Victory goes to the player who makes the next-to-last mistake
June 29, 2013 5 Comments
When we fire ls –all in linux cli, files may be listed in different colours
Blue: Directory file White: Normal file Green: Executable file Yellow: Device file Magenta: Picture file Cyan: link file Red: Compressed file
-(Hyphen) = Normal file d=directory l=link file b=Block device file c=character device file
June 29, 2013 5 Comments
There may be incidents which we accidently delete necessary files from hadoop. Sometimes the entire file system may get deleted. For doing recovery process the below steps may help you.
For doing this recovery method trash should be enabled in hdfs. Trash can be enabled by setting the property fs.trash.interval greater than 0. By default the value is zero. Its value is number of minutes after which the checkpoint gets deleted. If zero, the trash feature is disabled. We have to set this property in core-site.xml.
<property> <name>fs.trash.interval</name> <value>30</value> <description>Number of minutes after which the checkpoint gets deleted. If zero, the trash feature is disabled. </description> </property>
There is one more property which is having relation with the above property called fs.trash.checkpoint.interval. It is the number of minutes between trash checkpoints. This should be smaller or equal to fs.trash.interval. Everytime the checkpointer runs, it creates a new checkpoint out of current and removes checkpoints created more than fs.trash.interval minutes ago.The default value of this property is zero.
<property> <name>fs.trash.checkpoint.interval</name> <value>15</value> <description>Number of minutes between trash checkpoints. Should be smaller or equal to fs.trash.interval. Every time the checkpointer runs it creates a new checkpoint out of current and removes checkpoints created more than fs.trash.interval minutes ago. </description> </property>
If the above properties are enabled in your cluster. Then the deleted files will be present in .Trash directory of hdfs. You have time to recover the files until the next checkpoint occurs. After the new checkpoint the deleted files will not be present in the .Trash. So recover before the new checkpoint. If this property is not enabled in your cluster, you can enable this for future recovery.. 🙂
June 22, 2013 2 Comments
For disabling the GUI login mode, do the following steps
Open the terminal and do the following commands as root user or sudo user.
Edit /etc/inittab, enter:
# nano /etc/inittab
Find:
id:5:initdefault:
Replace with:
id:3:initdefault:
Save and close the file.
For stopping the currently running GUI session:
# init 3
To get back into the GUI mode temporarily, type the following in the terminal.
# init 5
June 22, 2013 Leave a comment
If you want to change the default GUI mode boot of ubuntu machine, just do the following changes. Here actually we are not removing the GUI, just changing the default booting as text mode. By doing this you can reduce the memory consumption.
Open the terminal as root user or sudo user.
open /etc/default/grub as and add text to the
sudo nano /etc/default/grub
And in the file, change this line:
GRUB_CMDLINE_LINUX_DEFAULT="text"
Then update your Grub
sudo update-grub
To view the changes, reboot the system. Then the system will always boot in text mode. This is applicable to Ubuntu OS only.
If you want to get the UI without changing the configuration from text mode, execute startx command in the terminal. Then the UI will be loaded.
June 6, 2013 Leave a comment
Java provides built-in support for multithreaded programming. A multithreaded program contains two or more parts that can run concurrently. Each part of such a program is called a thread, and each thread defines a separate path of execution.
Here I am explaining a simple multi-threaded program.
The main thread writes 5000 to 1 in a file named MainThread.txt and the child thread writes 1 to 5000 in a file named childthread.txt.
Both will happen at the same time. That is it will run in parallel.
We are creating a child thread class by implementing a method Runnable.
This class will contain a method named run() where we do our functionality.
We will instantiate this thread class in the main method, so it will run along with the main thread.
The child thread class is
package com.amal.thread;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
public class ThreadTest implements Runnable {
Thread t;
ThreadTest()
{
t=new Thread(this,"My Test");
System.out.println("My test thread");
t.start();
}
public void run() {
File file=new File("childthread.txt");
try {
FileWriter fwt = new FileWriter(file.getAbsoluteFile());
BufferedWriter bwt = new BufferedWriter(fwt);
for(int i=0; i<5000; i++)
{
bwt.write("thread "+i);
bwt.newLine();
}
bwt.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
The main class is
package com.amal.thread;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
public class MainClass {
public static void main(String[] args) throws IOException {
new ThreadTest();
File file1=new File("MainThread.txt");
FileWriter fw = new FileWriter(file1.getAbsoluteFile());
BufferedWriter bw = new BufferedWriter(fw);
for (int i=5000;i>1;i--)
{
bw.write("main "+i);
bw.newLine();
}
bw.close();
}
}
June 3, 2013 Leave a comment
One handy feature of pig’s Grunt shell is completion mechanism, which will try to complete
Pig Latin keywords and functions when you press the Tab key. For example, consider
the following incomplete line:
grunt> a = foreach b ge
If you press the Tab key at this point, ge will expand to generate, a Pig Latin keyword:
grunt> a = foreach b generate
We can customize the completion tokens by putting our necessary tokens in a file named autocomplete and put it in the pig class path or in the directory where we are invoking the grunt shell
For example: I created a file named autocomplete which contains the tokens
Julie India Software Engineer Hadoop Bigdata
Then after saving this if u press the corresponding alphabet and press tab, it will display the choices for autocompletion.
