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

[Paul Jarc]

Alexis Huxley <ahuxley@gmx.net> 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