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Max Koretskyi
Asked: 2014-11-09 01:54:15 +0800 CST

Why doesn't redirecting output from nginx work?

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I want to list all modules of nginx server by line, so I type the following:

nginx -V | tr -- - '\n'

But it doesn't put modules in lines. If I do it like this:

2>&1 nginx -V | tr -- - '\n'

it works. But why?

output

1 Answers

  1. Best Answer

    What you're observing indicates that nginx -V writes its output to the standard error stream (aka stderr - on file descriptor 2) rather than the standard output stream (aka stdout - file descriptor 1). Only stdout gets passed through the pipe | so your tr command doesn't receive it.

    The 2>&1 syntax tells the shell to redirect stderr to stdout so that it then gets piped to tr. FYI the placement you have used is syntactically legal but unconventional - most often that kind of redirection would be written as 

    nginx -V 2>&1 | tr -- - '\n'

    BTW the pipe | is really redirecting the output of the first command rather than the input of the second, which is coming from the standard input stream (stdin - file descriptor 0) as usual.

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