Note: The tokens that I mentioned above is not related to pig commands or funtions. It is just for an example only. Like this you can create your own custom scripts or tokens for making the scripting handy
June 1, 2013 3 Comments
Type hostname or ipaddress
Then in the left side part of putty, click on SSH and expand.
Then you can see a section auth
Click on auth
There you will get a window with browse button.
Load your private key file (.ppk) and press open.
Then enter username and passphrase-key (if given) and login
This is the method we usually use to login to unix cloud instances .
June 1, 2013 Leave a comment
On a Windows machine, you can use PuttyGen to generate a public/private key pair.
PuttyGen can be downloaded from http://www.putty.org/
The private key is what you need on the client machine – for use with Putty for example. The public key goes to the host machine.
Open PuTTY Key Generator (puttygen.exe in the putty folder) which should look something like this.
PuTTYGen supports 3 key types:
SSH-2 contains more features than SSH-1. SSH-1 has some design flaws which make it more vulnerable than SSH-2. Only choose SSH-1 if the server/client you want to connect to does not support SSH-2. The default SSH-2 RSA is probably better than SSH-2 DSA.
The Number of bits in a genereted key sets the size of your key, and thus the security level. For SSH-2 RSA, it’s recommended to set this at a minimum of 2048. PuTTYGen defaults to 1024. Setting this to 4096 would provide an even stronger key, but is probably overkill for most uses.
Click Generate to start the key generation. You will see something like the figure below ( move your mouse as suggested above the progress bar):
The result of the key generation is shown below. (in the box labelled Public key for pasting into OpenSSH authorized_keys file).
The Key comment enables you to generate multiple keys and easily tell them apart. It’s general recommended to set this to username@hostname, where the username is the username used for login, and hostname is, as it says on the tin, the name of the host machine. For example, for a user ‘amal’ on domain ‘example.com’, set this to amal@example.com.
The Key passphrase is an additional way to protect your private key, and is never transmitted over the internet. The strength of your key is not affected by the passphrase in any way. If you set one, you will be asked for it before any connection is made via SSH . Setting it might gain you a few extra moments if your key falls into the wrong hands, as the culprit tries to guess your passphrase. Obviously if your passphrase is weak, it rather defeats the purpose of having it.
If you don’t want the passphrase key, you can leave it empty.
Note that if your set a passphrase and forget it, there is no way to recover it. When you reload a previously saved private key (using the Load button), you will be asked for the passphrase if one is set.
Here is what PuTTYGen looks like after editing the key comment and the passphrase.
Now save your keys – one private and one public – using the Save private key and Save public key buttons respectively. You can save the public key in any format – *.txt is good. The private key is saved in PuTTY’s format – *.PPK. PuTTY will need this private key for authentication.
The public key in the highlighted box is all in one line as expected by OpenSSH, and is in the correct format (unlike the version you just saved). If you are using OpenSSH, this is what you paste in your .ssh/authorized_keys file.
June 1, 2013 Leave a comment
Access to Linux and Unix systems via Secure Shell (SSH) is standard practice. It offers encrypted access to enable you to administer your server which is vital over the big bad internet.
There are different ways to access SSH: password, user keys and host-based keys. Passwords are the most common but are less secure than key-based access. Passwords are susceptible to keylogger attacks, as well as more likely to fool users into a “man-in-the-middle” attack (one where you think you’re logging onto your server, but you are actually proxying your connection through another server which has been compromised and is recording every keystroke and data transfer.)
Key based access is more secure as it requires two parts of a key to be present before access is granted. When dealing with cloud based services such as Rackspace and Amazon Web Services, key based access is enabled by default. Key based access is also known as “passwordless access” as access is granted by your key, not by asking for any passwords. The exception to this is if you put a password on your key (but you can enable services that ask for this password once and it is cached for the rest of your session).
