Jontas » unix

Add a user to a group

jontas — Fri, 03 Dec 2010 10:14:33 +0000

To add a existing user to a group

usermod -a -G thegroup theuser

To add a user to a group when the user is created

useradd -G thegroup theuser

.forward to several recipients

jontas — Tue, 28 Sep 2010 08:08:03 +0000

On *nix environments it is possible to forward the mail to one account to another user (or another email address if external mail is supported).
This is done by adding the recipient in a file called .forward in the home directory.

Example To simply forward to another address:

user@server$ echo recipient@mail.com > ~/.forward
user@server$ chmod go-w ~/.forward

In order to forward to several addresses coma (,) is used as a delimiter between the different recipients.

Example To forward to two addresses at the same time:

user@server$ echo recipient1@mail.com,recipient2@mail.com > ~/.forward
user@server$ chmod go-w ~/.forward

Linux send a log (or output from a command) as a mail from the linux console

jontas — Wed, 18 Aug 2010 11:00:22 +0000

Sometimes it is nice to send a log or some other file as a email to someone (or a part of a file)

cat myfile.txt | mail -s 'SUBJECT' example@mail.com;

Change myfile.txt, SUBJECT and example@mail.com to something useful.

It is also possible to send the output of a command to someone

ps -ef|mail -s 'SUBJET' example@mail.com;

Pine: Flags

jontas — Wed, 23 Jun 2010 19:18:34 +0000

To mark a message as unread in pine this can be done using flags.
First flags have to be turned on in setup
[M]ain [S]etup [C]onfig
Then find “enable-flag-cmd” and turn it on.
After this marking a message as unread is done by “* N” in the list view or when looking at the message.

All the flags are:

[N]New
[D]Deleted
[*] Important
[A] Answered

[^T] Can be used to see all the flags and set/unset them one by one (or all).

OpenSSH and password-less logins

jontas — Fri, 21 May 2010 08:27:07 +0000

Using SSH you can run commands at remote servers.

By default the remote server will ask for credentials but for instance when writing a script it is not a good idea to store the login credentials in a script.
The solution to this is to create a key pair at the origin machine (where the script is) and then send this key to the remoteserver (where the script needs to login).

First create a key pair (only has to be done once for this machine; it can be reused for other machines if you wish to be able to login to several computers)

 jonas@jonas-desktop:~$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/home/jonas/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/jonas/.ssh/id_rsa.
Your public key has been saved in /home/jonas/.ssh/id_rsa.pub.
The key fingerprint is:
11:f9:5a:8f:7d:74:e4:68:3b:4b:22:1c:78:e6:be:2d jonas@jonas-desktop
The key's randomart image is:
+--[ RSA 2048]----+
|        ..       |
|        ..      .|
|        .o     + |
|        ..*   + o|
|        SB = o o |
|        . = + =  |
|         . . + o |
|          E.  .  |
|          .o.    |
+-----------------+

Passphrase is needed to “unlock” this key before being able to use it. Leaving passphrase blank means that the key can be used without any inputs.

Next send the key to the server that you wish to be able to login to.

 jonas@jonas-desktop:~$ ssh-copy-id -i .ssh/id_rsa.pub user@remoteserver
Warning: Permanently added 'remoteserver,11.222.333.222' (RSA) to the list of known hosts.
user@remoteserver's password: 
Now try logging into the machine, with "ssh 'user@remoteserver'", and check in:
 
  .ssh/authorized_keys
 
to make sure we haven't added extra keys that you weren't expecting.

-i is the identity file we created with ssh-keygen

This is it. Now you are able to login to remoteserver as user without a password.

tar extracting to a target directory

jontas — Wed, 19 May 2010 12:03:25 +0000

By default untaring will put the contents into the current directory.
Sometimes it is not desirable to change directory to the target (such as in a scrip).

tar -xvvf /source/tarfile.tar -C /target

This will extract(x), very verbosly(vv) the file(f) /source/tarfile.tar to the directory(-C) /target (it will change the output folder to /target (so make sure it exists).

What program is using port x?

jontas — Thu, 18 Feb 2010 14:30:55 +0000

netstat can be used to figure out what program is using a port.

$ netstat -nlp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      1096/sshd
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      1733/apache2
tcp6       0      0 :::22                   :::*                    LISTEN      1096/sshd
Active UNIX domain sockets (only servers)
Proto RefCnt Flags       Type       State         I-Node   PID/Program name    Path
unix  2      [ ACC ]     STREAM     LISTENING     6338     1071/gdm-simple-sla @/tmp/gdm-greeter-LvjLQPzT
unix  2      [ ACC ]     STREAM     LISTENING     5165     1072/X              /tmp/.X11-unix/X0

If you have a lot open then grep can be used to only find what you are looking for

netstat -nlp|grep :22
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      1096/sshd
tcp6       0      0 :::22                   :::*                    LISTEN      1096/sshd

For windows, simply start with

netstat -n

Showing (and/or) Clearing the arp cache

jontas — Tue, 16 Feb 2010 14:00:37 +0000

The arp cache should (normally) be cleared within 20 minutes (or less, this depends on what platform is used).

If you don’t know what ARP is and what is is good for, then perhaps you should take a moment and read the man page:

DESCRIPTION
Arp manipulates or displays the kernels IPv4 network neighbor cache. It can add entries to the table, delete one or display the current content.
 
ARP stands for Address Resolution Protocol, which is used to find the media access control address of a network neighbor for a given IPv4 Address.

On a windows platform to show the arp cache

C:\> arp -a
Interface: 192.168.1.49 --- 0xb
  Internet Address      Physical Address      Type
  192.168.1.29          00-1d-92-08-49-25     dynamic
  192.168.1.34          00-24-81-85-ab-b6     dynamic

To clear the cache

C:\>netsh interface ip delete arpcache
Ok.

To show the arp cache on a nix machine:

$  arp -vn; cat /proc/net/arp
Address                  HWtype  HWaddress           Flags Mask            Iface
192.168.1.254            ether   00:08:ae:09:20:a0   C                     eth0
IP address       HW type     Flags       HW address            Mask     Device
192.168.1.254    0x1         0x2         00:08:ae:09:20:a0     *        eth0

To clear the apr cache on a nix machine.

$  ip neigh flush all

unqualified host name ([something]) unknow; sleeping for retry

jontas — Mon, 30 Nov 2009 14:30:32 +0000

sendmail expects to the machine to have a FQDN (fully qualified domain name).

If it does not have that (type “hostname” to check what name the machine has) sendmail will complain about this and take a loot of extra time trying to find it.

To use a FQDN edit “/etc/hosts” and change [something] to [something.yourdomain.com] and this will go away. Updating “/etc/hostname” at the same time might be a good idea; don’t forget to run “/bin/hostname -F /etc/hostname” after updating /etc/hostname.