VI Tips

After lots of request from my friends, I finally end up with writing something on VI or VIM ( Vi IMproved ). This is one of the most powerful editor in UNIX environment.

Try this
# vi test.txt
[Press ESC]
:help

Please read through that briefly. There are various .txt files just like each chapters in text book.

You can access those .txt file using

For example
:help usr_01

I want you guys to read and learn by yourself. That really helps. You will know what you are doing.

However, I will mention some frequently used shortcuts/commands:

h j k l move left move up move down move right

:x [Enter] To save and exit file
:q To quit
:qw Same as :x
:q! Ignore the changes and quit

:set number Display line number on each line
:set nonumber Don't display line number on each line

:set ruler Display the cursor position at the bottom of the screen

CTRL + g Display the cursor position with percentage of the page

CTRL + d Scroll half window down
CTRL + u Scroll half window up

/findthis Search 'findthis' string
[ Press n or N for searching backward and forward in the file]

The characters .*[]^%/\?~$ have special meanings. If you want to use them in a search you must put a \ in front of them.

:set ignorecase To ignorecase while searching the string/pattern
:set noignorecase

G To go to the bottom of the page
gg To go to the top of the page

yy To copy/yank the line
dd To delete the line
p To paste the yanked/copied line

$ To go to the end of the line
0 To go to the front of the line

{ To go to the end of the paragraph
{ To go to the front of the paragraph

:syntax on To enable syntax highlight
:syntax off


:line_number To go to particular line e.g To go to line 22 , do the following :22 [enter]


SEARCHING FOR A WORD IN THE TEXT

Suppose you see the word "TheLongFunctionName" in the text and you want to find the next occurrence of it. You could type "/TheLongFunctionName", but that's a lot of typing. And when you make a mistake Vim won't find it. There is an easier way: Position the cursor on the word and use the "*" command. Vim will grab the word under the cursor and use it as the search
string. The "#" command does the same in the other direction. You can prepend a
count: "3*" searches for the third occurrence of the word under the cursor.

HIGHLIGHTING MATCHES

While editing a program you see a variable called "nr". You want to check where it's used. You could move the cursor to "nr" and use the "*" command and press "n" to go along all the matches.

There is another way. Type this command:

:set hlsearch

If you now search for "nr", Vim will highlight all matches. That is a verygood way to see where the variable is used, without the need to type commands.

To switch this off:
:set nohlsearch


REPLACE THE WORD

:%s/old_word/new_word/g

This will substitute old_word with new_word globally.

TUNING SEARCHES

There are a few options that change how searching works. These are the essential ones:

:set incsearch

This makes Vim display the match for the string while you are still typing it. Use this to check if the right match will be found. Then press to really jump to that location. Or type more to change the search string.

:set nowrapscan

This stops the search at the end of the file. Or, when you are searching backwards, at the start of the file. The 'wrapscan' option is on by default, thus searching wraps around the end of the file.


Bonus Tips:
If you like one of the options mentioned before, and set it each time you use Vim, you can put the command in your Vim startup file.
Edit the file, as mentioned at |not-compatible|. Or use this command to find out where it is:

:scriptnames

Edit the file, for example with:

:edit ~/.vimrc

Then add a line with the command to set the option, just like you typed it in
Vim. Example:

Go:set hlsearch

Sending email using Command line + Scripting with example

Trial 1:

mail -s "This is a subject" john.doe@test.net < /root/myBodyFile

This simple command will send the email to john.doe@test.net with the subject "This is a subject" and body with the content of the file /root/myBodyFile

Trial 2:

You could check if your linux machine delievered the message to john doe or not.

# mail

If you see any MAILER-DAEMON@ , check if it is for john.doe@test.net (i.e email recepient) or not.

There could be various possible issues.

In my case I am using PostFix as my MTA.
I have configured mailrelayhost for my MTA.

For postfix, you can go to

#vi /etc/postfix/main.ca
------
-----
mailrelayhost= < specify ur mailrelay host or IP address of your mail server >
-----
-----

And also make sure that your linux machine's IP address is allowed to send email using email server (contact your email server administrator). He will add IP address of your linux machine as trusted host in his email server

Trail 3:

I would like to create alias so that I can send email to the group of people. It's easy.

