Hadoop - MapReduce

MapReduce is a framework using which we can write applications to process huge amounts of data, in parallel, on large clusters of commodity hardware in a reliable manner.

Hadoop - Command Reference

There are many more commands in "$HADOOP_HOME/bin/hadoop fs" than are demonstrated here, although these basic operations will get you started. Running ./bin/hadoop dfs with no additional arguments will list all the commands that can be run with the FsShell system. Furthermore, $HADOOP_HOME/bin/hadoop fs -help commandName will display a short usage summary for the operation in question, if you are stuck.

A table of all the operations is shown below. The following conventions are used for parameters:

"<path>" means any file or directory name. 
"<path>..." means one or more file or directory names. 
"<file>" means any filename. 
"<src>" and "<dest>" are path names in a directed operation. 
"<localSrc>" and "<localDest>" are paths as above, but on the local file system. 

All other files and path names refer to the objects inside HDFS.

1.	ls <path> Lists the contents of the directory specified by path, showing the names, permissions, owner, size and modification date for each entry.
2.	lsr <path> Behaves like -ls, but recursively displays entries in all subdirectories of path.
3.	du <path> Shows disk usage, in bytes, for all the files which match path; filenames are reported with the full HDFS protocol prefix.
4.	dus <path> Like -du, but prints a summary of disk usage of all files/directories in the path.
5.	mv <src><dest> Moves the file or directory indicated by src to dest, within HDFS.
6.	cp <src> <dest> Copies the file or directory identified by src to dest, within HDFS.
7.	rm <path> Removes the file or empty directory identified by path.
8.	rmr <path> Removes the file or directory identified by path. Recursively deletes any child entries (i.e., files or subdirectories of path).
9.	put <localSrc> <dest> Copies the file or directory from the local file system identified by localSrc to dest within the DFS.
10.	copyFromLocal <localSrc> <dest> Identical to -put
11.	moveFromLocal <localSrc> <dest> Copies the file or directory from the local file system identified by localSrc to dest within HDFS, and then deletes the local copy on success.
12.	get [-crc] <src> <localDest> Copies the file or directory in HDFS identified by src to the local file system path identified by localDest.
13.	getmerge <src> <localDest> Retrieves all files that match the path src in HDFS, and copies them to a single, merged file in the local file system identified by localDest.
14.	cat <filen-ame> Displays the contents of filename on stdout.
15.	copyToLocal <src> <localDest> Identical to -get
16.	moveToLocal <src> <localDest> Works like -get, but deletes the HDFS copy on success.
17.	mkdir <path> Creates a directory named path in HDFS. Creates any parent directories in path that are missing (e.g., mkdir -p in Linux).
18.	setrep [-R] [-w] rep <path> Sets the target replication factor for files identified by path to rep. (The actual replication factor will move toward the target over time)
19.	touchz <path> Creates a file at path containing the current time as a timestamp. Fails if a file already exists at path, unless the file is already size 0.
20.	test -[ezd] <path> Returns 1 if path exists; has zero length; or is a directory or 0 otherwise.
21.	stat [format] <path> Prints information about path. Format is a string which accepts file size in blocks (%b), filename (%n), block size (%o), replication (%r), and modification date (%y, %Y).
22.	tail [-f] <file2name> Shows the last 1KB of file on stdout.
23.	chmod [-R] mode,mode,... <path>... Changes the file permissions associated with one or more objects identified by path.... Performs changes recursively with R. mode is a 3-digit octal mode, or {augo}+/-{rwxX}. Assumes if no scope is specified and does not apply an umask.
24.	chown [-R] [owner][:[group]] <path>... Sets the owning user and/or group for files or directories identified by path.... Sets owner recursively if -R is specified.
25.	chgrp [-R] group <path>... Sets the owning group for files or directories identified by path.... Sets group recursively if -R is specified.
26.	help <cmd-name> Returns usage information for one of the commands listed above. You must omit the leading '-' character in cmd.