I/O Redirection allows us to change destination of data. Reasons to use it are countless. Some of them are clean output when looking for files. Commands like:

find / -iname '*.java'
grep -irl 'Hello World' /

Produces huge amount of errors when we try to look into directory (or files) without proper permission.

find: /.DocumentRevisions-V100: Permission denied
find: /.fseventsd: Permission denied
find: /.Spotlight-V100: Permission denied
find: /.Trashes: Permission denied

Other reason can be saving different outputs to different log files or advance piping.

Most of uses:

# Hide / remove error stream
command 2>/dev/null

# Combine output and error stream as one output stream
command 2>&1
http://www.tldp.org/LDP/abs/html/io-redirection.html http://unix.stackexchange.com/questions/70963/difference-between-2-2-dev-null-dev-null-and-dev-null-21#70971
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