Handy Linux commands/scripts

1. Security (Authentication and others)

1.1. Login check

#on fedora or ubuntu distr. check for last logins with dates

last

#on fedora distri. check for last invalid login attemps. (Note: run using root or sudo)

lastb

#send alert/email on ssh login
one simple way is to add sendmail line into ~/.bashrc file if you want to do user-specific alert. For instance, you want to get alert only if someone logged into a machine with a specific user.

IP="$(echo $SSH_CONNECTION | cut -d " " -f 1)"
HOSTNAME=$(hostname)
NOW=$(date +"%e %b %Y, %a %r")

echo 'Someone from '$IP' logged into '$HOSTNAME' on '$NOW'.' | mail -s 'SSH Login Notification' YOUR_EMAIL_ADDRESS




or you can write a monitoring and parsing program that will keep an eye on /var/log/auth.log for ssh login logs. and send alerts as soon as it detects any malicious access.




1.2. Useful OpenSSL commands

see certificate subject

openssl x509 -in certificateName -subject

see certitifcate in text form

openssl x509 -in certificateName -text

connect to a server acting as a client using openssl

openssl s_client -connect [server_ip/server_name]:port

if you have to pass certificate to this connect request

openssl s_client -connect [server_ip/server_name]:port -cert path-to-certificate

if you also know the CAfile or CApath you can also provide them

openssl s_client -connect [server_ip/server_name]:port -CApath path_to_CAdirectory

2. Networking related

#obtain your external ip

curl ifconfig.me

#port forwarding n tunneling using SSH

ssh userName@hostnam/ip -N -L localport:host:hostport -L localport:host:hostport

now add host with 127.0.0.1(loopback address for localhost) mapping into your /etc/hosts file

#dynamic port forwarding using SSH

to be done

#Setting system-wide proxy settings

sudo export http_proxy='http://IP:port'

where IP is the IP address of your proxy server and port is the port of that proxy server.

#dig to get detail DNS query information

dig +noall +answer www.google.com

dig +noall +answer -x 209.85.227.105

For other handy tools and tips, please follow the given link.

http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch04_:_Simple_Network_Troubleshooting#Sources_of_a_Lack_of_Connectivity

*Update hosts information in Windows as well

http://accs-net.com/hosts/how_to_use_hosts.html

Windows 95/98/Me c:\windows\hosts

Windows NT/2000/XP Pro c:\winnt\system32\drivers\etc\hosts

Windows XP Home c:\windows\system32\drivers\etc\hosts (I have tested this one)

3. User Management

#change the default shell for a user account

chsh

this command will ask for your password, and then the path to the new shell.

#password less sudo access

To allow a user with sudo access without password, edit /etc/sudoers on (ubuntu) and add the following line at the end 

<username> = (ALL:ALL) NOPASSWD: ALL

sample:

khawar = (ALL:ALL) NOPASSWD: ALL

reboot the system and you can access sudo without being asked for passwords

4. Process Management

#search for a specific process name and get its id and kill it.

ps -ef | awk '/Startup.py/ {print $2}' | xargs kill -9

5. Other Utilities

#output with nice formatting (if output has multiple columns)

Let's take the example of mount command. its output is not properly formatted. 

mount | column -t

#list directories only

ls -l | grep '^d' | awk '{print $NF;}'

#Split a big file

if you want to split a file on linux, use split command. lets say you have a big file of size 5GB, and want to split it in 3GBs.

split -b 3G  bigger_filename

where bigger_filename is the source file you want to split.

#format a drive

mkfs.vfat /dev/sdb1

#print file contents in reverse (opposite of cat) 

tac filename

#print file contents with line numbers

cat -n filename

we can also use nl command

nl filename

#file sizes of the given directory

du -h -d 1 directory_name

#convert output of your command to an image file

ls | convert label:@- PATH_TO_IMAGEFILE.png

This will generate an image file of the ls output. For this, X11 should be installed on your system. Not available on Mac OS X latest releases.

#send email (you should have sendmail installed)

echo "test msg" | mail -s test EMAIL_ADDRESS

this will send "test msg" (mail body) with subject "test" (by using -s flag) to the given EMAIL_ADDRESS

6. Search files and perform operations in one command

#find and remove files in one command

find . -type f -name "*.bak" -exec rm -i {} \;

#find files and replace text within it using sed

for file in `find src -name 'YOUR_FILE_NAME'`; do sed 's/SEARCH_STRING/REPLACE_WITH/g'

"$file" > tmp_file; mv tmp_file $file; echo "$file done"; done

#To find and delete empty directories

find -depth -type d -empty -exec rmdir {} \;

#Delete specific files using ls and grep within one directory

cd your_directory

ls -la | grep "username" | awk '{print $colposition}'|while read line do rm $line; done

where colposition is the column number of the filenames. At current display format of ls, it is 9th column that contains the filenames.

7. Installation related (on Ubuntu or Debian)

I experienced a situation in which a failed install script changed my start-stop-daemon scripts and did not rollback the changes. After some searching,  I found that the following command fixes such problem.

#reconfigure an improper installed package on Ubuntu or Debian distributions.

sudo apt-get install dpkg --reinstall