feed output of command to another command as file

Question

I can make config file of tor with cat <<EOF command as follows :

cat <<EOF
SocksPort 9050
ControlPort 9150
DataDirectory /usr/var/lib/tor/torrc
EOF

but it can save output with redirect > to file. And then use tor -f file

But how feed directly to tor -f ? I don't want to make file or any kind of temp file and delete afterwards.

• I want something like

  tor -f < $(cat <<EOF
  ...
  ...
  EOF)
  

  but it gives EOF): ambiguous redirect
• With direct redirect ? tor -f <<EOF... not worked
• And pipeline will give error.

Thanks

Answer 1

What you're looking for is called process substitution; it's not a standard feature of the shell, but supported in Bash/ksh/zsh. An example:

$ echo <(echo foo)
/dev/fd/63
$ cat <(echo foo)
foo

The <(cmd) expands to the name of a file which the shell arranges to have the output from cmd available to read. On Linux, it's likely /dev/fd/NN, but it might be a named pipe too, depends on the system and the shell.

So, I suppose you could use

tor -f  <(cat <<EOF
...
...
EOF
)

Note that above with cat, I could have used cat < <(echo foo) with the redirection instead of cat <(echo foo), it's the same difference as with cat < somefile and cat somefile. cat works with either, since it reads stdin by default, but if the tool expects a filename, you wouldn't add the redirection.

---

Anyway, the above is a bit hairy, it'd be better to have tor read the here-doc directly, without another process in the middle. You'll just have to make it read stdin.

A few utilities read stdin (or write to stdout) if the filename given is -, but if tor doesn't support that, you can use /dev/stdin (resp. /dev/stdout), so I suppose this might work:

tor -f /dev/stdin <<EOF
...
...
EOF

Unless of course it needs stdin for something else. In that case, you can should be able to use another fd with the here-doc:

tor -f /dev/fd/9 9<<EOF
...
...
EOF

(though you need a system that supports /dev/fd/NN)