Setting this up on your Linux server is very simple, and most installations of SSH (OpenSSH) enable both password and key-based access by default. Let’s assume user@client needs to access user@server
Debian/Ubuntu
sudo apt-get install openssh-server
CentOS/Fedora/RedHat/Oracle Enterprise Linux
sudo yum install openssh-server
RSAAuthentication yes PubkeyAuthentication yes
Debian/Ubuntu
sudo /etc/init.d/ssh restart
CentOS/Fedora/RedHat/Oracle Enterprise Linux
sudo /etc/init.d/sshd restart
On your Linux client (desktop or other server you’ll be using to connect to the server configured in steps 1-3)
ssh-keygen -t rsa
You will see output like the following:
Generating public/private rsa key pair. Enter file in which to save the key (/home/user/.ssh/id_rsa): Created directory ‘/home/user/.ssh’. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/user/.ssh/id_rsa. Your public key has been saved in /home/user/.ssh/id_rsa.pub. The key fingerprint is: 79:e1:08:77:c2:0d:c4:ff:35:22:64:9a:4d:03:b8:67 user@client The key’s randomart image is: +–[ RSA 2048]—-+ | ++. | | …o= | | ..+O+. | | .oE*+.. o | | oS oo o . | | . . | | | | | | | +—————–+
This produces two important pieces of data. Your PRIVATE KEY (~/.ssh/id_rsa) and your PUBLIC KEY (~/.ssh/id_rsa.pub). You must keep your PRIVATE KEY safe. Your public key can be given to anyone. Without your private key your public key is just a string of characters and you can’t generate a private key from a public key. Equally, you can’t generate a public key from a private key. Together they make your key-pair.
To enable your private key to access the server running SSH configured in steps 1-3 (server) you simply copy the contents of your public key onto the server.
Copy the public key from your client machine to server
scp .ssh/id_rsa.pub user@server: (enter your password)
Login to server
ssh user@server (enter your password)
Copy the public key to authorized_keys
cat .ssh/id_rsa.pub >> .ssh/authorized_keys
Change the permission of authorized_keys file to 600 (rw——-)
chmod 0600 .ssh/authorized_keys
This creates the directory .ssh/ and relevant authorized_keys file with the correct permissions (anything less strict will not work). You can put in a number of public keys in here, line-by-line. When there are multiple entries it allows multiple people to connect to that account using their keys. This becomes useful when a team of system administrators require access to systems with minimal accounts installed, but each are accountable for audit purposes as to who logged onto the system.
Log out of that session and log back in again and you shouldn’t be asked for a password.
June 1, 2013 1 Comment
FS Shell
FS shell means FileSystem shell. The file system may be hdfs or the local file system(linux file system.
For HDFS the scheme is hdfs,
Eg: hdfs://namenodehost:<port>/user/test
For the local filesystem the scheme is file.
Eg: file:///testfile
If no schema is specified, the default scheme specified in the configuration is used. By default it is hdfs.
Majority of the commands in FS shell behave like corresponding Unix commands. Differences are described with each of the commands.
cat
Usage: hadoop fs -cat URI [URI …]
Copies source paths to stdout.
Example:
Exit Code:
Returns 0 on success and -1 on error.
chgrp
Usage: hadoop fs -chgrp [-R] GROUP URI [URI …]
Change group association of files. With -R, make the change recursively through the directory structure. The user must be the owner of files, or else a super-user. Additional information is in the HDFS Admin Guide: Permissions.
chmod
Usage: hadoop fs -chmod [-R] <MODE[,MODE]… | OCTALMODE> URI [URI …]
Change the permissions of files. With -R, make the change recursively through the directory structure. The user must be the owner of the file, or else a super-user. Additional information is in the HDFS Admin Guide: Permissions.
chown
Usage: hadoop fs -chown [-R] [OWNER][:[GROUP]] URI [URI ]
Change the owner of files. With -R, make the change recursively through the directory structure. The user must be a super-user. Additional information is in the HDFS Admin Guide: Permissions.
copyFromLocal
Usage: hadoop fs -copyFromLocal <localsrc> URI
Similar to put command, except that the source is restricted to a local file reference.
copyToLocal
Usage: hadoop fs -copyToLocal [-ignorecrc] [-crc] URI <localdst>
Similar to get command, except that the destination is restricted to a local file reference.
count
Usage: hadoop fs -count [-q] <paths>
Count the number of directories, files and bytes under the paths that match the specified file pattern. The output columns are:
DIR_COUNT, FILE_COUNT, CONTENT_SIZE FILE_NAME.
The output columns with -q are:
QUOTA, REMAINING_QUATA, SPACE_QUOTA, REMAINING_SPACE_QUOTA, DIR_COUNT, FILE_COUNT, CONTENT_SIZE, FILE_NAME.
