* Continuously check Apache error log file:
$ tail -f /var/log/httpd/error_log
* View first 15 lines from MySQL log:
$ head -15 /var/log/mysqld.log
* vi keyboard shortcuts
=> jump to end of line
$
=> start of line
0
=> Delete rest of line
D
=> Repeat the last command given:
. (dot)
=> add ‘maal’ to the end of every line. 1 is line 1, $ is the last line
:1,$ s/$/maal/
=> put ‘bingo’ at the start of lines 5-10
:5,10 s/^/bingo/
=> change foo to bar for all occurrences in the rest of the file from where the cursor is
:s/foo/bar/g
=> Delete current line and got into insert mode.
C
=> Remove the ^M from files that came from windows:
:se ff=unix
=> Turn on/off display of line numbers:
:set nu
:set nonu
=> if you want actual line numbers in your file:
:%!cat -n
=> find the word under cursor
* (star)
* screen command
Just type screen and your problems are solved 
$ cat ~/.screenrc
# no annoying audible bell, please
vbell on
# detach on hangup
autodetach on
# don’t display the copyright page
startup_message off
# emulate .logout message
pow_detach_msg “Screen session of \$LOGNAME \$:cr:\$:nl:ended.”
# advertise hardstatus support to $TERMCAP
termcapinfo xterm* ti@:te@
# make the shell in every window a login shell
shell -$SHELL
defscrollback 10000
# Extend the vt100 desciption by some sequences.
termcap vt* AF=\E[3%dm:AB=\E[4%dm
caption always
caption string '%{= wk}[ %{k}%H %{k}][%= %{= wk}%?%-Lw%?%{r}(%{r}%n*%f%t%?(%u)%?%{r})%{k}%?%+Lw%?%?%= %{k}][%{b} %d/%m %{k}%c %{k}]‘
# keybindings
bind -k F5 prev
bind -k F6 next
netstat
* Display total number of internet (port 80) connections:
$ netstat -an |grep :80 |wc -l
* Display all ports your machine listening on:
$ netstat -ant | grep LISTEN
nmap
* Scan a machine on your LAN with nmap and know which ports are open on it:
$ nmap ip
find
* find top 10 largest files in /var:
$ find /var -type f -ls | sort -k 7 -r -n | head -10
* find all files having size more than 5 GB in /var/log/:
$ find /var/log/ -type f -size +5120M -exec ls -lh {} \;
* find all today’s files and copy them to another directory:
$ find /home/me/files -ctime 0 -print -exec cp {} /mnt/backup/{} \;
* find all temp files older than a week and delete:
$ find /temp/ -mtime +7-type f | xargs /bin/rm -f
* find and rename all mp3 files by changing their uppercase names to lowercase:
$ find /home/me/music/ -type f -name *.mp3 -exec rename ‘y/[A-Z]/[a-z]/
grep
* Print Apache’s documentroot directory name:
$ grep -i documentroot /etc/httpd/conf/httpd.conf
* View file contents without comments and empty lines:
$ grep -Ev “^$|^#” /etc/my.cnf
* print only IP address assigned to the interface:
$ ifconfig eth0 | grep ‘inet addr:’ | cut -d’:’ -f2 | awk ‘{ print $1}’
* How many email messages sent for a particular date:
$ cat /var/log/maillog | grep “status=sent” | grep “May 25″ | wc -l
* Find out a running process/daemon from process list (thanks to staranneph for recalling this):
ps -ef | grep mysql
* You can also note cpu/mem usage by using above. like in below command output, you can see that Plesk’s statistics process is utilizing more than 18% cpu alone:
[root@myserver ~]# ps aux | grep statistics
root 8183 18.4 0.0 58384 2848 ? D 04:05 3:00 /usr/local/psa/admin/sbin/statistics
.