Bash script execution with and without shebang in Linux and BSD

Question

How and who determines what executes when a Bash-like script is executed as a binary without a shebang?

I guess that running a normal script with shebang is handled with binfmt_script Linux module, which checks a shebang, parses command line and runs designated script interpreter.

But what happens when someone runs a script without a shebang? I've tested the direct execv approach and found out that there's no kernel magic in there - i.e. a file like that:

$ cat target-script
echo Hello
echo "bash: $BASH_VERSION"
echo "zsh: $ZSH_VERSION"

Running compiled C program that does just an execv call yields:

$ cat test-runner.c
void main() {
        if (execv("./target-script", 0) == -1)
                perror();
}
$ ./test-runner
./target-script: Exec format error

However, if I do the same thing from another shell script, it runs the target script using the same shell interpreter as the original one:

$ cat test-runner.bash
#!/bin/bash
./target-script

$ ./test-runner.bash 
Hello
bash: 4.1.0(1)-release
zsh: 

If I do the same trick with other shells (for example, Debian's default sh - /bin/dash), it also works:

$ cat test-runner.dash   
#!/bin/dash
./target-script

$ ./test-runner.dash 
Hello
bash: 
zsh: 

Mysteriously, it doesn't quite work as expected with zsh and doesn't follow the general scheme. Looks like zsh executed /bin/sh on such files after all:

greycat@burrow-debian ~/z/test-runner $ cat test-runner.zsh 
#!/bin/zsh
echo ZSH_VERSION=$ZSH_VERSION
./target-script

greycat@burrow-debian ~/z/test-runner $ ./test-runner.zsh 
ZSH_VERSION=4.3.10
Hello
bash: 
zsh: 

Note that ZSH_VERSION in parent script worked, while ZSH_VERSION in child didn't!

How does a shell (Bash, dash) determines what gets executed when there's no shebang? I've tried to dig up that place in Bash/dash sources, but, alas, looks like I'm kind of lost in there. Can anyone shed some light on the magic that determines whether the target file without shebang should be executed as script or as a binary in Bash/dash? Or may be there is some sort of interaction with kernel / libc and then I'd welcome explanations on how does it work in Linux and FreeBSD kernels / libcs?

Answer 1

Since this happens in dash and dash is simpler, I looked there first.

Seems like exec.c is the place to look, and the relevant functionis are tryexec, which is called from shellexec which is called whenever the shell things a command needs to be executed. And (a simplified version of) the tryexec function is as follows:

STATIC void
tryexec(char *cmd, char **argv, char **envp)
{
        char *const path_bshell = _PATH_BSHELL;

repeat:

        execve(cmd, argv, envp);

        if (cmd != path_bshell && errno == ENOEXEC) {
                *argv-- = cmd;
                *argv = cmd = path_bshell;
                goto repeat;
        }
}

So, it simply always replaces the command to execute with the path to itself (_PATH_BSHELL defaults to "/bin/sh") if ENOEXEC occurs. There's really no magic here.

I find that FreeBSD exhibits identical behavior in bash and in its own sh.

The way bash handles this is similar but much more complicated. If you want to look in to it further I recommend reading bash's execute_command.c and looking specifically at execute_shell_script and then shell_execve. The comments are quite descriptive.

Answer 2

(Looks like Sorpigal has covered it but I've already typed this up and it may be of interest.)

According to Section 3.16 of the Unix FAQ, the shell first looks at the magic number (first two bytes of the file). Some numbers indicate a binary executable; #! indicates that the rest of the line should be interpreted as a shebang. Otherwise, the shell tries to run it as a shell script.

Additionally, it seems that csh looks at the first byte, and if it's #, it'll try to run it as a csh script.