Example:
Exit Code:
Returns 0 on success and -1 on error.
cp
Usage: hadoop fs -cp URI [URI …] <dest>
Copy files from source to destination. This command allows multiple sources as well in which case the destination must be a directory.
Example:
Exit Code:
Returns 0 on success and -1 on error.
du
Usage: hadoop fs -du URI [URI …]
Displays aggregate length of files contained in the directory or the length of a file in case its just a file.
Example:
hadoop fs -du /user/hadoop/dir1 /user/hadoop/file1 hdfs://nn.example.com/user/hadoop/dir1
Exit Code:
Returns 0 on success and -1 on error.
dus
Usage: hadoop fs -dus <args>
Displays a summary of file lengths.
expunge
Usage: hadoop fs -expunge
Empty the Trash. Refer to HDFS Architecture for more information on Trash feature.
get
Usage: hadoop fs -get [-ignorecrc] [-crc] <src> <localdst>
Copy files to the local file system. Files that fail the CRC check may be copied with the -ignorecrc option. Files and CRCs may be copied using the -crc option.
Example:
Exit Code:
Returns 0 on success and -1 on error.
getmerge
Usage: hadoop fs -getmerge <src> <localdst> [addnl]
Takes a source directory and a destination file as input and concatenates files in src into the destination local file. Optionally addnl can be set to enable adding a newline character at the end of each file.
ls
Usage: hadoop fs -ls <args>
For a file returns stat on the file with the following format:
filename <number of replicas> filesize modification_date modification_time permissions userid groupid
For a directory it returns list of its direct children as in unix. A directory is listed as:
dirname <dir> modification_time modification_time permissions userid groupid
Example:
hadoop fs -ls /user/hadoop/file1 /user/hadoop/file2 hdfs://nn.example.com/user/hadoop/dir1 /nonexistentfile
Exit Code:
Returns 0 on success and -1 on error.
lsr
Usage: hadoop fs -lsr <args>
Recursive version of ls. Similar to Unix ls -R.
mkdir
Usage: hadoop fs -mkdir <paths>
Takes path uri’s as argument and creates directories. The behavior is much like unix mkdir -p creating parent directories along the path.
Example:
Exit Code:
Returns 0 on success and -1 on error.
moveFromLocal
Usage: dfs -moveFromLocal <localsrc> <dst>
Similar to put command, except that the source localsrc is deleted after it’s copied.
moveToLocal
Usage: hadoop fs -moveToLocal [-crc] <src> <dst>
Displays a “Not implemented yet” message.
mv
Usage: hadoop fs -mv URI [URI …] <dest>
Moves files from source to destination. This command allows multiple sources as well in which case the destination needs to be a directory. Moving files across filesystems is not permitted.
Example:
Exit Code:
Returns 0 on success and -1 on error.
put
Usage: hadoop fs -put <localsrc> … <dst>
Copy single src, or multiple srcs from local file system to the destination filesystem. Also reads input from stdin and writes to destination filesystem.
Exit Code:
Returns 0 on success and -1 on error.
rm
Usage: hadoop fs -rm URI [URI …]
Delete files specified as args. Only deletes non empty directory and files. Refer to rmr for recursive deletes.
Example:
Exit Code:
Returns 0 on success and -1 on error.
rmr
Usage: hadoop fs -rmr URI [URI …]
Recursive version of delete.
Example:
Exit Code:
Returns 0 on success and -1 on error.
setrep
Usage: hadoop fs -setrep [-R] <path>
Changes the replication factor of a file. -R option is for recursively increasing the replication factor of files within a directory.
Example:
Exit Code:
Returns 0 on success and -1 on error.
stat
Usage: hadoop fs -stat URI [URI …]
Returns the stat information on the path.
Example:
Exit Code:
Returns 0 on success and -1 on error.
tail
Usage: hadoop fs -tail [-f] URI
Displays last kilobyte of the file to stdout. -f option can be used as in Unix.
Example:
Exit Code:
Returns 0 on success and -1 on error.
test
Usage: hadoop fs -test -[ezd] URI
Options:
-e check to see if the file exists. Return 0 if true.
-z check to see if the file is zero length. Return 0 if true
-d check return 1 if the path is directory else return 0.
Example:
text
Usage: hadoop fs -text <src>
Takes a source file and outputs the file in text format. The allowed formats are zip and TextRecordInputStream.
touchz
Usage: hadoop fs -touchz URI [URI …]
Create a file of zero length.
Example:
Exit Code:
Returns 0 on success and -1 on error.
Official Linux User
