Hadoop commands

Question

I have Hadoop installed in this location

/usr/local/hadoop$

Now I want to list the files inside the dfs. The command I used is :

hduser@ubuntu:/usr/local/hadoop$ bin/hadoop dfs -ls

This gave me the files in the dfs

Found 3 items
drwxr-xr-x   - hduser supergroup          0 2014-03-20 03:53 /user/hduser/gutenberg
drwxr-xr-x   - hduser supergroup          0 2014-03-24 22:34 /user/hduser/mytext-output
-rw-r--r--   1 hduser supergroup        126 2014-03-24 22:30 /user/hduser/text.txt

Next time, I tried the same command in a different manner

hduser@ubuntu:/usr/local/hadoop$ hadoop dfs -ls

It also gave me the same result.

Could some one please explain why both are working despite of executing the ls command from different folders. I hope you guys understood my question.Just explain me difference between these two :

hduser@ubuntu:/usr/local/hadoop$ bin/hadoop dfs -ls
hduser@ubuntu:/usr/local/hadoop$ hadoop dfs -ls

Karlynkarma

looks like you set /usr/local/hadoop/bin in your PATH. check the output of echo $PATH. – Voucher

Answer 1

In unix an executable file can be executed in two ways, either by giving the absolute/relative path or commands in system executables path(path should be specified in PATH variable)

When you execute bin/hadoop dfs -ls should be inside the directory /usr/local/hadoop. Or /usr/local/hadoop/bin/hadoop dfs -ls will also work

There is one environment variable PATH in unix which keeps in the list of executable location by default it keeps the following path /usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin: . Whenever we execute any command like ls, mkdir etc it is taking from the one location in PATH variable. When you give the command hadoop(it will be taken from the path /usr/local/hadoop/bin/). Since you have specified the path /usr/local/hadoop/bin/ in PATH variable. Use the following command to check the value of your PATH variable

echo $PATH

Preciosity

Answer 2

You set a hadoop global path HADOOP_HOME in your ~/.bashrc file so that Hadoop commands will works in anywhere in Terminal.

Daddylonglegs

Answer 3

In both case you got same result because you already set HADOOP_HOME/bin in bashrc file you can check the entry by sudo nano ~/.bashrc , we do this because it give us convenience to execute command from terminal irrespective of current file directory

if you remove HADOOP_HOME/bin entry from bashrc file you will not get the same result

Szeged

Answer 4

First of all, you are executing the same command. bin/hadoop in hadoop installation dir and hadoop are same, For this check your .bashrc file where you must have specified the executable path for hadoop. if you call hadoop it means you are calling /usr/local/hadoop/bin/hadoop command. If you are having problem with output of ls- you are executing ls on hadoop file system, not on local file system. It will show you the content available in hadoop file system. for more details go to localhost:50070 and check the content

Caltanissetta

Answer 5

In both case you got same result because you already set HADOOP_HOME/bin in bashrc file. Because in unix an executable file can be executed in two ways, either by giving the absolute/relative path or commands in system's executable path.

When you execute - "bin/hadoop dfs -ls" should be inside the directory /usr/local/hadoop.

Toomey

Answer 6

To locate where the executable file associated with the hadoop command, just run:

which hadoop

This will print out the location of the hadoop command used.

Microscopy

Answer 7

At a certain point during the Hadoop installation you configure the hdfs filesystem. And eventually you format it using hdfs namenode -format. From that point on dfs does not refer to your own filesystem, but the hdfs filesystem. When you execute hadoop dfs -ls it displays the user's home directory on the hdfs filesystem. It doesn't matter from where you are located on the host filesystem when you execute the command because it's not being used.

However it is possible not to configure hdfs and it'll use the local filesystem. Either way hadoop dfs -ls displays the contents of the user's home directory.

With that note, if you remove the user directory /user/hduser and execute hadoop dfs -ls it will give you an error because the user directory does not exist.

Source: https://amodernstory.com/2014/09/23/installing-hadoop-on-mac-osx-yosemite/

Another:

Blithesome

Answer 8

This is related to OS and not Hadoop.

When ever you run a command without an explicit path the OS will search the locations in the PATH variable. In your case during installation of Hadoop you must have set some what following variables in your user profile (.bashrc or .profiles)

export HADOOP_INSTALL=/usr/local/hadoop
export PATH=$HADOOP_INSTALL/bin:$PATH

So whenever you type following it will check in $HADOOP_INSTALL/bin as you have set this path in OS PATH variable.

hadoop dfs -ls

And when you type following then it will use current folder path which is in your snippet is /usr/local/hadoop and just under your current folder there is bin/hadoop file

bin/hadoop dfs -ls 

Hence in both the cases it is using same file for execution but identification of it done via PATH variable in one case and current directory content path (absolute path) in another case.

Moralez

I'd just add that you can use which hadoop do see which executable is found (in case there are multiple executables with the same name in your path). – Vaticinate

And that the path is searched left to right, and only until a match is found. – Vaticinate

Answer 9

Its because PATH Variable you set while configuring Hadoop. PATH contains path to your HADOOP home so thats why even if you not specified the /bin path, it is taking it from PATH variable.

Hymeneal

Answer 10

Both the commands are doing the same thing.

Check your /etc/bashrc or /root/.bashrc

There you will find the HADOOP_HOME set and bin path is added along with the path variable. While setting this,we will be able to execute the hadoop commands anywhere from the command line.No other use on it..!!!

Imbalance

Answer 11

It is working because you are not executing shell "ls" command instead passing "ls" as parameter to the command "hadoop" so in both case same exact command with same parameters is executed i.e. 'hadoop dfs -ls' only thing changing is that in one case you are qualifying it with path in another case you are not and that is working because 'hadoop' must be in set in you $PATH environment variable.

