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Re: "cmd | cat > file" does not create 'file' before running 'cmd'


From: Paul Jarc
Subject: Re: "cmd | cat > file" does not create 'file' before running 'cmd'
Date: Tue, 24 Sep 2002 11:14:27 -0400
User-agent: Gnus/5.090008 (Oort Gnus v0.08) Emacs/21.2 (i686-pc-linux-gnu)

Alexis Huxley <address@hidden> wrote:
>       However, adding a pipe to the command like this:
>
>       rm -f fred
>       ls fred | cat > fred
>       cat fred
>
>       gives an unexpected failure:
>
>       ls: fred: No such file or directory
>
>       Surely the output file should *still* be created
>       before the command runs, and so the 'ls' should
>       find it when it is executed, and so there should
>       not be an error?

The redirection is bound to the command "cat", not the command
"ls fred | cat".  The redirection is done before the command it is
bound to, but all the elements of the pipeline run concurrently.
Sometimes fred might be created before ls looks for it; sometimes not.
You can make sure it's created first like this:
{ ls fred | cat; } > fred


paul




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