Login as a root in your linux machine

#vi /etc/aliases
-----
----
#myEmailGroup: List of all emails separated by comma
-----
-----
ServerAlert: user1@gmail.com,user2@hotmail.com,214000000@txt.att.net
-----
-----

Next step is to load all the aliases

#newaliases

#echo $?
If it returns 0, then above command was executed succesfully


Finally you can send email to the group of users by

#mail -s "Hi.. all of you" ServerAlert < /root/myBodyFile


Application of email alert for the Asterisk PRI monitoring:

1. Create the script file

#vi astPRIcontrol.sh
#!/bin/bash
#author DShah erdevendra@gmail.com
#created on 10/22/10
#This script checks the PRI status; If PRI is down, it pages the admins

pristatus=echo /usr/sbin/asterisk -rx "pri show spans" |grep -i down

if [ -z $pristatus ]
then
#echo "PRI up"
notify=0
else
#echo "PRI down"
notify=1
fi

if [ $notify -eq 1 ]
then
mail -s " PRI down" Server_Alert < /root/pristatus

fi


2. Change the permission (make the script executable)

#chmod 700 /root/astPRIstatus.sh

3. Create a file called /root/pristatus

#vi /root/pristatus
PRI at asterisk server is down

4. Add the script file in crontab: it runs the script every minute

#crontab -e

*/1 * * * * bash /root/astPRIstatus.sh >/dev/null

Basic guide for Logrotate in Linux

Log files in Linux usually reside at /var/log... It keeps on growing so log management is essential. Log management is usually achieved using logrotate. Logrotate is managed by cronjobs in Linux.

For logrotate, you can configure /etc/logrotate.conf or create the individual configuration files for each application or each log file in /etc/logrotate.d

step 1:

Let's say, I have VOIP application 'asterisk' running on my system. Asterisk generates various log files under /var/log/asterisk directory. I would create astlog under /etc/logrotate.d to manage the log files.

#cd /etc/logrotate.d
#vi astlog
/var/log/asterisk/full /var/log/asterisk/messages /var/log/asterisk/debug /var/log/asterisk/*.log {
nocompress
daily
rotate 5
missingok
copytruncate
}

Here we listed all the log files to be managed and provided the attributes of the log management. Don't compress the log file, rotate the log file daily, max number of log rotation 5 ( i.e logfilexxx.1, logfilexxx.2, .... , logfilexxx.5). It only keeps 5 log files. With copytruncate option, the original log file is truncated in place after creating a copy, instead of moving the old log file and optionally creating a new one. It is useful when some program cannot be told its logfile and thus might continue writing(apending) to the previous log file.

[you can use #stat < filename > or # ls -l < filename > to check the inode number
copytruncate helps the log file to preserver it's inode(unique file number) ]

If you don't want to use copytruncate option, then you have to tell the program that log file has been recreated (with new INODE number). For example, in my case I could have done

#cd /etc/logrotate.d
#vi astlog
/var/log/asterisk/full /var/log/asterisk/messages /var/log/asterisk/debug /var/log/asterisk/*.log {
nocompress
daily
rotate 5
missingok
create

        postrotate
                /usr/sbin/asterisk -rx 'logger reload' > /dev/null 2> /dev/null
    endscript

}

Here, we are telling our program 'asterisk' to reload logger as new log file has been created after log rotation.

step 2:

By default, Logrotate is scheduled daily. You can find 'logrotate' under /etc/cron.daily

Let's look at /etc/crontab

# less /etc/crontab
SHELL=/bin/sh
PATH=/usr/bin:/usr/sbin:/sbin:/bin:/usr/lib/news/bin
MAILTO=root
#
# check scripts in cron.hourly, cron.daily, cron.weekly, and cron.monthly
#
-*/15 * * * * root test -x /usr/lib/cron/run-crons && /usr/lib/cron/run-crons >/dev/null 2>&1


The time to execute the scripts is managed by crontab. /usr/lib/cron/run-crons script controls the cron.hourly, cron.daily, cron.weekly and cron.monthly. run_crons runs every 15 minutes and ensures that cron jobs are taken care of.

In SLES, if you need to change the default daily time , you can go to YAST --> System --> /etc/sysconfig editor --> System --> Cron --> DAILY_TIME and change the time.

Let's say, I want logrotation to be done at OFF hours (10:30 pm) to avoid the possible load on the server, then, I can change DAILY_TIME to 22:30

For more information:

http://www.linuxtopia.org/online_books/suse_linux_guides/SLES10/suse_enterprise_linux_server_installation_admin/sec_suse_pakete.html

NFS (Network File System) in SUSE Linux

NFS server allows transparent acess to programs,files or storage space on the server.

Service:Program/Daemon:Start Script

Port mapper: /sbin/portmap : /etc/init.d/portmap

NFS Server: /usr/sbin/rpc.nfsd and /usr/sbin/rpc.mountd : /etc/init.d/nfsserver

NFS server configuration overview:

All configurations for NFS server are stored in the file /etc/exports. Client-side configuration takes place using the file /etc/fstab.
For the NFS server to start automatically when the computer is booted, the coressponding symbolic links in the runlevel directories must be generated. If you configure the NFS server with YaST, this is done automatically; otherwise, you need to generate them with

#insserv nfsserver

or
#chkconfig nfsserver on


In SUSE Linux, it's very easy to setup NFS server using YaST.