Saharan

Answer 12

The command hadoop dfs ls is equal to ls command in the hdfs file system. We can treat that as the ls command in linux/unix.

Superload

I guess the above makes sense, but the grammar isn't well. Would someone care to fix this? – Cindelyn

Can't you fix it yourself? – Atavistic

Answer 13

When you are log in as hadoop user just type hado in terminal and press TAB key you will get hadoop means your hadoop setup is working properly.. So your ~./bashrc file is also set properly.. it means that when you use this command from any directory structure hadoop dfs -ls / it will gives you list of all files which are present in hdfs

Fishbowl

Answer 14

The hadoop executable is within the /bin/ folder so it is the exact same command as long as /bin/hadoop/ is set as a the 'hadoop' variable in $PATH. You can find the $PATH variable defined in the ~/.bashrc file. Try cat .bashrc from you root directory so you can take a look.

Klecka

Answer 15

There is one simple concept in LINUX Operating System. You can say bashrc file is linked to the terminal. So when you open the terminal then it loads all the variable from the bashrc file. Any executable appended in the path variable defined in the bashrc file can be loaded from any directory you presently are.

Now to answer your question. Since hadoop fs -ls / was working fine for you so that means the executable is in your PATH variable. and for the latter case bin/hadoop fs -ls / you are manullay going to the folder where the executable is present. This is similar to running any executable in LINUX.

Brockie

Answer 16

just set all the hadoop, yarn and other path in .bashrc file. it will run from anywhere.

standard hadoop>bin/hadoop fs -ls see the below hadoop forum for more details on hadoop.

http://tekzak.com/forum/viewforum.php?f=2&sid=5d01e2e3c27aebc6e7ee95447ef328a4

Boletus

Answer 17

The variable HADOOP_HOME might have been set with the bin path of hadoop binary. In such a case, both the above commands irrespective of from where the command hadoop is executed will work.

Colorcast

Answer 18

You will have to give the absolute path followed by bin/hadoop dfs -ls

 Absolute_path/bin/hadoop dfs -ls

Immersionism

Answer 19

Because You set your hadoop path in .bashrc file of your hadoop user.SO you dont need to navigate your path to bin folder.The command which work from bin folder also works from root folder of current user.

Aparejo

Answer 20

1. Command hadoop fs -ls is to list all files and directories in root folder in your hadoop file system (HDFS) rather then your current file system. Without any modifications of files in root of HDFS, result should be the same using this command.

2. you have already added hadoop/bin in the path when you installed hadoop in your machine. Command would directed to the same path wherever you run it in your current system.

Therefore, you dont make any change in your HDFS and you use the same command. That is why you get exactly the same result.

Malenamalet

Answer 21

Both the commands bin/hadoop dfs -ls and hadoop dfs -ls are working for you because you have the hadoop executable (/usr/local/hadoop/bin/hadoop) set in $PATH variable for the user "hduser".

To understand it further, you can open a Terminal, and remove the value ($HADOOP_HOME/bin or /usr/local/hadoop/bin) from $PATH using the export command in linux. If you do this, the second command(hadoop dfs -ls) won't work for that Terminal Session.

Abutilon

Answer 22

In your case hadoop is present at bin/ (within /usr/local/hadoop), so you may execute it as bin/hadoop from /usr/local/hadoop(which is the current location in above example).

You are also able to execute it directly without specifying the relative/absolute path as the location of hadoop is added to the PATH.

You may check this by running which hadoop and printing PATH(echo $PATH).

Hop

Answer 23

when you install Hadoop its binaries are added to /usr/bin folder.

any Binary in folder /bin, /sbin, /usr/bin are available from any path and user in UNIX. proof: 1: https://askubuntu.com/questions/571617/what-is-the-purpose-of-the-bin-directory

Just to add, there are differences between folders /bin, /sbin etc and differences are explained here (https://askubuntu.com/questions/308045/differences-between-bin-sbin-usr-bin-usr-sbin-usr-local-bin-usr-local)

Caterer

Answer 24

Its because of bashrc linux env setup

1) export HADOOP_PREFIX=/usr/local/hadoop/

2) export PATH=$PATH:$HADOOP_PREFIX/bin

After doing this we need to run command

exec bash 

Synod

Answer 25

Its quite likely that you have exported

$HADOOP_HOME/bin in PATH variable.

Like if it is EMR then it would be export HADOOP_PREFIX=/usr/lib/hadoop export PATH=$PATH:$HADOOP_PREFIX/bin

You can check the path and find out

Rutherfurd

Answer 26

It is because you have already exported hadoop path when you did install hadoop . Now you can either go to the exact hadoop path or just type hadoop from whereever you are. It will work both ways.

Blowing

Answer 27

I will answer your question in 2 perspectives,

hduser@ubuntu:/usr/local/hadoop$ bin/hadoop dfs -ls
hduser@ubuntu:/usr/local/hadoop$ hadoop dfs -ls

1. How these 2 commands works in a same way?
   It is because your environment is aware of hadoop commands. It is because your $PATH variable contains hadoop installation directory.
2. Why both returns the same result?
   It is because you are trying to list the directory which is residing in hdfs. When you execute hadoop dfs -ls command it will list the current user's home directory items from hdfs. In your case it lists hduser user directory data.

Hope it answers your question.

Aperture

Answer 28

Both the commands gives the same result because we will already give the home path while installing hadoop itself. So it will work even if we give the original path and also will work if it is given directly.

Impignorate