Category Archives: Linux Tips

Linux: How do you find out what your server’s outgoing ip is?

13 Replies

There are many times when I needed to find out my outgoing (or external) IP for the servers which are behind load balancers or firewalls.  I used to just login to another external server from the server in question and find out by looking at “who” what my external ip is.  Even though it works and I am so used to it, today I decided to figure out a more graceful way of finding my outgoing ip.  As most of us already know, whatismyip.com is the quickest way to find out your outgoing ip from the browser.  So I decided to use the same way on the servers.  So I issued a wget:

wget http://www.whatismyip.org

Well that does the trick.  But being lazy as I am, I did not want to have to cat the output file to find out the ip (plus there is no point of creating extra files and doing extra work to remove them).  So if you are ssh’ed in, you can issue following command (I am sure there is another way of doing it, but this is the quickest way I could think of):

wget -q -O - http://www.whatismyip.org

-O tells wget to redirect output to the following file (- being the standard out ).  So it basically echo’s output to our console.

-q makes wget run in  quiet mode so you do not see all of the connection/download/etc output.

That is it!  I am curious to know what other ways people use to get the same information.  Please share your way if possible.

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DISCLAIMER: Please be smart and use code found on internet carefully. Make backups often. And yeah.. last but not least.. I am not responsible for any damage caused by this posting. Use at your own risk.

Quick tip: how do you rename all files so spaces are converted to underscores?

4 Replies

My friend today asked me how to convert all spaces in filenames under a specified directory to underscores. Also, at the same time lowercase all of the filenames. Here is a quick script to do what is needed. Let us start with creating some test data in a temp directory:

mkdir temp
cd temp
touch Foo FooO "Foo Bar" "FOO BAaR"
\ls | while read -r FILENAME
do
mv -v "$FILENAME" `echo $FILENAME| tr ' ' '_'| tr '[A-Z]' '[a-z]'`
done

Note:  I intentionally have slash in front of ls (\ls).  \ls means that we want to make sure there is no ls alias overwriting our command. This is needed if your system has alias setup to display ls in a different way instead of default listing.  mv -v shows us the filenames being renamed as your script goes through the whole dir.  Your output should be like:

`Foo' -> `foo'
`FOO BAaR' -> `foo_baar'
`Foo Bar' -> `foo_bar'
`FooO' -> `fooo'

One of the very powerful commands in this post is the “tr” command. This command is not as popular as sed or awk but it is very useful and simple to use (read more about tr).

If you only needed to convert spaces to underscores and you are using CentOS/Fedora/Redhat, you can use this simpler method. NOTE: this command is not available on all distributions: rename " " "_" *

Learn more about rename command
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DISCLAIMER: Please be smart and use code found on internet carefully. Make backups often. And yeah.. last but not least.. I am not responsible for any damage caused by this posting. Use at your own risk.

Linux: How do you rename a user account in linux?

18 Replies

In Linux, there is no command which will rename a user account. If you make a mistake creating a user account, user changes their name or if user does not like his user name, there is no real easy way of going and making the change. Only thing I know you can do is to go through some files and rename user manually. Let us say that we have a user who is named joe and we want to rename him to john.

Note: you must be logged in as root to do following.

vi /etc/passwd
find joe and change it to john, save/exit

vi /etc/group
find joe and change it to john, save/exit

vi /etc/shadow
find joe and change it to john. This file is read only and you have to force overwrite it. In vi it is :w! once saved, quit.

cd /home
mv joe john

And that should do the trick.

[Edited] Right after I posted this post, I was contacted and was told to look at utility called usermod. Read more about it man usermod. Got to love the quick responses.

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DISCLAIMER: Please be smart and use code found on internet carefully. Make backups often. And yeah.. last but not least.. I am not responsible for any damage caused by this posting. Use at your own risk.

MySQL: How do you set up master-master replication in MySQL? (CentOS, RHEL, Fedora)

16 Replies

Setting up master-master replication in MySQL is very similar to how we set up master/slave replication. You can read up about how to setup master/slave replication in my previous post: How to set up master/slave replication in MySQL. There is obviously pros and cons about using master/master replication. But this is not a post which discuses advantages and disadvantages for using master/master replication. One of the differences between master/master set up and master/slave is that in master/master set up, you have true redundancy. If one server dies, second server can take all the inserts/selects. In master/slave setup, if master dies, you will have to go through steps to make slave become the master. Master/master set up we are going to set up is essentially master/slave and slave/master. Meaning, if you had two servers, db0 and db1, your setup will be db0(master)/db1(slave) and also db0(slave)/db1(master). Here are some assumptions:

Master1 server ip: 10.0.0.1
Master2 server ip: 10.0.0.2
Slave username: slaveuser
Slave pw: slavepw
Your data directory is: /usr/local/mysql/var/

Let us go through the steps you must take on Master1 to enable it to act as master and slave by using following configuration which goes under [mysqld] section:

# let's make it so auto increment columns behave by having different increments on both servers
auto_increment_increment=2
auto_increment_offset=1
# Replication Master Server
# binary logging is required for replication
log-bin=master1-bin
binlog-ignore-db=mysql
binlog-ignore-db=test
# required unique id between 1 and 2^32 - 1
server-id = 1
#following is the slave settings so this server can connect to master2
master-host = 10.0.0.2
master-user = slaveuser
master-password = slavepw
master-port = 3306

Following is the configuration which goes on master2 under [mysqld] section:

# let's make it so auto increment columns behave by having different increments on both servers
auto_increment_increment=2
auto_increment_offset=2
# Replication Master Server
# binary logging is required for replication
log-bin=master2-bin
binlog-ignore-db=mysql
binlog-ignore-db=test
# required unique id between 1 and 2^32 - 1
server-id = 2
#following is the slave settings so this server can connect to master2
master-host = 10.0.0.1
master-user = slaveuser
master-password = slavepw
master-port = 3306

On master1 server, go to mysql> prompt and add the appropriate user:

mysql> grant replication slave on *.* to slaveuser@'10.0.0.2' identified by 'slavepw';

On master2 server do the same but allow right ip:

mysql> grant replication slave on *.* to slaveuser@'10.0.0.1' identified by 'slavepw';

Restart both of the master servers and check slave status:

mysql> show slave status\G

That is all you have to do to set up the replication. Of course there are a lot more configuration options but this should get your replication going and you can tweak from here on.

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DISCLAIMER: Please be smart and use code found on internet carefully. Make backups often. And yeah.. last but not least.. I am not responsible for any damage caused by this posting. Use at your own risk.