Dash and C: eval "$(

Question

I would like to use the system() function in C to evaluate an expression within a file cmdfile but I'm not getting the same results as when I do so on the command line directly. The content of cmdfile is the following:

$ cat cmdfile
echo hello

and when I evaluate its content on the command line directly, it works:

$ eval "$(<cmdfile)"
hello

To do the same in C, I'm using system(). This is my code:

$ cat systest.c
#include <stdio.h>
#include <string.h>

int main (int argc, char* argv[])
{
    char* cmd = argv[1];
    printf("%s\n", cmd);
    system(cmd);
    return 0;
}

The trouble is that I don't see any output when using the above code:

$ ./systest "eval \"\$(<cmdfile)\""
eval "$(<cmdfile)"

There should be hello printed right after the printf output but it doesn't work. Still, I know that system() is definitely doing something, because if I give it a non-existing filename, dash complains:

$ ./systest "eval \"\$(<cmdfileFF)\""
eval "$(<cmdfileFF)"
sh: 1: cannot open cmdfileFF: No such file

and if I just evaluate echo hello without involving cmdfile, it works too:

$ ./systest "eval \"echo hello\""
eval "echo hello"
hello

I'd like to know what is causing this difference in behaviour. Is there any other way of executing the content of cmdfile in dash? I'm restricted to only using the built-in commands of dash on the command line, so options such as ./systest "eval \"\$(cat cmdfile)\"" are not possible. Further, the expansion of "$(<cmdfile)" should only happen within system(), not before (thus ./systest "eval \"$(<cmdfile)\"" won't work.

I tested this with dash 0.5.10.2-6 and dash 0.5.8-2.1ubuntu2.

Thank you for any insight!

Edit

Thanks to Jonathan Leffler's comment, I now realise that dash doesn't understand the $(<file) syntax. So what would be a dash-compatible equivalent?

Wrap-up

So my confusion was due to the fact that system(...) always uses /bin/sh, but when testing my expressions on the command line, I was accidentally invoking bash instead of dash. Hence the results were different.

Thanks for the comment, @JonathanLeffler. /bin/sh (and /usr/bin/sh) are sym links pointing to dash on my system. If dash doesn't understand $( — borizzzzz, Sep 08 '19 at 07:34
@alk, so that the expression within the brackets only gets expanded in system(), not before. — borizzzzz, Sep 08 '19 at 07:38
I expected the C compiler to complain about the escaped `$`. — alk, Sep 08 '19 at 07:39
@alk — the C compiler doesn’t see the dollar sign, escaped or unescaped. The C runtime might see it; it depends on what the she’ll does. — Jonathan Leffler, Sep 08 '19 at 07:42
@JonathanLeffler: Ah well yes, I assumed this went literally into the C code. Which it probably does not. — alk, Sep 08 '19 at 07:45
Using single quotes around the argument probably makes life easier than using double quotes. There’s no need to escape anything except single quotes. Make sure you see what is being passed to you program. Add `fflush(stdout);` before the call to `system()`. — Jonathan Leffler, Sep 08 '19 at 07:46
I *think* dot-sourcing is equivalent to `eval`…? `./systest '. ./cmdfile'` — Ry-, Sep 08 '19 at 07:50
@oguzismail I wanted to read the file without changing the rights. I just tested Ry- 's suggestion and it does seem to work with my version of dash. So I guess that's the only option for me for now. Thanks for helping! — borizzzzz, Sep 08 '19 at 08:06
@Ry-, your version did work for me. If you want to type it into an answer, I'd be happy to accept. — borizzzzz, Sep 08 '19 at 08:07
`system` **always** runs `/bin/sh`, that's what POSIX says. Depending on things it might be more or less strictly POSIX-compliant... — Antti Haapala -- Слава Україні, Sep 08 '19 at 08:45

score 3 · Accepted Answer · answered Sep 08 '19 at 08:11

$(< …) substitution isn’t POSIX-sh-compatible, but your sh is restricted to about that. A general alternative is to replace < cmdfile with cat cmdfile:

./systest "eval \"\$(cat cmdfile)\""

but I think dot-sourcing is equivalent in this case:

./systest '. ./cmdfile'

score 1 · Answer 2 · answered Sep 08 '19 at 09:30

The proper fix is to put a shebang line in the script and mark it as executable.

#!/bin/sh
echo "hello"

The shebang needs to be the absolutely first line of the file (its first two bytes should be # and !). The quoting around the argument to echo is not strictly necessary here, but good practice. (See also When to wrap quotes around a shell variable?)

Changing the permissions only needs to be done once, when you have just created the file:

chmod +x ./cmdfile

Now, you can simply use

system("./cmdfile")

Dash and C: eval "$(

Edit

Wrap-up

2 Answers2