Subshell and process substitution

Question

Apologies if this is a basic question - I'm stuck trying to solve a larger problem, and it's come down to how a shell script is invoked - directly (shellScript.sh) or using sh shellScript.sh.

Here's a model for the problem:

When I execute on bash:

cat <(echo 'Hello')

I see the output

Hello

But when I use:

sh -c "cat <(echo 'Hello')"

I see errors:

sh: -c: line 0: syntax error near unexpected token `('
sh: -c: line 0: `cat <(echo 'Hello')'

I've tried escaping the <, ( and ) in various combinations, but I don't see the output anywhere. What am I missing here?

My actual problem is that I'm passing a <() as an input argument to a python script within a shell script, and while it works fine when I invoke the shell script using just the name, if I use sh to invoke it, I get errors similar to what I've shown above.

Thank you!

Answer 1

Process substitution is a feature that originated in the Korn shell in the 80s (first documented in ksh86). At the time, it was only available on systems that had support for /dev/fd/<n> files.

Later, the feature was added to zsh (from the start: 1990) and bash (in 1993). zsh was using temporary named pipes to implement it, while bash was using /dev/fd/<n> where available and named pipes otherwise. zsh switched to using /dev/fd/<n> where available in 2.6-beta17 in 1996.

Support for process substitution via named pipes on systems without /dev/fd was only added to ksh in ksh93u+ in 2012. The public domain clone of ksh doesn't support it.

To my knowledge, no other Bourne-like shell supports it (rc, es, fish, non-Bourne-like shells support it but with a different syntax). yash has a <(...) construct, but that's for process redirection.

While quite useful, the feature was never standardized by POSIX. So, one can't expect to find it in sh, so shouldn't use it in a sh script.

Though the behaviour for <(...) is unspecified in POSIX, (so there would be no harm in retaining it), bash disables the feature when called as sh or when called with POSIXLY_CORRECT=1 in its environment.

So, if you have a script that uses <(...), you should use a shell that supports the feature to interpret it like zsh, bash or AT&T ksh (of course, you need to make sure the rest of the syntax of script is also compatible with that shell).

In any case:

cat <(cmd)

Can be written:

cmd | cat

Or just

cmd

For a command other than cat (that needs to be passed data via a file given as argument), on systems with /dev/fd/x, you can always do:

something | that-cmd /dev/stdin

Or if you need that-cmd's stdin to be preserved:

{ something 3<&- | that-cmd /dev/fd/4 4<&0 <&3 3<&-; } 3<&0