50 quick linux command tips

visit ilovetux.com
1. How to create a return email address when using mutt ?
# export REPLYTO=youremail@yourdomain.com
2. How to dump a website into your terminal screen ?
# elinks -dump “http://www.gmanews.tv/forex.php”
# lynx -dump “http://www.gmanews.tv/forex.php”
# links -dump “http://www.gmanews.tv/forex.php”
3. How to dump a website into a file ?
# elinks -dump “http://www.gmanews.tv/forex.php” > file1
# lynx -dump “http://www.gmanews.tv/forex.php” > file2
# links -dump “http://www.gmanews.tv/forex.php” > file3
3. How to sort batch of files containing alpha numeric characters in a file with one command ?
# cat filename101*.txt sort -n
4. How to remove duplicate lines from a file ?
# uniq < filename01.txt
5. How to force rotate of all your defined log files located in /var/log/ ?
# logrotate -f /etc/logrotate.conf
6. How to change the interval time of your daily and weekly system cron jobs ?
# vi /etc/anacrontab
7. How to start X / Gnome from terminal ?
# startx
8. How to do a whole new xorg.conf setup for your nonworking/epileptic X which was caused recent xorg.conf misconfiguration ?
# mv /etc/X11/xorg.conf /etc/X11/xorg.conf.old# gdmsetup
9. Do you have a very old and nice webtool for newbies or junior system admins?
Click here
10. How to launch and run a program into background ?
# program-name &
10. How to locate and find file(s) in CLI ?
# find /foldername -name name-of-file
Ex.# find /home -name *vertito*# locate *vertito*
11. For script kiddies who successfully hacked a box, LOL. How to know your current dropped user, shell id, group id, groups your shell belongs to after successfully hacking into a linux box?
# id
12. How to know who is currently active from ssh connection from your server ?
# w
13. How to know all your opened UDP ports and connections ?
# ss -u -a
14. What are the most commonly known world writeable and readable folders?
# cd /tmp# cd /dev/shm
So watch out for any scripts or file changes there!
15. How to build rpm package from source file (*.src.rpm) ?
# rpmbuild –rebuild *.src.rpm
Most probably, you’ll find the sources in /root/rpmbuild/SOURCES and the RPM file in /root/rpmbuild/RPMS . If not, try /usr/src/redhat/SOURCES and /usr/src/redhat/RPMS
16. How to find all files from a particular folder that contains this string and do it recursively ?
# find /home/vertito -exec grep -li myownstring {} \;
17. How to find a specific filename and delete it ?
# find / -name specific-filename.txt -exec rm -rf {} \;
18. How to trim my temporary files and delete the ones that are more than 1 month old?
# find /tmp -mtime +31 -exec rm -rf {} \;
19. How to know the descriptions, functions, and more info a a linux command ?Ex.
# man elinks# apropos elinks# whatis elinks# info elinks
20. How to know from which rpm package does a particular library belongs to ?
Ex. /lib/libcap.so
# rpm -qf /lib/libcap.so
21. How to find out interesting port of a particular host ?
# nmap host-IP-Address-here
22. How to ping one subnet class of IP address or group of IP address ?
# fping -g 192.168.100.0/27$ fping -g 192.168.1.1 192.168.1.254
23. How to know if a port is open ?
Example: port 25
# telnet IP-address 25# nc -dz IP-address 25
24. How to watch for file and file size changes of one folder live and not by email ?
# cd your-folder# watch ls -la
26. How to print line numbers of each output line of a text file ?
# cat -n yourfile.txt
27. How to negate a grep result?
# grep -v “null” yourfilename
28. How to list statistics from your network ?
# netstat -s
29. What is the country code for a particular country ?
# grep -i philippines /usr/share/zoneinfo/iso3166.tab
30. How to find out who owns and manages a particular domain ?
# whois google.com
31. How to get the resolved IP of a domain ?
# nslookup google.com# host google.com# dig google.com
32. How to say hello and goodbye to your system log file ?
# logger ‘Hello and Goodbye’
33. How to list out all your USB current connections ?
# lsusbverbose# lsusb -v
34. How to list out all your PCI card connections ?
# lspciverbose# lspci -v
35. How to lock/unlock a bash enabled user shell account ?
# passwd -l useraccount# passwd -u useraccount# passwd -S useraccount
36. How to limit and change available shell accounts for user’s shell assignment ?
# vi /etc/shells
37. How to change the default values when adding new user accounts ?
# vi /etc/default/useradd
38. How to create default files automatically every time a new user accounts is created ?
# cp /home/vertito/whateverfile.txt /etc/skel# useradd -d /home/newuser newuser
39. How to go the easy way to home folder of a particular user if you have multinested virtual home folders?
# cd ~hisusername
40. How to browse and download the whole pages of a particular WWW site in one shot ?
# wget -p –progress=dot http://www.google.com
41. How to safely mark badblocks from another ext2/ext3 linux harddisk ?
# umount /dev/other-harddisk2# e2fsck -c /dev/other-harddisk2
42. How to create a new ext3 filesytem with bad-block checking from your secondary harddisk?
# mkfs.ext3 -c /dev/other-harddisk2
43. How to compare bzip2 compressed files ?
# bzdiff file1 file2
44. How to detect hardware monitoring chips and load modules related to newly detected chips?
# sensors-detect>45. How to know if your postfix or sendmail is running ?
# ps axuw grep sendmail# ps axuw grep postfix
46. How to change the default port when spamassassin is launched?
# vi /etc/sysconfig/spamassassin
47. How to disable postfix permanently after reboot ?
# chkconfig –levels 345 postfix off
48. How to know the number of children spawned by apache ?
# ps axuw grep http wc -l
49. How to avoid a module from being loaded by kernel during startup and blacklist it permanently?
# vi /etc/modprobe.d/blacklist
50. How to quickly count all your queued mail when you box is really hogging due to spam bruteforce attacks and spam mailer?
# mailq wc -l

Securing SSH server

Disable root ssh login :

edit /etc/ssh/sshd-config

locate #PermitRootLogin yes to PermitRootLogin no

save file and restart the ssh : service sshd restart

just use sudo

Trace ssh users with most Failed Login Attempts :

#cat secure* grep 'Failed password' awk '{print $9}' sort -rn uniq -c

insert this head -10 - to list top 10 failed ssh