Why does ksh fail to capture standard error during nested command substitutions?

I have the following shell script.

$ cat foo.sh 
foo()
{
    A=$(uname)
    printf "hello "
    bogus
}

echo output: "$(foo 2>&1)"

It produces the following output in bash, zsh, dash and posh. It makes sense because there is no such command called bogus on the system.

$ bash foo.sh
output: hello foo.sh: line 5: bogus: command not found
$ zsh foo.sh
output: hello foo:4: command not found: bogus
$ dash foo.sh
output: hello foo.sh: 5: foo.sh: bogus: not found
$ posh foo.sh
output: hello foo.sh:8: bogus: not found

But in ksh on Debian, it does not print the error message due to invoking the bogus command.

$ ksh foo.sh
output: hello 

What went wrong?

In case, you want to know about the system details and the version details of the shell, please see the following outputs.

$ uname -a
Linux debian1 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u2 (2016-01-02) x86_64 GNU/Linux
$ cat /etc/debian_version 
8.3
$ ksh --version
  version         sh (AT&T Research) 93u+ 2012-08-01
$ dpkg -l bash zsh dash posh ksh | tail -n 5
ii  bash           4.3-11+b1      amd64        GNU Bourne Again SHell
ii  dash           0.5.7-4+b1     amd64        POSIX-compliant shell
ii  ksh            93u+20120801-1 amd64        Real, AT&T version of the Korn shell
ii  posh           0.12.3         amd64        Policy-compliant Ordinary SHell
ii  zsh            5.0.7-5        amd64        shell with lots of features

On a CentOS system I see the expected output.

$ cat /etc/centos-release 
CentOS release 6.7 (Final)
$ ksh --version
  version         sh (AT&T Research) 93u+ 2012-08-01
$ ksh foo.sh
output: hello foo.sh[5]: bogus: not found [No such file or directory]

If I remove the command substitution in the function foo, then all shells produce similar output on Debian.

$ cat bar.sh
foo()
{
    printf "hello "
    bogus
}

echo output: "$(foo 2>&1)"
$ bash bar.sh
output: hello bar.sh: line 4: bogus: command not found
$ zsh bar.sh
output: hello foo:3: command not found: bogus
$ dash bar.sh
output: hello bar.sh: 4: bar.sh: bogus: not found
$ posh bar.sh
output: hello bar.sh:7: bogus: not found
$ ksh bar.sh
output: hello bar.sh[4]: bogus: not found [No such file or directory]

Why doesn’t ksh print the error due to the bogus command in the first example on Debian but produces it on CentOS?

I found the following text in the POSIX.1-2008 standard:

With the $(command) form, all characters following the open parenthesis to the matching closing parenthesis constitute the command. Any valid shell script can be used for command, except a script consisting solely of redirections which produces unspecified results.

I suspect that the part of the text that I have highlighted in bold font is responsible for causing unspecified behaviour in the above examples. However, I am not entirely sure because I couldn’t find a definition for “a script consisting solely of redirections” in the standard.

I have two questions.

1. Can we prove with sufficient references to the standard or man pages whether the output of ksh in the first example is a bug or not?
2. How else can I capture the standard error written by a shell function where the shell function is performing command substitution?

Answer

I searched about this issue and found following:

According to standard, this is an error because whenever a command is not found a “command not found” error is given.

This is a bug which has been resolved in beta branch of ksh. Use that branch. I tested it in ubuntu and it works for me.

Here’s how I did that:

git clone https://github.com/att/ast.git
cd ast
git checkout beta
bin/package make
cp arch/linux.i386-64/bin/ksh /usr/local/bin/ksh

There are many bugs in master branch’s ksh. Just check commit logs in https://github.com/att/ast/commits/beta .