Configure and start NFS server:

To use YaST to configure the NFS server, start Yast and then select Network Services > NFS Server.

or
you can do everything manually
You need to set permissions for exported directories in /etc/exports

#vi /etc/exports
/var/backup *(ro)
/var/work *(rw,sync)


[
Note: if you are specifying the write permission, make sure that the directory has write permission for OTHER user. You can add the write permission to the directory/file by
#chmod o+w /var/work
]

Restart the server to reflect the changes

#rcnfsserver restart


Configure and start NFS Client:

To configure the NFS server, start Yast and then select Network Services > NFS Client. Add all the NFS server and mount point information.

I would recommend YaST for client setup, as it will automatically populate /etc/fstab with the provided NFS server information. You can check /etc/fstab file after you configure NFS client using YaST.

Router ARP Cache not releasing Server IP Address

I moved all IP address from old to a new server. However, I can't ping those servers as those IP addresses are not get updated due to arp cache issues ( IPs are cached on the router). How to solve this issue?

As ARP stands Address Resolution Protocol, it is used to resolve IP address to the corresponding Ethernet address. ARP maintains the mapping betweeen IP address and MAC address in a table in a memory called ARP cache. The entries in this table are dynamicaly added and removed. This is common and well known issue as most network admin configure their routers with a long ARP cache timeout. As a result my requests are going to the old server. If I move IP address, it may take hours before server can communicate. To get rid of this problem, we need to request the MAC address for it's own IP which will cause routers and other hardware update ARP cache. This is called a 'unsolicited ARP' or 'gratuitous ARP'

We can use 'arping command' to send an ARP request to resolve its won IP address.

#arping -U -I [Interface Name] [IP Address]

e.g
#arping -U -I eth1 192.168.1.2

where,
-U : Unsolicited ARP mode to update neighbours ARP cache. No replies are expected
-I eth1: Name of network device where to send ARP request packets.

Pure-FTP on SLES OES2

I will be talking about PURE-FTP

Installation:

You can install pure-ftp using YAST ( interactive tool for software installation on SLES/OES2)

# rpm -qagrep ftp
pure-ftpd-1.0.20-24.13

Configuration:

Configuration files:

/etc/pure-ftpd/pure-ftpd.conf
/etc/ftpusers


Make sure that service is running and will startup on next reboot

#chkconfig --list grep ftp
pure-ftpd 0:off 1:off 2:off 3:on 4:off 5:on 6:off

pure-ftpd is the name of the deamon/service. If there were 'off' on runleve 3 and 5, you could run the command

#chkconfig pure-ftpd on


/etc/pure-ftpd/pure-ftpd.conf file is very clean and nicely described.

Some of the parameters are:
ChrootEveryone yes
TrustedGID 100

NoAnonymous yes
UnixAuthentication yes

NoRename yes
and many more


Make sure that, after making some changes on this file, you have to restart the service.

#service pure-ftpd restart



Additional Tips:

What should I do to give ftp user (say ftpuser) access to some different directory?

Ans: Say, ftpuser is chrooted to /home/ftpuser. So, he won't be able to access other than /home/ftpuser. Say, ftpuser need access to /tmp/backup. You can create a shortcut for /tmp/backup in /home/ftpuser. However, 'ln' command will not help in FTP. You will have to use 'mount --bind ' command

#mount --bind /tmp/backup /home/ftpuser/backup

You can go to /etc/bash.bashrc and add the above line to mount the drive automatically at every reboot.


Have a fun :)

User add/remove in SLES

[Note: Don't use [ ] bracket in command line. You have been warned! ]

In SLES, adding user and removing user from the system requires some special flags to be used in command line.

To add user

#useradd -m [username]

-m flag enables to create /home/[username] directory

#useradd -r -m [username]

-r flag create a system account. A system account is an user with an UID between SYSTEM_UID_MIN and SYSTEM_UID_MAX as defined in /etc/login.defs, if no UID is specified. The GROUPS entry in /etc/default/useradd is ignored, too.

#passwd [username]
to create/change the password



Check out this after you add the user

#tail /etc/passwd --> This file contains user's information
testuser:x:1001:100::/home/testuser:/bin/bash
testuserdev:x:103:100::/home/testuserdev:/bin/false

testuser is created using only -m flag. Since, it is regular user, it is allowed to make login in the shell. It uses /bin/bash shell. [Remember, without shell you can't login. As shell is the middleware between user and kernel]

testuserdev is created using -r -m flag. Since, it is system user, it is not allowed to login in the shell. That's the reason, it has /bin/false




To remove the user
#userdel [username]

This will simply remove the user account credentials, but all the files in /home/[username] directory is not removed

#userdel -r [username]

-r flag forces to remove /home/[username] directory as well while deleting the user.