Redirect the standard IO streams

[ping83416]

I can easily understand the statements like

1) bzip2 --version >/tmp/list_right 2>/tmp/list_error
2) bzip2 --version >/tmp/list 2>&1

But I really don't understand the statement below. Please help.

bzip2 --version 2>&1 < /dev/null | head -n1

 

[rstanley]

I can easily understand the statements like

1) bzip2 --version >/tmp/list_right 2>/tmp/list_error
2) bzip2 --version >/tmp/list 2>&1

But I really don't understand the statement below. Please help.

bzip2 --version 2>&1 < /dev/null | head -n1

The command calls bzip2,

bzip2

with the version option,

--version

redirects stderr (2) to stdout (1),

2>&1

redirects stdin (0) from /dev/null (Not needed),

< /dev/null

pipes stdout to the head command,

| head

with the number of lines option,

-n1

and outputs only the first line of the multi line version and copyright notice, to stdout.

On my system it displays,

$ bzip2 --version 2>&1 < /dev/null | head -n1
bzip2, a block-sorting file compressor. Version 1.0.6, 6-Sept-2010.

See this article on File Redirection for more information about the predefined stdin, stdout, and stderr files.