I have the following shell script. 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. But in ksh on Debian, it does not print the error message due to invoking the bogus command. What went wrong? In case, you want to know about the

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.

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?
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 .

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Answer