Some More useful Commands

Monitoring and debugging
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

Others useful commands
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