh -C user@ip_addr 'cd /home/backup-home && tar x -p'  	copy a local directory on remote directory via ssh
tar cf - . | (cd /tmp/backup ; tar xf - ) 	local copy preserving permits and links from a directory to another
find /home/user1 -name \*.txt | xargs cp -av --target-directory=/home/backup/ --parents 	find and copy all files with '.txt' extention from a directory to another
find /var/log -name *.log | tar cv --files-from=- | bzip2 > log.tar.bz2 	find all files with '.log' extention and make an bzip archive
dd if=/dev/hda of=/dev/fd0 bs=512 count=1 	make a copy of MBR (Master Boot Record) to floppy
dd if=/dev/fd0 of=/dev/hda bs=512 count=1 	restore MBR from backup copy saved to floppy
cdrecord -v gracetime=2 dev=/dev/cdrom -eject blank=fast -force  	clean a rewritable cdrom
mkisofs /dev/cdrom > cd.iso 	create an iso image of cdrom on disk
mkisofs /dev/cdrom | gzip > cd_iso.gz 	create a compressed iso image of cdrom on disk
mkisofs -J -allow-leading-dots -R -V "Label CD" -iso-level 4 -o ./cd.iso data_cd 	create an iso image of a directory
cdrecord -v dev=/dev/cdrom cd.iso 	burn an ISO image
gzip -dc cd_iso.gz | cdrecord dev=/dev/cdrom - 	burn a compressed ISO image
mount -o loop cd.iso /mnt/iso 	mount an ISO image
cd-paranoia -B 	rip audio tracks from a CD to wav files
cd-paranoia -- "-3" 	rip first three audio tracks from a CD to wav files
cdrecord --scanbus 	scan bus to identify the channel scsi
dd if=/dev/hdc | md5sum 	perform an md5sum on a device, like a CD
ifconfig eth0  	show configuration of an ethernet network card
ifup eth0 	activate an interface 'eth0'
ifdown eth0 	disable an interface 'eth0'
ifconfig eth0 192.168.1.1 netmask 255.255.255.0 	configure IP Address
ifconfig eth0 promisc 	configure 'eth0' in promiscuous mode to gather packets (sniffing)
dhclient eth0 	active interface 'eth0' in dhcp mode
route -n 	show routing table
route add -net 0/0 gw IP_Gateway 	configura default gateway
route add -net 192.168.0.0 netmask 255.255.0.0 gw 192.168.1.1 	configure static route to reach network '192.168.0.0/16'
route del 0/0 gw IP_gateway 	remove static route
echo "1" > /proc/sys/net/ipv4/ip_forward 	activate ip routing
hostname 	show hostname of system
host www.example.com 	lookup hostname to resolve name to ip address and viceversa(1)
nslookup www.example.com 	lookup hostname to resolve name to ip address and viceversa(2)
ip link show 	show link status of all interfaces
mii-tool eth0 	show link status of 'eth0'
ethtool eth0 	show statistics of network card 'eth0'
netstat -tup 	show all active network connections and their PID
netstat -tupl 	show all network services listening on the system and their PID
tcpdump tcp port 80 	show all HTTP traffic
iwlist scan 	show wireless networks
iwconfig eth1 	show configuration of a wireless network card
hostname 	show hostname
host www.example.com 	lookup hostname to resolve name to ip address and viceversa
nslookup www.example.com 	lookup hostname to resolve name to ip address and viceversa
whois www.example.com 	lookup on Whois database 
nbtscan ip_addr  	netbios name resolution
nmblookup -A ip_addr 	netbios name resolution
smbclient -L ip_addr/hostname 	show remote shares of a windows host
smbget -Rr smb://ip_addr/share 	like wget can download files from a host windows via smb
mount -t smbfs -o username=user,password=pass //WinClient/share /mnt/share 	mount a windows network share
iptables -t filter -L  	show all chains of filtering table
iptables -t nat -L 	show all chains of nat table
iptables -t filter -F 	clear all rules from filtering table
iptables -t nat -F 	clear all rules from table nat
iptables -t filter -X 	delete any chains created by user
iptables -t filter -A INPUT -p tcp --dport telnet -j ACCEPT 	allow telnet connections to input
iptables -t filter -A OUTPUT -p tcp --dport http -j DROP 	block HTTP connections to output
iptables -t filter -A FORWARD -p tcp --dport pop3 -j ACCEPT 	allow POP3 connections to forward chain
iptables -t filter -A INPUT -j LOG --log-prefix "DROP INPUT" 	logging sulla chain di input  Logging on chain input
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE 	configure a PAT (Port Address Traslation) on eth0 masking outbound packets
iptables -t nat -A PREROUTING -d 192.168.0.1 -p tcp -m tcp --dport 22 -j DNAT --to-destination 10.0.0.2:22 	redirect packets addressed to a host to another host
top  	display linux tasks using most cpu
ps -eafw 	displays linux tasks
ps -e -o pid,args --forest 	displays linux tasks in a hierarchical mode
pstree  	Shows a tree system processes
kill -9 ID_Processo 	force closure of the process and finish it
kill -1 ID_Processo 	force a process to reload configuration
lsof -p $$ 	display a list of files opened by processes
lsof /home/user1 	displays a list of open files in a given path system
strace -c ls >/dev/null 	display system calls made and received by a process
strace -f -e open ls >/dev/null 	display library calls
watch -n1 'cat /proc/interrupts' 	display interrupts in real-time
last reboot 	show history reboot
lsmod 	display kernel loaded
free -m 	displays status of RAM in megabytes
smartctl -A /dev/hda 	monitoring reliability of a hard-disk through SMART
smartctl -i /dev/hda 	check if SMART is active on a hard-disk
tail /var/log/dmesg 	show events inherent to the process of booting kernel
tail /var/log/messages 	show system events
apropos ...keyword 	display a list of commands that pertain to keywords of a program , useful when you know what your program does, but you don't know the name of the command
man ping 	display the on-line manual pages for example on ping command - use '-k' option to find any related commands
whatis ...keyword 	displays description of what a program does
mkbootdisk --device /dev/fd0 `uname -r` 	create a boot floppy
gpg -c file1 	encrypt a file with GNU Privacy Guard
gpg file1.gpg 	decrypt a file with GNU Privacy Guard
wget -r www.example.com 	download an entire web site
wget -c www.example.com/file.iso 	download a file with the ability to stop the download and resume later
echo 'wget -c www.example.com/files.iso' | at 09:00 	start a download at any given time
ldd /usr/bin/ssh 	show shared libraries required by ssh program
alias hh='history' 	set an alias for a command - hh = history
chsh 	change shell command
chsh --list-shells 	nice command to know if you have to remote into another box
who -a 	show who is logged on, and print: time of last system boot, dead processes, system login processes, active processes spawned by init, current runlevel, last system clock change
